Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
The Last 1000 Years
ABSTRACT: (1) The pollen and plant macrofossils in sediments from three high-altitude lakes in the Sierra Nevada, California, document vegetation changes that have occurred over the last 12 500 years. (2) Trees became established around the lakes by c. 10 000 years ago. Early Holocene forests were structurally different from those of today; they were more open than at present, with more montane chaparral shrubs. Several tree species characteristic of modern subalpine forests were rare and restricted to mesic habitats. Significantly drier conditions are inferred. (3) By c. 6000 B.P., effective precipitation had increased, as shown by an increase in subalpine conifers, principallyTsuga mertensiana (mountain hemlock) andAbies magnifica (red fir). (4) The upper altitudinal limits of many subalpine conifers began to fall c. 2500 B.P., coincident with the beginning of Neoglacial cooling. (5) The climatic interpretations are consistent with those previously described for the Pacific North-west region of North America, but not consistent with those for the American South-west.
Gaffen, D.J., T. J. Crowley, S. K. Baum, K.-Y. Kim, W. T. Hyde (2003). Detection of volcanic, solar and greenhouse gas signals in paleo-reconstructions of Northern Hemispheric temperature. Geophysical Research Letters 30 (5): 1242, doi:10.1029/2002GL016635
ABSTRACT: We apply a multiple regression method to estimate the response to anthropogenic and natural climate forcings simultaneously from a number of paleo-reconstructions of Northern Hemispheric average temperature. These long records (600 to 1000 years) provide a unique opportunity to distinguish between different external influences on climate. The response to volcanic forcing is reliably detected in all reconstructions, and the simulated temperature response to volcanic eruptions compares favorably with observations. The response to solar forcing is detected in Hemispheric mean data only over some periods in some records, and appears weak. Although most records can be used only to the middle of the 20th century, the temperature response to CO2 can be detected by this time in most records.
ABSTRACT: Seasonal and annual temperature reconstructions derived from western North American semi-arid site tree-ring chronologies are used to examine the possible spatial response of North American climate to volcanic eruptions within the period 1602 to 1900. Low-latitude eruptions appear to give the strongest response. Cooling of the annual average temperatures in the central and eastern United States is reconstructed to follow volcanic eruptions with warming in the western states. The magnitude and spatial extent of the reconstructed cooling and warming varies seasonally. The warming that occurs in the west is strongest and most extensive in winter while the cooling in the east is most marked in summer. These results are based on reconstructed climate records which contain error terms unrelated to climatic factors. The suggested pattern of response to volcanic forcing is, however, supported by four independent temperature/proxy temperature series within the area of the temperature reconstructions. Additional support is provided by three independent series lying outside the area which suggest that the temperature spatial response may extend to the north beyond the area covered by the tree-ring reconstructions.
D. T. Shindell, G. A. Schmidt, M. E. Mann, G. Faluvegi (2004). Dynamic winter climate response to large tropical volcanic eruptions since 1600. Journal of Geophysical Research 109 (D05104): doi:10.1029/2003JD004151
ABSTRACT: We have analyzed the mean climate response pattern following large tropical volcanic eruptions back to the beginning of the 17th century using a combination of proxy-based reconstructions and modern instrumental records of cold-season surface air temperature. Warm anomalies occur throughout northern Eurasia, while cool anomalies cover northern Africa and the Middle East, extending all the way to China. In North America, the northern portion of the continent cools, with the anomalies extending out over the Labrador Sea and southern Greenland. The analyses confirm that for two years following eruptions the anomalies strongly resemble the Arctic Oscillation/Northern Annular Mode (AO/NAM) or the North Atlantic Oscillation (NAO) in the Atlantic-Eurasian sector. With our four-century record, the mean response is statistically significant at the 95% confidence level over much of the Northern Hemisphere land area. However, the standard deviation of the response is larger than the mean signal nearly everywhere, indicating that the anomaly following a single eruption is unlikely to be representative of the mean. Both the mean response and the variability can be successfully reproduced in general circulation model simulations. Driven by the solar heating induced by the stratospheric aerosols, these models produce enhanced westerlies from the lower stratosphere down to the surface. The climate response to volcanic eruptions thus strongly suggests that stratospheric temperature and wind anomalies can affect surface climate by forcing a shift in the AO/NAM or NAO.
ABSTRACT: We examine the climate response to solar irradiance changes between the late 17th-century Maunder Minimum and the late 18th century. Global average temperature changes are small (about 0.3° to 0.4°C) in both a climate model and empirical reconstructions. However, regional temperature changes are quite large. In the model, these occur primarily through a forced shift toward the low index state of the Arctic Oscillation/North Atlantic Oscillation as solar irradiance decreases. This leads to colder temperatures over the Northern Hemisphere continents, especially in winter (1° to 2°C), in agreement with historical records and proxy data for surface temperatures.
ABSTRACT: There has been confirmation in the last two decades, through instrumental measurements onboard satellites, that the ‘solar constant’ does, as has long been hypothesized, vary over different timescales and with identifiable periodicities. This being the case, it is necessary to explore how these variations play a role in terrestrial climate change. While there is no consensus as to the best method for estimating past variations in solar output, it seems likely that over the last 500 years the sun has played a role in the changing climate. However, there is little evidence to suggest that changes in irradiance are having a large impact on the current warming trend.
ABSTRACT: El Niño events—anomalous warmings of the tropical Pacific with associated climatic and economic impacts around the globe—have occurred at several-year intervals since before written records began with the logs of Francisco Pizarro in 1525. In this review, the history of El Niño research is traced from its beginnings through the key innovations of Bjerknes and Wyrtki to the unusual 1982-1983 event. Recent research is then reviewed, with detailed discussions of two important processes: instability growth and vacillation between climate states. Throughout the paper there are adjunct discussions of extraregional teleconnections, ecological impacts, and research on El Niño in the ancient record. The final section discusses the present paradigm for vacillations between El Niño and non-El Niño states and speculates on the possibly chaotic nature of El Niño. El Niño and its atmospheric counterpart, the Southern Oscillation, appear to occur as an internal cycle of positive and negative feedbacks within the coupled ocean-atmosphere climate system of the tropical Pacific, although hypotheses based on external forcing also exist. All events are preceded by westerly wind anomalies on the equator near the date line. Baroclinic equatorial Kelvin waves are generated, propagating eastward toward South America where they depress the thermocline and raise sea level, while the deep, upper ocean reservoir of warm water in the western Pacific is depleted. Sea surface temperature (SST) anomalies in the cool eastern Pacific occur primarily because the normal source of cold water is depressed below the reach of mixing and upwelling processes. In the central equatorial Pacific, eastward advection by anomalous zonal flows is the principal mechanism. Nonlinear heat transfer to the lower atmosphere creates a positive ocean-atmosphere feedback resulting in the unstable growth of anomalies along the equator. Much of the present research aims at determining how the ocean-atmosphere system vacillates between the El Niño and non-El Niño states. Coupled models suggest that a longer time scale, negative-feedback process produces the transitions: at the apex of an El Niño development an anomalous atmospheric convection above the areas of maximum SST produces areas of reduced upper layer thickness in the off-equatorial ocean, which slowly propagate westward to the western boundary as Rossby waves and back to the central equatorial Pacific as upwelling Kelvin waves, reestablishing the normal cooling process. A similar negative feedback of opposite sign completes the second half of an oscillation, returning again to the El Niño state. However, the notion that El Niño-Southern Oscillation variability results only from an internal feedback process is still highly contentious, and a number of external forcing mechanisms have been proposed.
ABSTRACT: The instrumental record is too brief for evaluation of the El Niño-Southern Oscillation (ENSO) system and its long-term response to climate forcing. To supplement these data, we use a new reconstruction of December–February Niño-3 sea surface temperatures based on subtropical North American tree-ring records to investigate aspects of ENSO variability over the past six centuries (AD 1408–1978). Spectral analyses reveal that the reconstruction best resolves variability within the “classical” ENSO band of 2–8 years. A low amplitude ENSO epoch in the 17th to 18th centuries broadly coincides with “Little Ice Age” conditions over much of the globe. The detailed behavior shows good agreement with shorter tree-ring reconstructions of ENSO over the past few centuries, but differs at times from other longer coral ENSO records and recent model simulations of past ENSO behavior. We discuss possible reasons for these discrepancies.
ABSTRACT: This study uses proxy climate records derived from paleoclimate data to investigate the long-term behaviour of the Pacific Decadal Oscillation (PDO) and the El Niño Southern Oscillation (ENSO). During the past 400 years, climate shifts associated with changes in the PDO are shown to have occurred with a similar frequency to those documented in the 20th Century. Importantly, phase changes in the PDO have a propensity to coincide with changes in the relative frequency of ENSO events, where the positive phase of the PDO is associated with an enhanced frequency of El Niño events, while the negative phase is shown to be more favourable for the development of La Niña events.
ABSTRACT: The authors have analyzed global station data and created a gridded dataset of monthly precipitation for the 1900-1988 period. Statistical analyses suggest that discontinuities associated with instrumental errors are large for many high-latitude station records although they are unlikely to be significant for the majority of the stations. The first leading EOF in global precipitation fields is an ENSO-related pattern concentrating mostly in the low latitudes. The second leading EOF depicts a linear increasing trend (~2.4 mm/decade) in global precipitation fields during the 1900-1988 period. Consistent with the zonal precipitation trends identified in previous analyses, the EOF trend is seen as a long-term increase mostly in North America, mid- to high-latitude Eurasia, Argentina and Australia. The spatial patterns of the trend EOF and the rate of increase are generally consistent with those of the precipitation changes in increasing CO2 GCM experiments.
The North Atlantic Oscillation accounts for ~10% of December-February precipitation variance over North Atlantic surrounding regions. The mode suggests that during high-NAO-index winters, precipitation is above normal in northern (>50°N) Europe, the eastern United States, northern Africa and the Mediterranean; while below-normal precipitation occurs in southern Europe, eastern Canada and western Greenland.
Wet and dry months of one standard deviation occur at probabilities close to those of a normal distribution in midlatitudes. In the subtropics, the mean interval between two extreme events is longer. The monthly wet and dry events seldom (probability <5%) last longer than 2 months. ENSO is the single largest cause for global extreme precipitation events. Consistent with the upward trend in global precipitation, globally, the averaged mean interval between two dry months increased by ~28% from 1900-1944 to 1945-1988. The percentage of wet areas over the U.S. has more than doubled (from ~12% to >24%) since the 1970s while the percentage of dry areas has decreased by a similar amount since the 1940s. Severe droughts and floods comparable to the midwest U.S. 1988 drought and 1993 flood have occurred 2-9 times in each of several other regions of the world during this century.
ABSTRACT: Anticipating the consequences of climatic change for fire requires understanding of the causes of variation in historical fire regimes. We assessed the influence of annual and decadal variation in climate on fire regimes of ponderosa pine-dominated forests in eastern Oregon and Washington using existing, annually dated tree-ring reconstructions (1687–1994). In four watersheds, we compared the extent of low-severity fires (total area burned each year) to precipitation and the Southern Oscillation Index, a measure of variation in El Niño-Southern Oscillation (ENSO), which affects weather in this region. At the annual scale, large fires burned during dry years and El Niño years (low SOI) in all watersheds while small fires burned regardless of variation in these climate parameters. Large fires also burned during relatively wet years and La Niña years (high SOI) in one watershed, indicating that local factors can override regional climate controls in some locations. Climate from previous years did not influence current year's fire extent. The influence of ENSO on fire regimes in this region has not previously been demonstrated at these multicentury, regional scales. At the decadal scale, fire extent varied with precipitation, perhaps in response to variation in such climate features as the Pacific Decadal Oscillation. Several decades of low fire extent in the watersheds during the early 1800s was synchronous with a lack of fire at other sites in North and South America, probably in response to a change in the global climate that included a lessening in the frequency and/or intensity of ENSO events.
ABSTRACT: Climate in the North Pacific and North American sectors has experienced interdecadal shifts during the 20th century. A network of recently developed tree-ring chronologies for Southern and Baja California extends the instrumental record, and reveals decadal-scale variability back to AD 1661. The Pacific Decadal Oscillation (PDO) is closely matched by the dominant mode of tree-ring variability, which provides a preliminary view of multi-annual climate fluctuations spanning the past four centuries. The reconstructed PDO index features a prominent bidecadal oscillation, whose amplitude weakened in the late 1700s to mid-1800s. A comparison with proxy records of ENSO suggests that the greatest decadal-scale oscillations in Pacific climate between 1706 and 1977 occurred around 1750, 1905, and 1947.
ABSTRACT: Annual growth increments from trees and coral heads provide an opportunity to develop proxy records of North Pacific climatic variability that extend back in time well beyond the earliest instrumental records, and in regions where records have not been kept. Here we combine five published proxy records of North Pacific climatic variability in order to identify the extent to which these records provide a coherent picture of Pacific Basin climatic variability. This composite chronology is well correlated with the Pacific Decadal Oscillation (PDO) index, and provides a better record of PDO variability than any of the constituent chronologies back to 1840. A comparison of these records suggests that the PDO may not have been an important organizing structure in the North Pacific climate system over much of the 19th century, possibly indicating changes in the spatial pattern of sea-level pressure and consequent surface climate patterns of variability over the Americas.
ABSTRACT: Ecological theory asserts that the climate of a region exerts top-down controls on regional ecosystem patterns and processes, across space and time. To provide empirical evidence of climatic controls, it would be helpful to define climatic regions that minimized variance in key climate attributes, within climatic regions—define the periods and features of climatic regimes, and then look for concordance between regional climate and ecosystem patterns or processes. In the past, these steps have not been emphasized. Before we evaluated the recent climate of the northwestern United States, we established a Northwest climatic region by clustering time series of the Palmer Drought Severity Index (PDSI) for the period of 1675–1978, for the western United States. The background climatic regime and anomalies of the recent northwestern U.S. climate were then identified through temporal pattern analysis involving application of correspondence analysis to the same PDSI time series.
Our analysis distinguished 10 distinct periods and four unique types of regimes (climatic signals). Five of the 10 periods (79% of the 300-year record) were marked by mild and equitable moisture conditions (Pacific regime), the “background” climate of the Northwest. The remaining periods were anomalies. Two periods displayed a high-variance, mixed signal marked by switching between severe to extreme annual to interannual dry and wet episodes (High/Mixed regime; 9% of the record). Two more periods displayed a moderate-variance, mixed signal marked by switching between moderate to severe annual to interannual dry and wet episodes (Moderate/Mixed regime; 5%). Only one period was unidirectional and relatively low variance, marked by persistent yet mild to moderate drought (Low/Dry regime, 7%).
Our method distinguished decadal- to interdecadal-scale regimes, defined regime periods, and detected both mixed and unidirectional anomalies from the background climate. The ability to distinguish the variance, direction, and period of sequential climatic regimes provides a plausible basis for examining the role of past climate within terrestrial ecosystems of the Northwest. For example, we found concordance between the period of the Low/Dry anomaly and a period of tree establishment in the Olympic Mountains of Washington, close alignment between tree growth with the Moderate/Mixed and High/Mixed signals in Oregon, and a mixed fire response to mixed climatic signals in northeastern Oregon. Linking historical climatic regimes to particular ecosystem patterns and processes also aids in the prediction of future ecosystem changes by providing evidence of the kinds of interactions that may be anticipated.
ABSTRACT: March-August sea surface temperatures (SST) are reconstructed for the Gulf of Alaska (GOA) from 1750-1983 based on tree-ring data from coastal and south-central Alaska and the Pacific Northwest. Some of the trends resemble those documented in other northern instrumental and proxy records, including cooler SSTs in the early and middle 1800s, during the Little Ice Age. There is overall warming in this century, including a positive trend from the mid-1970s to 1980s, following cooler 1960s-1970s. The twentieth century warming exceeds maxima in the reconstructed SSTs back to AD 1750 and is consistent with other evidence for unusual Northern Hemisphere warming. Changes over the period of recorded North Pacific SST have been linked to a pattern of variability known as the Pacific Decadal Oscillation (PDO). Maps comparing the reconstruction to the North Pacific SST field and other analyses suggest that it may reflect variations related to the PDO over several centuries.
Benson, L., B. Linsley, J. Smoot, S. Mensing, S. Lund, S. Stine, A. Sarna-Wojcicki (2003). Influence of the Pacific Decadal Oscillation on the climate of the Sierra Nevada, California and Nevada. Quaternary Research 59 (2): 151-159
ABSTRACT: Mono Lake sediments have recorded five major oscillations in the hydrologic balance between A.D. 1700 and 1941. These oscillations can be correlated with tree-ring-based oscillations in Sierra Nevada snowpack. Comparison of a tree-ring-based reconstruction of the Pacific Decadal Oscillation (PDO) index  with a coral-based reconstruction of Subtropical South Pacific sea-surface temperature  indicates a high degree of correlation between the two records during the past 300 yr. This suggests that the PDO has been a pan-Pacific phenomena for at least the past few hundred years. Major oscillations in the hydrologic balance of the Sierra Nevada correspond to changes in the sign of the PDO with extreme droughts occurring during PDO maxima. Four droughts centered on A.D. 1710, 1770, 1850, and 1930 indicate PDO-related drought reoccurrence intervals ranging from 60 to 80 yr.
ABSTRACT: Low-frequency (periodicities lower than 20 years) hydrologic variability in the western United States over the past 500 years is studied using available tree-ring reconstructions of Palmer Drought Severity Index (PDSI), streamflow, and climate indices. Leading rotated principal component (RPC) scores of a gridded tree-ring reconstruction of the PDSI from 1525 to 1975 are significantly correlated with indices representing large-scale climate variations from the Pacific and Atlantic Oceans. RPC1 (31%) is related to the influence of North Pacific sea surface temperature (SST) variations, indexed by the Pacific Decadal Oscillation (PDO). RPC2 (24%) is apparently related to North Atlantic SST variations, indexed by the Atlantic Multidecadal Oscillation (AMO). RPC3 (19%) is moderately correlated with a smoothed version of the Southern Oscillation Index. Consistent with recent studies of instrumental data, RPC1 (PDO) and RPC2 (AMO) explain a large part of the multidecadal hydrologic variability of the interior western United States. Western U.S. PDSI variability exhibits significant pentadecadal (and longer) oscillations in the epochs from circa 1525 to 1650 and 1850 to 1975, while bidecadal oscillations are prevalent in the middle epoch from circa 1650 to 1850. The changes in spectral characteristics of western U.S. PDSI were related to similar changes in the PDO (and therefore in RPC1). In contrast, RPC2 had a regular periodicity of 51 years for the past ~500 years. This regularity is intriguing, and although RPC2 was primarily related to the AMO in this study, the influence from Pacific climate cannot be discarded.
INTRODUCTION: It has long been believed that the 20th warming was preceded by the "Little Ice Age" and the "Medieval Warm Period" as a result of the natural variability of climate. But as Bradley explains in this Perspective, recent research has shown that these climate patterns may not have been global, and are much more variable in time and space than previously assumed. However, one fact remains indisputable, namely that end-20th century temperatures were higher than at any time during the last millenium.
INTRODUCTION: During the Medieval Warm Period (800 to 1200 A.D.), the Vikings colonized Greenland. In his Perspective, Broecker discusses whether this warm period was global or regional in extent. He argues that it is the last in a long series of climate fluctuations in the North Atlantic, that it was likely global, and that the present warming should be attributed in part to such an oscillation, upon which the warming due to greenhouse gases is superimposed.
ABSTRACT: The western United States is experiencing a severe multiyear drought that is unprecedented in some hydroclimatic records. Using gridded drought reconstructions that cover most of the western United States over the past 1200 years, we show that this drought pales in comparison to an earlier period of elevated aridity and epic drought in AD 900 to 1300, an interval broadly consistent with the Medieval Warm Period. If elevated aridity in the western United States is a natural response to climate warming, then any trend toward warmer temperatures in the future could lead to a serious long-term increase in aridity over western North America.
ABSTRACT: Recent reconstructions of Northern Hemisphere temperatures and climate forcing over the past 1000 years allow the warming of the 20th century to be placed within a historical context and various mechanisms of climate change to be tested. Comparisons of observations with simulations from an energy balance climate model indicate that as much as 41 to 64% of preanthropogenic (pre-1850) decadal-scale temperature variations was due to changes in solar irradiance and volcanism. Removal of the forced response from reconstructed temperature time series yields residuals that show similar variability to those of control runs of coupled models, thereby lending support to the models' value as estimates of low-frequency variability in the climate system. Removal of all forcing except greenhouse gases from the ~1000-year time series results in a residual with a very large late-20th-century warming that closely agrees with the response predicted from greenhouse gas forcing. The combination of a unique level of temperature increase in the late 20th century and improved constraints on the role of natural variability provides further evidence that the greenhouse effect has already established itself above the level of natural variability in the climate system. A 21st-century global warming projection far exceeds the natural variability of the past 1000 years and is greater than the best estimate of global temperature change for the last interglacial.
ABSTRACT: We review evidence for climate change over the past several millennia from instrumental and high-resolution climate “proxy” data sources and climate modeling studies. We focus on changes over the past 1 to 2 millennia. We assess reconstructions and modeling studies analyzing a number of different climate fields, including atmospheric circulation diagnostics, precipitation, and drought. We devote particular attention to proxy-based reconstructions of temperature patterns in past centuries, which place recent large-scale warming in an appropriate longer-term context. Our assessment affirms the conclusion that late 20th century warmth is unprecedented at hemispheric and, likely, global scales. There is more tentative evidence that particular modes of climate variability, such as the El Niño/Southern Oscillation and the North Atlantic Oscillation, may have exhibited late 20th century behavior that is anomalous in a long-term context. Regional conclusions, particularly for the Southern Hemisphere and parts of the tropics where high-resolution proxy data are sparse, are more circumspect. The dramatic differences between regional and hemispheric/global past trends, and the distinction between changes in surface temperature and precipitation/drought fields, underscore the limited utility in the use of terms such as the “Little Ice Age” and “Medieval Warm Period” for describing past climate epochs during the last millennium. Comparison of empirical evidence with proxy-based reconstructions demonstrates that natural factors appear to explain relatively well the major surface temperature changes of the past millennium through the 19th century (including hemispheric means and some spatial patterns). Only anthropogenic forcing of climate, however, can explain the recent anomalous warming in the late 20th century.
ABSTRACT: Spatially resolved global reconstructions of annual surface temperature patterns over the past six centuries are based on the multivariate calibration of widely distributed high-resolution proxy climate indicators. Time-dependent correlations of the reconstructions with time-series records representing changes in greenhouse-gas concentrations, solar irradiance, and volcanic aerosols suggest that each of these factors has contributed to the climate variability of the past 400 years, with greenhouse gases emerging as the dominant forcing during the twentieth century. Northern Hemisphere mean annual temperatures for three of the past eight years are warmer than any other year since (at least) AD 1400.
ABSTRACT: Climate exerts a profound influence on landscape by determining the flux of both energy (solar radiation) and mass (rain, snow, and water vapor). If climate changes significantly, the landscape can be expected to respond geomorphologically, hydrologically, and biologically. These individual responses, in turn, can feed on one another, creating a cascade of landscape perturbations.
Around 1850, just as large numbers of Europeans descended on the Sierra Nevada for the first time, the region experienced a marked shift in climate, from the abnormally cool and moderately dry conditions of the previous two centuries (the “Little Ice Age”), to the relatively warm and wet conditions that have characterized the past 145 years. This climatic shift should concern land managers for two interrelated reasons: First, the landscape changes that have occurred since 1850 may not be entirely anthropogenic but rather attributable in part to the shift in climate. Second, the landscape of the immediate pre–gold rush period should not be considered an exact model for what the Sierra would be today had Europeans never colonized the region. Thus, attempts to restore “natural conditions” as part of an overall Sierra Nevada management plan should focus not on the pre-European landscape but rather on the landscape that would have evolved during the past century and a half in the absence of Europeans.
Using proxy climatic records, this chapter explores the Sierra Nevada climate of the period 1650–1850 and compares it to that of the modern (post-1850) period. The focus is on climate at the decade to century scale, rather than on individual years or meteorological events. Emphasis is placed on records from lakes, glaciers, tree lines, and tree rings that can be resolved to time scales of multiple decades or less. Other types of proxy indicators, such as pollen and pack-rat records, while indispensable for illuminating multiple-century to millennial changes in climate, are not included in this analysis.
ABSTRACT: Hydrologically sensitive tree-ring chronologies fromPinus flexilis in California and Alberta were used to produce an AD 993–1996 reconstruction of the Pacific Decadal Oscillation (PDO) and to assess long-term variability in the PDO's strength and periodicity. The reconstruction indicates that a ~50 to 70 year periodicity in the PDO is typical for the past 200 years but, was only intermittently a strong mode of variability prior to that. Between AD 1600 and 1800 there is a general absence of significant variability within the 50 to 100 year frequency range. Significant variability within in the frequency range of 50 to 100 years reemerges between AD 1500 and 1300 and AD 1200 to 1000. A prolonged period of strongly negative PDO values between AD 993 and 1300 is contemporaneous with a severe medieval megadrought that is apparent in many proxy hydrologic records for the western United States and Canada.
ABSTRACT: Winter climatic anomalies in the North Pacific sector and western North America are statistically calibrated with tree-ring data in western North America and reconstructed back to AD 1700. The results are verified using climatic data from the last half of the 19th century, which is prior to the calibration period. Climatic conditions reconstructed for 18th and 19th century winters are then summarized and compared with the 20th century record.
Blasing, T.J., Stahle, D. W., Duvick, D. N. (1988). Tree ring-based reconstruction of annual precipitation in the south-central United States from 1750 to 1980. Water Resources Research 24 (1): 163-171
ABSTRACT: A 231-year reconstruction of annual precipitation, from 1750 through 1980 A.D., was developed from 10 tree ring chronologies (9 post oak,Quercus stellata , and 1 white oak,Q. alba , series) in the south-central United States. Straight line regression was used to calibrate regionally averaged precipitation with ring width data, and the derived reconstruction was verified with independent climatic data and historical evidence. A variance trend in the tree ring data, which may have resulted from nonclimatic factors, was removed. The reconstructed precipitation series indicates that (1) a drought which appears to have been more severe than any in the instrumental record occurred about 1860 and (2) severe and prolonged droughts comparable to twentieth century events have occurred at roughly 15- to 25-year intervals throughout the past 231 years. It follows that serious droughts in the south-central United States could be expected to recur even in the absence of projected CO2 -induced warming.
ABSTRACT: Long-term records of precipitation variation for three regions within the Pacific Northwest are reconstructed based on ring-width data from drought-sensitive conifers. Precipitation reconstructions are derived using multiple regression models that predict variation in annual precipitation as a function of standardized and prewhitened tree-ring chronologies. The precipitation reconstructions indicate that droughts similar in magnitude and duration to those observed in the 1920s and 1930s have occurred frequently, at least once per century, in the past. The timing of drought episodes varies spatially, most notably during the nineteenth century. During the first half of the nineteenth century, precipitation was above average in Washington and northern Oregon but below average in southern Oregon and northern California. During the latter half of the nineteenth century, southern Oregon and northern California experienced above average precipitation while Washington and northern Oregon experienced repeated droughts. In contrast, severe, single-year drought events (1973, 1929, 1899, 1839, 1739, 1721, 1717) have affected the Pacific Northwest as a whole, reflecting the scale and persistence of the circulation features that cause such extreme events.
S.T. Gray, C. L. Fastie, S.T. Jackson, Julio L. Betancourt (2004). Tree-ring-based reconstruction of precipitation in the Bighorn Basin, Wyoming, since 1260 A.D.. Journal of Climate 17 (19): 3855-3865
ABSTRACT: Cores and cross sections from 79 Douglas fir (Pseudotsuga menziesii ) and limber pine (Pinus flexilis ) trees at four sites in the Bighorn Basin of north-central Wyoming and south-central Montana were used to develop a proxy for annual (June–June) precipitation spanning 1260–1998 a.d. The reconstruction exhibits considerable nonstationarity, and the instrumental era (post-1900) in particular fails to capture the full range of precipitation variability experienced in the past 750 years. Both single-year and decadal-scale dry events were more severe before 1900. Dry spells in the late thirteenth and sixteenth centuries surpass both magnitude and duration of any droughts in the Bighorn Basin after 1900. Precipitation variability appears to shift to a higher-frequency mode after 1750, with 15–20-yr droughts becoming rare. Comparisons between instrumental and reconstructed values of precipitation and indices of Pacific basin variability reveal that precipitation in the Bighorn Basin generally responds to Pacific forcing in a manner similar to that of the southwestern United States (drier during La Niña events), but high country precipitation in areas surrounding the basin displays the opposite response (drier during El Niño events).
ABSTRACT: Cores and cross sections from 133 limber pine (Pinus flexilis James) and Douglas fir (Pseudotsuga menziesii (Mirbel) Franco) at four sites were used to estimate annual (July to June) precipitation in the Yellowstone National Park region for the period from AD 1173 to 1998. Examination of the long-term record shows that the early 20th century was markedly wet compared to the previous 700 yr. Extreme wet and dry years within the instrumental period fall within the range of past variability, and the magnitude of the worst-case droughts of the 20th century (AD 1930s and 1950s) was likely equaled or exceeded on numerous occasions before AD 1900. Spectral analysis showed significant decadal to multidecadal precipitation variability. At times this lower frequency variability produces strong regime-like behavior in regional precipitation, with the potential for rapid, high-amplitude switching between predominately wet and predominately dry conditions. Over multiple time scales, strong Yellowstone region precipitation anomalies were almost always associated with spatially extensive events spanning various combinations of the central and southern U.S. Rockies, the northern U.S.–Southern Canadian Rockies and the Pacific Northwest.
S.T. Gray, S.T. Jackson, J. L. Betancourt (2004). Tree-ring based reconstructions of interannual to decadal scale precipitation variability for northeastern Utah since 1226 A.D.. Journal of the American Water Resources Association 40 (4): 947-960
ABSTRACT: Samples from 107 piñon pines (Pinus edulis ) at four sites were used to develop a proxy record of annual (June to June) precipitation spanning the 1226 to 2001 AD interval for the Uinta Basin Watershed of northeastern Utah. The reconstruction reveals significant precipitation variability at interannual to decadal scales. Single-year dry events before the instrumental period tended to be more severe than those after 1900. In general, decadal scale dry events were longer and more severe prior to 1900. In particular, dry events in the late 13th, 16th, and 18th Centuries surpass the magnitude and duration of droughts seen in the Uinta Basin after 1900. The last four decades of the 20th Century also represent one of the wettest periods in the reconstruction. The proxy record indicates that the instrumental record (approximately 1900 to the Present) underestimates the potential frequency and severity of severe, sustained droughts in this area, while over representing the prominence of wet episodes. In the longer record, the empirical probability of any decadal scale drought exceeding the duration of the 1954 through 1964 drought is 94 percent, while the probability for any wet event exceeding the duration of the 1965 through 1999 wet spell is only 1 percent. Hence, estimates of future water availability in the Uinta Basin and forecasts for exports to the Colorado River, based on the 1961 to 1990 and 1971 to 2000 “normal” periods, may be overly optimistic.
ABSTRACT: A time series of annual flow of the Sacramento River, California, is reconstructed to A.D. 869 from tree rings for a longterm perspective on hydrologic drought. Reconstructions derived by principal components regression of flow on time-varying subsets of tree-ring chronologies account for 64 to 81 percent of the flow variance in the 1906 to 1977 calibration period. A Monte Carlo analysis of reconstructed n-year running means indicates that the gaged record contains examples of drought extremes for averaging periods of perhaps = 6 to 10 years, but not for longer and shorter averaging periods. For example, the estimated probability approaches 1.0 that the flow in A.D. 1580 was lower than the lowest single-year gaged flow. The tree-ring record also suggests that persistently high or low flows over 50-year periods characterize some parts of the long-term flow history. The results should contribute to sensible water resources planning for the Sacramento Basin and to the methodology of incorporating tree-ring data in the assessment of the probability of hydrologic drought.
Graumlich, L.J., M. F. J. Pisaric, L. A. Waggoner, J. S. Littell, J. C. King (2003). Upper Yellowstone River flow and teleconnections with Pacific Basin climate variability during the past three centuries. Climate Change 59 (1-2): 245-262
ABSTRACT: Climate variability, coupled with increasing demand is raising concerns about the sustainability of water resources in the western United States. Tree-ring reconstructions of stream flow that extend the observational record by several centuries provide critical information on the short-term variability and multi-decadal trends in water resources. In this study, precipitation sensitive Douglas-fir (Pseudotsuga menzeisii ) tree ring records are used to reconstruct annual flow of the Yellowstone River back to A.D. 1706. Linkages between precipitation in the Greater Yellowstone Region and climate variability in the Pacific basin were incorporated into our model by including indices Pacific Ocean interannual and decadal-scale climatic variability, namely the Pacific Decadal Oscillation and the Southern Oscillation. The reconstruction indicates that 20th century streamflow is not representative of flow during the previous two centuries. With the exception of the 1930s, streamflow during the 20th century exceeded average flows during the previous 200 years. The drought of the 1930s resulted in the lowest flows during the last three centuries, however, this probably does not represent a worst-case scenario for the Yellowstone as other climate reconstructions indicate more extreme droughts prior to the 18th century.
Gray, S.T., J. L. Betancourt, C. L. Fastie, S.T. Jackson (2003). Patterns and sources of multidecadal oscillations in drought-sensitive tree-ring records from the central and southern Rocky Mountains. Geophysical Research Letters 30 (6): 1316, doi:10.1029/2002GL016154
ABSTRACT: Tree-ring records spanning the past seven centuries from the central and southern Rocky Mountains were studied using wavelet analysis to examine multidecadal (>30–70 yr) patterns of drought variation. Fifteen tree-ring series were grouped into five regional composite chronologies based on shared low-frequency behavior. Strong multidecadal phasing of moisture variation was present in all regions during the late 16th century megadrought. Oscillatory modes in the 30–70 yr domain persisted until the mid-19th century in two regions, and wet-dry cycles were apparently synchronous at some sites until the 1950s drought. The 16th/17th century pattern of severe multidecadal drought followed by decades of wet conditions resembles the 1950s drought and post-1976 wet period. The 16th century megadrought, which may have resulted from coupling of a decadal (~20–30 yr) Pacific cool phase with a multidecadal warm phase in the North Atlantic, marked a substantial reorganization of climate in the Rocky Mountain region.
ABSTRACT: Fire scars in giant sequoia [Sequoiadendron giganteum (Lindley) Buchholz] were used to reconstruct the spatial and temporal pattern of surface fires that burned episodically through five groves during the past 2000 years. Comparisons with independent dendroclimatic reconstructions indicate that regionally synchronous fire occurrence was inversely related to yearly fluctuations in precipitation and directly related to decadal-to-centennial variations in temperature. Frequent small fires occurred during a warm period from about A.D. 1000 to 1300, and less frequent but more widespread fires occurred during cooler periods from about A.D. 500 to 1000 and after A.D. 1300. Regionally synchronous fire histories demonstrate the importance of climate in maintaining nonequilibrium conditions.
ABSTRACT: Interannual time-scale associations between fire occurrence and drought indices, the Southern Oscillation, and other synoptic patterns demonstrate that large-scale, long-term atmospheric features are precursors to regional fire activity. However, our knowledge of fire-climate relations over longer (century) timescales is fragmentary because of the rarity of comparable climate and fire time-series with sufficient resolution, length and regional extent. In this study, we develop reconstructions of wildfire occurrence from tree-ring data collected from northwestern New Mexico to compare with a millennium-length dendroclimatic reconstruction of precipitation. Reconstructions of both wildfires and climate show simultaneous changes since AD 1700 that indicate climate forcing of wildfire regimes on interannual to century timescales. Following a centuries-long dry period with high fire frequency (c. AD 1400-1790), annual precipitation increased, fire frequency decreased, and the season of fire shifted from predominantly midsummer to late spring. We hypothesize that these shifts in fire regimes reflect long-term changes in rainfall patterns associated with changes in synoptic-scale atmospheric circulation patterns and the Southern Oscillation. Our evidence supports century-scale climate forcing of fire regimes in the American Southwest, providing a useful analogue of future wildfire regimes expected under changing global climate conditions.
ABSTRACT: Evidence from woodrat middens and tree rings at Dutch John Mountain (DJM) in northeastern Utah reveal spatiotemporal patterns of pinyon pine (Pinus edulis Engelm.) colonization and expansion in the past millennium. The DJM population, a northern outpost of pinyon, was established by long-distance dispersal (~40 km). Growth of this isolate was markedly episodic and tracked multidecadal variability in precipitation. Initial colonization occurred by AD 1246, but expansion was forestalled by catastrophic drought (1250–1288), which we speculate produced extensive mortality of Utah Juniper (Juniperus osteosperma (Torr.) Little), the dominant tree at DJM for the previous ~8700 years. Pinyon then quickly replaced juniper across DJM during a few wet decades (1330–1339 and 1368–1377). Such alternating decadal-scale droughts and pluvial events play a key role in structuring plant communities at the landscape to regional level. These decadal-length precipitation anomalies tend to be regionally coherent and can synchronize physical and biological processes across large areas. Vegetation forecast models must incorporate these temporal and geographic aspects of climate variability to accurately predict the effects of future climate change.
ABSTRACT: Foy Lake in northwestern Montana provides a record of annual-to-decadal-scale landscape change. Sedimentary charcoal and pollen analyses were used to document fire and vegetation changes over the last 3800 years, which were then compared to similar records from AD 1880 to 2000. The long-term record at Foy Lake suggests shifts between forest and steppe as well as changes in fire regime that are likely the result of climate change. Fire activity (inferred from the frequency of charcoal peaks) averaged 18 fire episodes/1000 years from 3800 to 2125 cal year BP, and increased from 16 fire episodes/1000 years at 2125 cal year BP to 22 episodes/1000 years at 750 cal year BP, a period when the pollen data suggest that steppe vegetation yielded to increasing patches of forest cover. Between 2125 and 750 cal year BP, increased forest cover produced more background charcoal than before and after this period, when vegetation was dominated by steppe. Between 750 and 75 cal year BP steppe has expanded and fire episode frequency averaged 33 episodes/1000 years, increasing to a maximum of 40 episodes/1000 years at ca. 300 cal year BP and then decreasing to present levels. Since AD 1880, the pollen record indicates an increase in shrubs and grasses from AD 1895 to 1960 as a result of vegetation changes associated with timber harvesting and livestock grazing. No fires have been documented in the Foy Lake watershed since AD 1880. Charcoal from the extralocal fires of AD 1910, burning over 4,111,249 ha in Idaho, Montana, and Wyoming, however, is present in Foy Lake. Between AD 1970 and 2000, increased arboreal pollen in the record is consistent with observations that the forest has become more closed. The activities of Euro–Americans have led to a decline in forest cover between AD 1880 and 1970, followed by a recent increase as trees are now growing in areas previously occupied by steppe. Euro–Americans are likely the cause of a reduction in fire activity in watershed since AD 1880.
ABSTRACT: A network of 32 drought sensitive tree-ring chronologies is used to reconstruct mean water year flow on the Columbia River at The Dalles, Oregon, since 1750. The reconstruction explains 30 percent of the variability in mean water year (October to September) flow, with a large portion of unexplained variance caused by underestimates of the most severe low flow events. Residual statistics from the tree-ring reconstruction, as well as an identically specified instrumental reconstruction, exhibit positive trends over time. This finding suggests that the relationship between drought and streamflow has changed over time, supporting results from hydrologic models, which suggest that changes in land cover over the 20th Century have had measurable impacts on runoff production. Low pass filtering the flow record suggests that persistent low flows during the 1840s were probably the most severe of the past 250 years, but that flows during the 1930s were nearly as extreme. The period from 1950 to 1987 is anomalous in the context of this record for having no notable multiyear drought events. A comparison of the flow reconstruction to paleorecords of the Pacific Decadal Oscillation (PDO) and El Niño/Southern Oscillation (ENSO) support a strong 20th Century link between large scale circulation and streamflow, but suggests that this link is very weak prior to 1900.
ABSTRACT: Data from prehistoric fluvial deposits can be used to extend the flood history of a river valley beyond historical records, thus increasing our understanding of variability in large, low-frequency flood events and providing a valuable means for paleoenvironmental reconstruction. We have applied this form of analysis to fluvial deposits from an archaeological site on the upper Columbia River in the state of Washington dating from 120 A.D.* to 1948 A.D. It was our expectation that, had flood frequencies remained constant, sedimentation event frequency would conform to an exponential function derived from the Wolman and Leopold model of vertical floodplain accretion. Our findings deviate from this model, showing that flood frequencies comparable to those of the twentieth century existed prior to 1020 A.D.* and after 1390 A.D.* Large floods were three to four times more common during the intervening centuries. On the basis of field evidence, we can rule out changing channel geometry, leaving climatic conditions as the most probable factors controlling this variation in flood frequency.
Jacobeit, J., R. Glaser, J. Luterbacher, H. Wanner (2003). Links between flood events in central Europe since A.D. 1500 and large-scale atmospheric circulation modes. Geophysical Research Letters 30 (4): 1172
ABSTRACT: Based on documentary sources incidence variations of flood events can be reconstructed back to AD 1500 for several catchment areas in Central Europe. Links to atmospheric circulation modes, derived from reconstructed large-scale sea level pressure (SLP) grids for the last 500 years, have been identified on climatic time scales (monthly to seasonal). These relations are expressed in terms of several indices describing the particular importance of atmospheric circulation modes as a dynamical background for the varying incidence of flood events. During winter, the zonal circulation mode covers the largest part of these events, in relation to mode-frequency, however, other circulation modes become important during historical periods of increased flood frequency, e.g. modes characterised by Atlantic low and Russian high pressure centres. Different subtypes of this mode reached maxima in flood-association during different climatic periods of the past.
Knapp, P. A., H. D. Grissino-Mayer, P. T. Soulé (2002). Climatic regionalization and the spatio-temporal occurrence of extreme single-year drought events (1500–1998) in the interior Pacific Northwest, USA. Quaternary Research 58 (3): 226-233
ABSTRACT: Tree-ring records from western juniper (Juniperus occidentalis var.occidentalis Hook.) growing throughout the interior Pacific Northwest identify extreme climatic pointer years (CPYs) (i.e., severe single-year droughts) from 1500–1998. Widespread and extreme CPYs were concentrated in the 16th and early part of the 17th centuries and did not occur again until the early 20th century. The 217-yr absence of extreme CPYs may have occurred during an extended period of low variance in the Pacific Decadal Oscillation. We mapped climatic boundaries for the interior Pacific Northwest based on the location of sites with similar precipitation variability indices. Three regions, the Northwest (based on chronologies from nine sites), the Southwest (four sites), and the East (five sites) were identified. Our results suggest that western juniper radial growth indices have substantial interannual variability within the northwestern range of the species (central Oregon), particularly when compared with western juniper growing in its eastern range (eastern Oregon, southeastern Idaho, and northern Nevada) and southwestern range (southern Oregon and northeast California). We suspect that the substantial differences in the variability of western juniper radial growth indices are linked to the influence of ENSO events on winter/spring precipitation amounts.
ABSTRACT: New tree-ring records of ring-width from remnant preserved wood are analyzed to extend the record of reconstructed annual flows of the Colorado River at Lee Ferry into the Medieval Climate Anomaly, when epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America. The most extreme low-frequency feature of the new reconstruction, covering A.D. 762-2005, is a hydrologic drought in the mid-1100s. The drought is characterized by a decrease of more than 15% in mean annual flow averaged over 25 years, and by the absence of high annual flows over a longer period of about six decades. The drought is consistent in timing with dry conditions inferred from tree-ring data in the Great Basin and Colorado Plateau, but regional differences in intensity emphasize the importance of basin-specific paleoclimatic data in quantifying likely effects of drought on water supply.
ABSTRACT: Studies from sites around the world1–5 have provided evidence for anomalous climate conditions persisting for several hundred years before about AD 1300. Early workers emphasized the temperature increase that marked this period in the British Isles, coining the terms 'Mediaeval Warm Epoch' and 'Little Climatic Optimum', but many sites seem to have experienced equally important hydrological changes. Here I present a study of relict tree stumps rooted in present-day lakes, marshes and streams, which suggests that California's Sierra Nevada experienced extremely severe drought conditions for more than two centuries before AD 1112 and for more than 140 years before AD 1350. During these periods, runoff from the Sierra was significantly lower than during any of the persistent droughts that have occurred in the region over the past 140 years. I also present similar evidence from Patagonia of drought conditions coinciding with at least the first of these dry periods in California. I suggest that the droughts may have been caused by reorientation of the mid-latitude storm tracks, owing to a general contraction of the circumpolar vortices and/or a change in the position of the vortex waves. If this reorientation was caused by mediaeval warming, future natural or anthropogenically induced warming may cause a recurrence of the extreme drought conditions.
ABSTRACT: Droughts are one of the most devastating natural hazards faced by the United States today. Severe droughts of the twentieth century have had large impacts on economies, society, and the environment, especially in the Great Plains. However, the instrumental record of the last 100 years contains only a limited subset of drought realizations. One must turn to the paleoclimatic record to examine the full range of past drought variability, including the range of magnitude and duration, and thus gain the improved understanding needed for society to anticipate and plan for droughts of the future. Historical documents, tree rings, archaeological remains, lake sediment, and geomorphic data make it clear that the droughts of the twentieth century, including those of the 1930s and 1950s, were eclipsed several times by droughts earlier in the last 2000 years, and as recently as the late sixteenth century. In general, some droughts prior to 1600 appear to be characterized by longer duration (i.e., multidecadal) and greater spatial extent than those of the twentieth century. The authors’ assessment of the full range of past natural drought variability, deduced from a comprehensive review of the paleoclimatic literature, suggests that droughts more severe than those of the 1930s and 1950s are likely to occur in the future, a likelihood that might be exacerbated by greenhouse warming in the next century. Persistence conditions that lead to decadal-scale drought may be related to low-frequency variations, or base-state shifts, in both the Pacific and Atlantic Oceans, although more research is needed to understand the mechanisms of severe drought.
ABSTRACT: The development of a 2° lat × 3° long grid of summer drought reconstructions for the continental United States estimated from a dense network of annual tree-ring chronologies is described. The drought metric used is the Palmer Drought Severity Index (PDSI). The number of grid points is 154 and the reconstructions cover the common period 1700–1978. In producing this grid, an automated gridpoint regression method called “point-by-point regression” was developed and tested. In so doing, a near-optimal global solution was found for its implementation. The reconstructions have been thoroughly tested for validity using PDSI data not used in regression modeling. In general, most of the gridpoint estimates of drought pass the verification tests used. In addition, the spatial features of drought in the United States have been faithfully recorded in the reconstructions even though the method of reconstruction is not explicitly spatial in its design.
The drought reconstructions show that the 1930s “Dust Bowl” drought was the most severe such event to strike the United States since 1700. Other more local droughts are also revealed in the regional patterns of drought obtained by rotated principal component analysis. These reconstructions are located on a NOAA Web site at the World Data Center-A in Boulder, Colorado, and can be freely downloaded from there.
ABSTRACT: Historical catch records suggest that climatic variability has had basin-wide effects on the northern Pacific and its fish populations, such as salmon, sardines and anchovies. However, these records are too short to define the nature and frequency of patterns. We reconstructed similar to 2,200-year records of sockeye salmon abundance from sediment cores obtained from salmon nursery lakes on Kodiak Island, Alaska. Large shifts in abundance, which far exceed the decadal-scale variability recorded during the past 300 years, occurred over the past two millennia. A marked, multi-centennial decline in Alaskan sockeye salmon was apparent from ~ 100 BC to AD 800, but salmon were consistently more abundant from AD 1200 to 1900. Over the past two millennia, the abundances of Pacific sardine and Northern anchovy off the California coast, and of Alaskan salmon, show several synchronous patterns of variability. But sardines and anchovies vary out of phase with Alaskan salmon over low frequency, which differs from the pattern detected in historical records. The coherent patterns observed across large regions demonstrate the strong role of climatic forcing in regulating northeastern Pacific fish stocks.
Finney, B. P., I. Gregory-Eaves, J. Sweetman, M.S.V. Douglas, J.P. Smol (2000). Impacts of climatic change and fishing on Pacific salmon abundance over the past 300 years. Science 290 (27 October): 795-799
ABSTRACT: The effects of climate variability on Pacific salmon abundance are uncertain because historical records are short and are complicated by commercial harvesting and habitat alteration. We use lake sediment records ofd15 N and biological indicators to reconstruct sockeye salmon abundance in the Bristol Bay and Kodiak Island regions of Alaska over the past 300 years. Marked shifts in populations occurred over decades during this period, and some pronounced changes appear to be related to climatic change. Variations in salmon returns due to climate or harvesting can have strong impacts on sockeye nursery lake productivity in systems where adult salmon carcasses are important nutrient sources.
Greenwald, D.N., L. B. Brubaker (2001). A 5000-year record of disturbance and vegetation change in riparian forests of the Queets River, Washington, U.S.A.. Canadian Journal of Forest Research 31 (8): 1375-1385
ABSTRACT: We used fossil pollen, charcoal, and sediment stratigraphy in three small hollows to investigate disturbance events and changes in the composition of riparian forests on a small section of the Queets River floodplain, Olympic Peninsula, Washington. The records ranged in age from approximately 500 years at two sites 300 and 550 m from the river, to 5000 years at a site 800 m from the river. Approximately 400–600 years BP, the two sites nearest the river were either inundated by a very large flood or covered by the active channel, which would have occupied a substantially different position than its present course. Following inundation or channel movement, the pollen record suggests thatAlnus rubra Bong., the primary mesic forest colonizer in the Pacific Northwest, increased and was then replaced byPicea sitchensis (Bong.) Carrière andTsuga heterophylla (Raf.) Sarg. At the site farthest from the river, two fires occurred within the last ca. 4500 years. One of the fires was followed by a period of shrub dominance and succession toTsuga heterophylla . The other fire did not cause a change in the pollen record. A recent unprecedented rise inTsuga heterophylla pollen, which began ca. 1000 years BP, might be in response to cooling during the Little Ice Age. Overall, the small hollow records highlight the complex effect of floods, fire, and possibly climate change on riparian forests of the Queets River.
P. F. Hessburg, J. K. Agee, J. F. Franklin (2005). Dry forests and wildland fires of the inland Northwest USA: Contrasting the landscape ecology of the pre-settlement and modern eras. Forest Ecology and Management 211 (1-2): 117-139
ABSTRACT: Prior to Euro–American settlement, dry ponderosa pine and mixed conifer forests (hereafter, the “dry forests”) of the Inland Northwest were burned by frequent low- or mixed-severity fires. These mostly surface fires maintained low and variable tree densities, light and patchy ground fuels, simplified forest structure, and favored fire-tolerant trees, such as ponderosa pine, and a low and patchy cover of associated fire-tolerant shrubs and herbs.
Low- and mixed-severity fires provided other important feedbacks and effects to ponderosa pine-dominated stands and landscapes. For example, in stands, frequent surface fires favored an ongoing yet piecemeal regeneration of fire-tolerant trees by periodically exposing patches of mineral soil. They maintained fire-tolerant forest structures by elevating tree crown bases and scorching or consuming many seedlings, saplings, and pole-sized trees. They cycled nutrients from branches and foliage to the soil, where they could be used by other plants, and promoted the growth and development of low and patchy understory shrub and herb vegetation. Finally, surface fires reduced the long-term threat of running crown fires by reducing the fuel bed and metering out individual tree and group torching, and they reduced competition for site resources among surviving trees, shrubs, and herbs. In landscapes, the patterns of dry forest structure and composition that resulted from frequent fires reinforced the occurrence of low- or mixed-severity fires, because frequent burning spatially isolated conditions that supported high-severity fires. These spatial patterns reduced the likelihood of severe fire behavior and effects at each episode of fire. Rarely, dry forest landscapes were affected by more severe climate-driven events.
Extant dry forests no longer appear or function as they once did. Large landscapes are homogeneous in their composition and structure, and the regional landscape is set up for severe, large fire and insect disturbance events. Among ecologists, there is also a high degree of concern about how future dry forests will develop, if fires continue to be large and severe. In this paper, we describe the key landscape pattern and process changes wrought by the sum of the settlement and management influences to date, and we point to an uncertain future for ecosystem management. Widespread selection cutting of the largest and oldest ponderosa pine and Douglas-fir in the 20th century has reduced much of the economic opportunity that might have been associated with restoration, and long-term investment will likely be needed, if large-scale restoration activities are attempted. An uncertain future for ecosystem management is based on the lack of current and improbable future social consensus concerning desired outcomes for public forestlands, the need for significant financial investment in ecosystem restoration, a lack of integrated planning and decision tools, and mismatches between the existing planning process, Congressional appropriations, and complex management and restoration problems.
ABSTRACT: A 17 000 yr fire history from Yellowstone National Park demonstrates a strong link between changes in climate and variations in fire frequency on millennial time scales. The fire history reconstruction is based on a detailed charcoal stratigraphy from Cygnet Lake in the rhyolite plateau region. Macroscopic charcoal particles were tallied from contiguous 1 cm samples of a 6.69-m-long core, and the data were converted to charcoal-accumulation rates at evenly spaced time intervals. Intervals of high charcoal-accumulation rates were interpreted as local fire events on the basis of information obtained from modern charcoal-calibration studies in the Yellowstone region. The record indicates that fire frequency was moderate (4 fires/1000 yr) during the late glacial period, reached highest values in the early Holocene (>10 fires/1000 yr), and decreased after 7000 calendar yr B.P. The present fire regime (2–3 fires/1000 yr) was established in the past 2000 yr. The charcoal stratigraphy correlates well with variations in July insolation through time, which suggests that regional climate changes are responsible for the long-term variations in fire frequency. In the early Holocene, summer insolation was near its maximum, which resulted in warmer, effectively drier conditions throughout the northwestern United States. At this time, the fire frequency near Cygnet Lake was at its highest. After 7000 calendar yr B.P., summer insolation decreased to present values, the regional climate became cooler and wetter, and fires were less frequent. The Cygnet Lake record suggests that long-term fire frequencies have varied continuously with climate change, even when the vegetation has remained constant
ABSTRACT: Fire was arguably the most important forest and rangeland disturbance process in the Inland Northwest United States for millennia. Prior to the Lewis and Clark expedition, fire regimes ranged from high severity with return intervals of one to five centuries, to low severity with fire-free periods lasting three decades or less. Indoamerican burning contributed to the fire ecology of grasslands and lower and mid-montane dry forests, especially where ponderosa pine was the dominant overstory species, but the extent of this contribution is difficult to quantify. Two centuries of settlement, exploitation, management, and climate variation have transformed the fire regimes, vegetation and fuel patterns, and overall functionality of these forests. We present a narrative that portrays conditions beginning at the first contact of Euro-American settlers with Indoamericans of the region and extending to the present. Due in part to its geographic isolation, the Inland Northwest was among the last regions to be discovered by Euro-Americans. In 200 years the region has undergone fur trapping and trading, sheep, cattle, and horse grazing, timber harvesting, mining, road construction, native grassland conversion to agricultural production, urban and rural area development, fire prevention, and fire suppression. We highlight key changes to forest landscape patterns and processes that occurred under these combined influences, discuss implications of the changes, and progress towards restoring sustainability. An adaptive ecosystem management model has been adopted by public land management agencies to remedy current conditions. Ecosystem management is a relatively new concept that emphasizes the integrity and sustainability of land systems rather than outputs from the land. Adaptive management emphasizes the twin notions that incomplete knowledge and high degrees of risk and uncertainty about earth and climate systems will always limit land and resource planning and management decisions, and that management is chiefly a learning and adapting process. We discuss current issues and future options associated with ecosystem management, including the low likelihood of social consensus concerning desired outcomes, the lack of integrated planning, analysis, and decision support tools, and mismatches between existing land management planning processes, Congressional appropriations, and complex management and restoration problems.
Hereford, R. (2002). Valley-fill alluviation during the Little Ice Age (ca. A.D. 1400–1880), Paria River basin and southern Colorado Plateau, United States. Geological Society of America Bulletin 114 (12): 1550-1563
ABSTRACT: Valley-fill alluvium deposited from ca. A.D. 1400 to 1880 is widespread in tributaries of the Paria River and is largely coincident with the Little Ice Age epoch of global climate variability. Previous work showed that alluvium of this age is a mappable stratigraphic unit in many of the larger alluvial valleys of the southern Colorado Plateau. The alluvium is bounded by two disconformities resulting from prehistoric and historic arroyo cutting at ca. A.D. 1200–1400 and 1860–1910, respectively. The fill forms a terrace in the axial valleys of major through-flowing streams. This terrace and underlying deposits are continuous and interfinger with sediment in numerous small tributary valleys that head at the base of hillslopes of sparsely vegetated, weakly consolidated bedrock, suggesting that eroded bedrock was an important source of alluvium along with in-channel and other sources. Paleoclimatic and high-resolution paleoflood studies indicate that valley-fill alluviation occurred during a long-term decrease in the frequency of large, destructive floods. Aggradation of the valleys ended about A.D. 1880, if not two decades earlier, with the beginning of historic arroyo cutting. This shift from deposition to valley entrenchment near the close of the Little Ice Age generally coincided with the beginning of an episode of the largest floods in the preceding 400–500 yr, which was probably caused by an increased recurrence and intensity of flood-producing El Niño events beginning at ca. A.D. 1870.
Miller, J., D. Germanoski, K. Waltman, R. Tausch, J. Chambers (2001). Influence of late Holocene hillslope processes and landforms on modern channel dynamics in upland watersheds of central Nevada. Geomorphology 38 (3-4): 373-391
ABSTRACT: Stratigraphic, geomorphic, and paleoecological data were collected from upland watersheds in the Great Basin of central Nevada to assess the relationships between late Holocene climate change, hillslope processes and landforms, and modern channel dynamics. These data indicate that a shift to drier, warmer climatic conditions from approximately 2500 to 1300 YPB led to a complex set of geomorphic responses. The initial response was massive hillslope erosion and the simultaneous aggradation of both side-valley alluvial fans and the axial valley system. The final response was fan stabilization and axial channel incision as fine-grained sediments were winnowed from the hillslope sediment reservoirs, and sediment yield and runoff processes were altered. The primary geomorphic response to disturbance for approximately the past 1900 years has been channel entrenchment, suggesting that the evolutionary history of hillslopes has produced watersheds that are prone to incision. The magnitude of the most recent phase of channel entrenchment varies along the valley floor as a function of geomorphic position relative to side-valley alluvial fans. Radial fan profiles suggest that during fan building, fan deposits temporarily blocked the flow of sediment down the main stem of the valley, commonly creating a stepped longitudinal valley profile. Stream reaches located immediately upvalley of these fans are characterized by low gradients and alternating episodes of erosion and deposition. In contrast, reaches coincident with or immediately downstream of the fans exhibit higher gradients and limited valley floor deposition. Thus, modern channel dynamics and associated riparian ecosystems are strongly influenced by landforms created by depositional events that occurred approximately 2000 years ago.
DESCRIPTION: Various human groups have greatly affected the processes and evolution of Middle Rio Grande Basin ecosystems, especially riparian zones, from A.D. 1540 to the present. Overgrazing, clear-cutting, irrigation farming, fire suppression, intensive hunting, and introduction of exotic plants have combined with droughts and floods to bring about environmental and associated cultural changes in the Basin. As a result of these changes, public laws were passed and agencies created to rectify or mitigate various environmental problems in the region. Although restoration and remedial programs have improved the overall "health" of Basin ecosystems, most old and new environmental problems persist.
ABSTRACT: Warm-season (April–September) temperature models based on a network of coastal ring-width and maximum latewood density tree-ring chronologies are the first reconstructions for coastal stations along the Gulf of Alaska and the Pacific Northwest. These well-verified temperature models are consistent with long climatic series from coastal stations and other proxy data from the Pacific coast. Cool summers during the 1850s and late 1800s in the Gulf of Alaska correspond to general glacier advance from the region. The Pacific Northwest reconstruction shows summer temperatures cooling in the early 1800s, coincident with a maximum of glacier activity in the coastal Olympic Mountains, Washington. The two warm-season temperature records show intervals when anomalies are opposite in sign, most notably during the 1850s, when cooling is inferred for the Gulf and warming is inferred for the Pacific Northwest. The records are coherent, however, during other intervals, with both showing cooling in the early 1800s and warming around 1870. The phase of these two records may reflect decadal changes in large-scale circulation in the northeastern Pacific. These land temperature reconstructions are strongly correlated with nearby sea surface temperatures, indicating large-scale oceanic–atmospheric influences.
ABSTRACT: Streamflow since 1560 A.D. for four rivers within the Sacramento River Basin, California, has been reconstructed dendroclimatically. Both the highest and the lowest reconstructed streamflows occurred during the historical period, with high flows from 1854 to 1916 and low flows from 1917 to 1950. Prolonged (decade-scale) excursions from the mean have been the norm throughout the reconstructed period. The periods of high and low streamflow in the Sacramento Basin are generally synchronous with wet and dry periods reconstructed by dendroclimatic studies in the western United States. The record indicates a number of asynchronous droughts or wet years. The strongest contrasts are developed between northern (western Washington and Oregon or the Columbia Basin) and southern (the Sacramento Basin or central California) climate regions. These asynchronous events may be due to variation in the latitude of the subtropical high and in the latitudinal position of winter storms coming off the Pacific. No association was found with El Niño-Southern Oscillation events.
ABSTRACT: The climate system consists of the atmosphere, the oceans, the cryosphere (land ice, snow, sea ice), the lithosphere, and the biomass. The behavior of the individual components of the system is governed by processes occurring over a broad range of time and space scales. The components are coupled by physical, biological, and chemical processes, and the coupled system seems capable of undergoing fluctuations on all time scales. In addition to these "internal" climatic processes, external processes (such as variability in the solar irradiance or human activities) must also be considered. Space and time scales of climatic variability are reviewed, with emphasis on the Holocene. Regional patterns of climatic variability may be associated with changes in the amplitude and longitudinal position of the long waves in the westerlies of midlatitudes, and with changes in the intensity and latitude of meridional circulation features such as the Hadley cell. Possible examples of this are mentioned. The variance spectrum of climatic time series is described and certain implications for climate modeling are suggested.
ABSTRACT: We examine the relationships, over the past millennium, between tree-ring chronologies from long-lived pines at their upper and lower limits in four mountain ranges in and near to the semi-arid Great Basin. We confirm LaMarche's (1974a) finding, based on a single mountain range in this same region, and a much shorter period of comparison, that climate responses are frequency dependent. In particular, upper and lower forest border chronologies in each mountain range are strongly coherent at decadal periods and less, with particular strength in the 3–7 year band. This variability is significantly correlated with precipitation. Conversely, we find no significant correlation between the low frequency fluctuations (60 years and longer) of upper and lower forest border chronologies. There are, however, significant correlations between the low-frequency components of the upper forest border chronologies in the different ranges, consistent with their containing a growing season temperature signal on decadal time scales. The four upper forest border chronologies all show an anomalous increase in growth since the late 19th century, and an apparent change in climate control of ring growth.
INTRODUCTION: Preserving multicentennial climate variability in long tree-ring records is critically important for reconstructing the full range of temperature variability over the past 1000 years. This allows the putative "Medieval Warm Period" (MWP) to be described and to be compared with 20th-century warming in modeling and attribution studies. We demonstrate that carefully selected tree-ring chronologies from 14 sites in the Northern Hemisphere (NH) extratropics can preserve such coherent large-scale, multicentennial temperature trends if proper methods of analysis are used. In addition, we show that the average of these chronologies supports the large-scale occurrence of the MWP over the NH extratropics.
ABSTRACT: Updated proxy reconstructions of water year (October–September) streamflow for four key gauges in the Upper Colorado River Basin were generated using an expanded tree ring network and longer calibration records than in previous efforts. Reconstructed gauges include the Green River at Green River, Utah; Colorado near Cisco, Utah; San Juan near Bluff, Utah; and Colorado at Lees Ferry, Arizona. The reconstructions explain 72–81% of the variance in the gauge records, and results are robust across several reconstruction approaches. Time series plots as well as results of cross-spectral analysis indicate strong spatial coherence in runoff variations across the subbasins. The Lees Ferry reconstruction suggests a higher long-term mean than previous reconstructions but strongly supports earlier findings that Colorado River allocations were based on one of the wettest periods in the past 5 centuries and that droughts more severe than any 20th to 21st century event occurred in the past.
ABSTRACT: Effective planning for use of water resources requires accurate information on hydrologic variability induced by climatic fluctuations. Tree-ring analysis is one method of extending our knowledge of hydrologic variability beyond the relatively short period covered by gaged streamfiow records. In this paper, a network of recently developed tree-ring chronologies is used to reconstruct annual river discharge in the upper Gila River drainage in southeastern Arizona and southwestern Arizona since A.D. 1663. The need for data on hydrologic variability for this semi-arid basin is accentuated because water supply is inadequate to meet current demand. A reconstruction based on multiple linear regression (R^2=0.66) indicates that 20th century is unusual for clustering of high-discharge years (early 1900s), severity of multiyear drought (1950s), and amplification of low-frequency discharge variations. Periods of low discharge recur at irregular intervals averaging about 20 years. Comparison with other tree-ring reconstructions shows that these low-flow periods are synchronous from the Gila Basin to the southern part of the Upper Colorado River Basin.
ABSTRACT: A gray silt layer 1–2 cm thick in the central Santa Barbara Basin, dated by varve counts to A.D. 1605 ± 5 yr, implies an intensity of precipitation, flooding of regional rivers, and transport of terrigenous detritus unmatched in the last 1000 yr. The inferred flood may correlate with the reported rare occurrence of a perennial lake (14 C dated to 390 ± 90 B.P.) in California's Mojave Desert, 300 km east of the area draining into the Santa Barbara Basin. The dating of the A.D. 1605 ± 5 yr flood event is consistent with tree-ring evidence for a wet and cold paleoclimate elsewhere in the region. Regional and global climate evidence indicates that much of the world also experienced rapid, intense cooling around A.D. 1605. This cooling was probably accompanied by an equatorward shift of prevailing wind patterns and associated storm tracks.
ABSTRACT: Water resource planning is based primarily on 20th century instrumental records of climate and streamflow. These records are limited in length to approximately 100 years, in the best cases, and can reflect only a portion of the range of natural variability. The instrumental record neither can be used to gage the unusualness of 20th Century extreme low flow events, nor does it allow the detection of low-frequency variability that may underlie short-term variations in flow. In this study, tree rings are used to reconstruct mean annual streamflow for Middle Boulder Creek in the Colorado Front Range, a semi-arid region of rapid growth and development. The reconstruction is based on a stepwise regression equation that accounts for 70 percent of the variance in the instrumental record, and extends from 1703-1987. The reconstruction suggests that the instrumental record of streamflow for Middle Boulder Creek is not representative of flow in past centuries and that several low flow events in the 19th century were more persistent than any in the 20th century. The 1840s to early 1850s period of low flow is a particularly notable event and may have coincided with a period of low flow in the Upper Colorado River Basin.
J. Timilsena, T. C. Piechota, H. Hidalgo, G. Tootle (2007). Five hundred years of hydrological drought in the upper Colorado River Basin. Journal of the American Water Resources Association 43 (3): 798-812
ABSTRACT: This article evaluates drought scenarios of the Upper Colorado River basin (UCRB) considering multiple drought variables for the past 500 years and positions the current drought in terms of the magnitude and frequency. Drought characteristics were developed considering water-year data of UCRB’s streamflow, and basin-wide averages of the Palmer Hydrological Drought Index (PHDI) and the Palmer Z Index. Streamflow and drought indices were reconstructed for the last 500 years using a principal component regression model based on tree-ring data. The reconstructed streamflow showed higher variability as compared with reconstructed PHDI and reconstructed Palmer Z Index. The magnitude and severity of all droughts were obtained for the last 500 years for historical and reconstructed drought variables and ranked accordingly. The frequency of the current drought was obtained by considering two different drought frequency statistical approaches and three different methods of determining the beginning and end of the drought period (annual, 5-year moving, and ten year moving average). It was concluded that the current drought is the worst in the observed record period (1923-2004), but 6th to 14th largest in terms of magnitude and 1st to 12th considering severity in the past 500 years. Similarly, the current drought has a return period ranging from 37 to 103 years based on how the drought period was determined. It was concluded that if the 10-year moving average is used for defining the drought period, the current drought appears less severe in terms of magnitude and severity in the last 500 years compared with the results using 1- and 5-year averages.
ABSTRACT: Ecological responses to climatic variability in the Southwest include regionally synchronized fires, insect outbreaks, and pulses in tree demography (births and deaths). Multicentury, tree-ring reconstructions of drought, disturbance history, and tree demography reveal climatic effects across scales, from annual to decadal, and from local (<102 km2 ) to mesoscale (104 –106 km2 ). Climate–disturbance relations are more variable and complex than previously assumed. During the past three centuries, mesoscale outbreaks of the western spruce budworm (Choristoneura occidentalis) were associated with wet, not dry episodes, contrary to conventional wisdom. Regional fires occur during extreme droughts but, in some ecosystems, antecedent wet conditions play a secondary role by regulating accumulation of fuels. Interdecadal changes in fire–climate associations parallel other evidence for shifts in the frequency or amplitude of the Southern Oscillation (SO) during the past three centuries. High interannual, fire–climate correlations (r = 0.7 to 0.9) during specific decades (i.e., circa 1740–80 and 1830–60) reflect periods of high amplitude in the SO and rapid switching from extreme wet to dry years in the Southwest, thereby entraining fire occurrence across the region. Weak correlations from 1780 to 1830 correspond with a decrease in SO frequency or amplitude inferred from independent tree-ring width, ice core, and coral isotope reconstructions.
Episodic dry and wet episodes have altered age structures and species composition of woodland and conifer forests. The scarcity of old, living conifers established before circa 1600 suggests that the extreme drought of 1575–95 had pervasive effects on tree populations. The most extreme drought of the past 400 years occurred in the mid–twentieth century (1942–57). This drought resulted in broadscale plant dieoffs in shrublands, woodlands, and forests and accelerated shrub invasion of grasslands. Drought conditions were broken by the post-1976 shift to the negative SO phase and wetter cool seasons in the Southwest. The post-1976 period shows up as an unprecedented surge in tree-ring growth within millennia-length chronologies. This unusual episode may have produced a pulse in tree recruitment and improved rangeland conditions (e.g., higher grass production), though additional study is needed to disentangle the interacting roles of land use and climate. The 1950s drought and the post-1976 wet period and their aftermaths offer natural experiments to study long-term ecosystem response to interdecadal climate variability.
B.B. Wolfe, T.L. Karst-Riddoch, S.R. Vardy, M.D. Falcone, R. I. Hall, T.W.D. Edwards (2005). Impacts of climate and river flooding on the hydro-ecology of a floodplain basin, Peace-Athabasca Delta, Canada since A.D. 1700. Quaternary Research 64 (2): 147-162
ABSTRACT: Multi-proxy paleolimnological analyses on lake sediment cores from “Spruce Island Lake” (58° 50.82' N, 111° 28.84' W), a perched basin in the northern Peace sector of the Peace-Athabasca Delta (PAD), Canada, give insights into the relative roles of flow regulation of the Peace River and climatic variability on the basin hydro-ecology. Results indicate substantial variability in basin hydro-ecology over the past 300 years ranging from seasonal to periodic desiccation in the 1700s to markedly wetter conditions during the early 1800s to early 1900s. The reconstruction is consistent with (1) dry climatic conditions that defined the peak of the Little Ice Age and subsequent amelioration evident in conventional ring-width and isotopic analyses of tree-ring records located hydrologically and climatically upstream of the PAD, and (2) Peace River flood history inferred from sub-annual magnetic susceptibility measurements from another lake sediment record in the Peace sector of the PAD. Although regulation of the Peace River for hydroelectric power generation since 1968 has long been considered a major stressor of the PAD ecosystem leading to reduced frequency of ice-jam and open-water flooding and an extended period of drying, our results show that current hydro-ecological status is not unprecedented as both wetter and drier conditions have persisted for decades in the recent past under natural climatic variability. Furthermore, paleolimnological evidence from Spruce Island Lake indicates that recently observed dryness is part of a longer trend which began some 20–40 years prior to Peace River regulation.
ABSTRACT: Changes in solar constant over an 11 yr cycle suggest a certain, but limited, degree of solar forcing of climate. The high-resolution climate (oxygen isotope) record of the Greenland GISP2 (Greenland Ice Sheet Project 2) ice core has been analyzed for solar (and volcanic) influences. The atmospheric14 C record is used as a proxy of solar change and compared to the oxygen isotope profile in the GISP2 ice core. An annual oxygen isotope profile is derived from centimeter-scale isotope measurements available for the post-A.D. 818 interval. Associated extreme summer and winter isotope ratios were found to yield similar climate information over the last millennium. The detailed record of volcanic aerosols, converted to optical depth and volcanic explosivity change, was also compared to the isotope record and the oxygen isotope response calibrated to short-term volcanic influences on climate. This calibration shows that century-scale volcanic modulation of the GISP2 oxygen isotope record can be neglected in our analysis of solar forcing. The timing, estimated order of temperature change, and phase lag of several maxima in14 C and minima in18 O are suggestive of a solar component to the forcing of Greenland climate over the current millennium. The fractional climate response of the cold interval associated with the Maunder sunspot minimum (and14 C maximum), as well as the Medieval Warm Period and Little Ice Age temperature trend of the past millennium, are compatible with solar climate forcing, with an order of magnitude of solar constant change of ~0.3%. Even though solar forcing of climate for the current millennium is a reasonable hypothesis, for the rest of the Holocene the century-scale events are more frequent in the oxygen isotope record than in the14 C record and a significant correlation is absent. For this interval, oceanic/atmospheric circulation forcing of climate may dominate. Solar forcing during the surprisingly strong 1470 yr climate cycle of the 11,000–75,000 yr B.P. interval is rather hypothetical.
ABSTRACT: It is hypothesised that the Medieval Warm Period was preceded and followed by periods of moraine deposition associated with glacier expansion. Improvements in the methodology of radiocarbon calibration make it possible to convert radiocarbon ages to calendar dates with greater precision than was previously possible. Dating of organic material closely associated with moraines in many montane regions has reached the point where it is possible to survey available information concerning the timing of the medieval warm period. The results suggest that it was a global event occurring between about 900 and 1250 A.D., possibly interrupted by a minor readvance of ice between about 1050 and 1150 A.D.
ABSTRACT: Building on recent studies, we attempt hemispheric temperature reconstructions with proxy data networks for the past millennium. We focus not just on the reconstructions, but the uncertainties therein, and important caveats. Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, our results suggest that the latter 20th century is anomalous in the context of at least the past millennium. The 1990s was the warmest decade, and 1998 the warmest year, at moderately high levels of confidence. The 20th century warming counters a millennial-scale cooling trend which is consistent with long-term astronomical forcing.
ABSTRACT: Instrumental Palmer Drought Severity Indexes (PDSI) averaged over the western United States and Great Plains document three major decadal moisture regimes during the twentieth century: the early twentieth-century pluvial, the Dust Bowl drought, and the 1950s drought. Tree-ring reconstructed PDSI for the contiguous Unites States replicates these three twentieth-century moisture regimes, and have been used to search for possible analogs over the past 500 yr. The early twentieth-century wet regime from 1905 to 1917 appears to have been the wettest episode across the West since A.D.1500, but similar pluvials occurred in the nineteenth, seventeenth, and sixteenth centuries. The Dust Bowl drought (1929–40) was most severe over the northern Plains to the northern Rockies. No close analogs are found for the full severity and geographical focus of the Dust Bowl drought over the past 500 yr. The 1950s drought (1946–56) was concentrated over the Southwest and was replicated by some 12 droughts of similar spatial coverage and duration over the past 500 yr. One of these analogs, the sixteenth-century megadrought, was also focused over the Southwest and appears to have surpassed the Dust Bowl drought in coverage, duration, and severity.
ABSTRACT: Significant climate anomalies have characterized the last 1000 yr in the Sierra Nevada, California, USA. Two warm, dry periods of 150- and 200-yr duration occurred during AD 900–1350, which were followed by anomalously cold climates, known as the Little Ice Age, that lasted from AD 1400 to 1900. Climate in the last century has been significantly warmer. Regional biotic and physical response to these climatic periods occurred. Climate variability presents challenges when interpreting historical variability, including the need to accommodate climate effects when comparing current ecosystems to historical conditions, especially if comparisons are done to evaluate causes (e.g., human impacts) of differences, or to develop models for restoration of current ecosystems. Many historical studies focus on “presettlement” periods, which usually fall within the Little Ice Age. Thus, it should be assumed that ecosystems inferred for these historical periods responded to different climates than those at present, and management implications should be adjusted accordingly. The warmer centuries before the Little Ice Age may be a more appropriate analogue to the present, although no historic period is likely to be better as a model than an understanding of what conditions would be at present without intervention. Understanding the climate context of historical reconstruction studies, and adjusting implications to the present, should strengthen the value of historical variability research to management.
Stahle, D. W., E. R. Cook, M. K. Cleaveland, M.D. Therrell, D. M. Meko, H. D. Grissino-Mayer, E. Watson, B.H. Luckman (2000). Tree-ring data document 16th century megadrought over North America. EOS, Transactions of the American Geophysical Union 81 (12): 121-125
ABSTRACT: Ring width and density measurements from the same trees can produce distinctly different climatic information. Ring width variations and recorded data in central and northern Alaska indicate annual temperatures increased over the past century, peaked in the 1940s, and are still near the highest level for the past 3 centuries. Density variations indicate summer temperatures are now warm but not above some previous levels occurring prior to this century. The early cooler period, corresponding to the Little Ice Age, was interrupted by brief warm intervals. The recent increase in temperatures combined with drier years may be changing the tree response to climate and raising the potential for some forest changes in Alaskan and other boreal forests.
ABSTRACT: Decreased solar activity correlates with positive cosmogenic isotope anomalies, and with cool, wet climate in temperate regions of the world. The relationship of isotope anomalies to climate may be the opposite for areas influenced by monsoonal precipitation, i.e., negative anomalies may be wet and warm. Petersen (1988) has found evidence for increased summer precipitation in the American Southwest that can be shown to be coincident with negative14 C anomalies during the Medieval Warm Period. The present study compares palynological indicators of lake level for the Southwest with Petersen's data and with the14 C isotope chronology. Percentages of aquatic pollen and algae from three sites within the Arizona Monsoon record greater lake depth or fresher water from A.D. 700–1350, between the Roman IV and Wolf positive isotope anomalies, thereby supporting Petersens's findings. Maximum summer moisture coincides with maximum population density of prehistoric people of the Southwest. However, water depth at a more northern site was low at this time, suggesting a climate-isotope relationship similar to that of other temperate regions. Further analysis of latitudinal patterns is hampered by inadequate14 C dating.
Barton, A.M., T. W. Swetnam, C. H. Baisan (2001). Arizona pine (Pinus arizonica ) stand dynamics: local and regional factors in a fire-prone madrean gallery forest of Southeast Arizona, USA. Landscape Ecology 16 (4): 351-369
ABSTRACT: In southwestern North America, large-scale climate patterns appear to exert control on moisture availability, fire occurrence, and tree demography, raising the compelling possibility of regional synchronization of forest dynamics. Such regional signals may be obscured, however, by local, site-specific factors, such as disturbance history and land use. Contiguous sites with similar physical environments, lower and middle Rhyolite Canyon, Arizona, USA, shared nearly the same fire history from 1660-1801, but then diverged. For the next 50 years, fires continued to occur frequently in lower Rhyolite, but, probably as result of flood-induced debris deposition, largely ceased in middle Rhyolite. We related stand dynamics of Arizona pine (Pinus arizonica ) to fire history and drought severity and compared the dynamics in the two sites before and after the divergence in fire frequency. Fires occurred during unusually dry years, and possibly following unusually moist years. Arizona pine exhibited three age structure peaks: two (1810–1830 and 1870–1900) shared by the two sites and one (1610–1640) only in middle Rhyolite. The latter two peaks occurred during periods of unusually low fire frequency, suggesting that fire-induced mortality shapes age structure. Evidence was mixed for the role of favorable moisture availability in age structure. As expected, moisture availability had a prominent positive effect on radial growth, but the effect of fire was largely neutral. The two sites differed only moderately in stand dynamics during the period of divergence, exhibiting subtle age structure contrasts and, in middle Rhyolite only, reduced growth during a 50-year fire hiatus followed by fire-induced release. These results suggest that, despite local differences in disturbance history, forest responses to regional fire and climate processes can persist.
Meyer, G.A., J. L. Pierce (2003). Climatic controls on fire-induced sediment pulses in Yellowstone National Park and central Idaho: a long-term perspective. Forest Ecology and Management 178 (1-2): 89-104
ABSTRACT: Fire management addressing postfire erosion and aquatic ecosystems tends to focus on short-term effects persisting up to about a decade after fire. A longer perspective is important in understanding natural variability in postfire erosion and sedimentation, the role of these processes in structuring habitat, and future expectations in light of a warming climate and environmental change. In cool high-elevation forests of northern Yellowstone National Park, stand ages indicate infrequent large stand-replacing fires. In warmer low-elevation forests of the Payette River region of Idaho, fire-scarred tree-rings record frequent low-severity fires before 1900; stand-replacing fires and resulting debris flows in recent decades are usually attributed to 20th-century fire suppression, grazing, and other land uses. In both areas, however, tree-ring records extend back only about 500 years. We use14 C-dated geologic records to examine spatial and temporal patterns of fire-induced sedimentation and its relation to climate over the last 10 000 years. We review sedimentation processes in modern postfire events, which vary in magnitude and impact on stream systems depending on burn severity, basin geomorphology, and the timing and characteristics of postfire storms. Modern deposits also provide analogs for identification of fire-related deposits in alluvial fans. In Yellowstone, episodes of fire-induced sedimentation occurred at intervals of about 300–450 years during the last 3500 years, indicating a regime of infrequent high-severity fires. Millennial-scale variations in the fire-sedimentation record appear to relate to hemispheric-scale climatic change. Fire-related sedimentation is rare in Yellowstone during cooler episodes (e.g., the Little Ice Age ~1200–1900), probably because effectively wetter conditions prevented most fires from spreading. During some of the same cool periods, the Payette region experienced light surface fires and frequent, small pulses of fire-induced sediment. Between 900 and 1200, however, large fire-related debris flows occurred in both study areas, coincident with the Medieval Warm Period. During that time, drought may have limited grass growth in xeric Payette-region forests, restricting surface fire spread and allowing understory shrubs and trees to create ladder fuels. Although fire suppression and land-use effects are clearly involved in recent catastrophic fires in the Payette region, a warming climate and severe drought are probable contributors to major stand-replacing fires and postfire sedimentation, both past and present. Restoration and maintenance of conditions prior to European settlement may be unrealistic because of the potent influence of climate, and the incidence of severe fires will likely increase in both areas with future warming.
ABSTRACT: The floodplain along a 75-km segment of the Brazos River, traversing the Gulf Coastal Plain of Texas, has a complex late Quaternary history. From 18,000 to 8500 yr B.P., the Brazos River was a competent meandering stream that migrated from one side of the floodplain to the other, creating a thick layer of coarse-grained lateral accretion deposits. After 8500 yr B.P., the hydrologic regime of the Brazos River changed. The river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasionally avulse. Avulsion occurred four times; first at 8100 yr B.P., then at 2500 yr B.P., again around 500 yr B.P., and finally around 300 yr B.P. The depositional regime on the floodplain also changed after 8500 yr B.P., with floodplain construction dominated by vertical accretion. Most vertical accretion occurred from 8100 to 4200 yr B.P. and from 2500 to 1250 yr B.P. Two major and three minor periods of soil formation are documented in the floodplain sequence. The two most developed soils formed from 4200 to 2500 yr B.P. and from around 1250 to 500 yr B.P. These changes on the floodplain appear to be the result not of a single factor, but of the complex interplay among changes in climate, sediment yield, and intrinsic floodplain variables over time.
Wells, S. G., L.D. McFadden, J.C. Dohrenwend (1987). Influence of late Quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont, eastern Mojave Desert, California. Quaternary Research 27 (2): 130-146
ABSTRACT: Radiocarbon dating of late Quaternary deposits and shorelines of Lake Mojave and cation-ratio numerical age dating of stone pavements (Dorn, 1984) on the adjacent Soda Mountains piedmont provide age constraints for alluvial and eolian deposits. These deposits are associated with climatically controlled stands of Lake Mojave during the past 15,000 yr. Six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces, as well as on soil-geomorphic data. All but one of these stratigraphic units were deposited in response to time-transgressive climatic changes beginning approximately 10,000 yr ago. Increased eolian flux rates occurred in response to the lowering of Lake Mojave and a consequent increase in fine-sediment availability. Increased rates of deposition of eolian fines and associated salts influenced pedogenesis, stone-pavement development, and runoff-infiltration relations by (1) enhancing mechanical weathering of fan surfaces and hillslopes and (2) forming clay- and silt-rich surface horizons which decrease infiltration. Changes in alluvial-fan source areas from hillslopes to piedmonts during the Holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less-permeable soils. Inferred increased in early to middle Holocene monsoonal activity resulted in high-magnitude paleo-sheetflood events on older fan pavements; this runoff triggered piedmont dissection which, in turn, caused increased sediment availability along channel walls. Thus, runoff-infiltration changes during the late Quaternary have occurred in response to eolian deposition of fines, pedogenesis, increased sheetflood activity in the Holocene, and vegetational changes which are related to many complicated linkages among climatic change, lake fluctuations, and eolian, hillslope, and alluvial-fan processes.
ABSTRACT: The native population collapse in 16th century Mexico was a demographic catastrophe with one of the highest death rates in history. Recently developed tree-ring evidence has allowed the levels of precipitation to be reconstructed for north central Mexico, adding to the growing body of epidemiologic evidence and indicating that the 1545 and 1576 epidemics of cocoliztli (Nahuatl for "pest”) were indigenous hemorrhagic fevers transmitted by rodent hosts and aggravated by extreme drought conditions.
ABSTRACT: The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.
ABSTRACT: Solar total and ultraviolet (UV) irradiances are reconstructed annually from 1610 to the present. This epoch includes the Maunder Minimum of anomalously low solar activity (circa 1645–1715) and the subsequent increase to the high levels of the present Modern Maximum. In this reconstruction, the Schwabe (11-year) irradiance cycle and a longer term variability component are determined separately, based on contemporary solar and stellar monitoring. The correlation of reconstructed solar irradiance and Northern Hemisphere (NH) surface temperature is 0.86 in the pre-industrial period from 1610 to 1800, implying a predominant solar influence. Extending this correlation to the present suggests that solar forcing may have contributed about half of the observed 0.55°C surface warming since 1860 and one third of the warming since 1970.
ABSTRACT: About 8200 years ago, the climate of much of the Northern Hemisphere cooled abruptly for a period of about 200 years. In their Perspective, Clarke et al. examine the most likely culprit for this cooling: an outburst of fresh water from a vast, ice-dammed glacial lake in North America. The superlake had formed when the kilometers-thick ice sheet covering much of North America disintegrated. When the ice dam became unstable, fresh water flooded from the lake into the North Atlantic. It remains unclear how this fresh water affected ocean circulation or whether the outburst occurred in more than one stage, but the timing points strongly to the outburst flood as the trigger of the 8200-year climate event.
P.D. Jones, K.R. Briffa, T.J. Osborn, J.M. Lough, T.D. van Ommen, B.M. Vinther, J. Luterbacher, E.R. Wahl, F.W. Zwiers, M.E. Mann, G.A. Schmidt, C.M. Ammann, B.M. Buckley, K.M. Cobb, J. Esper, H. Goosse, N. Graham, E. Jansen, T. Kiefer, C. Kull, M. Küttel, E. Mosley-Thompson, J.T. Overpeck, N. Riedwyl, M. Schulz, A.W. Tudhope, R. Villalba, H. Wanner, E. Wolff, E. Xoplaki (2009). High-resolution palaeoclimatology of the last millennium: a review of current status and future prospects. The Holocene 19 (1): 3-49
ABSTRACT: This review of late-Holocene palaeoclimatology represents the results from a PAGES/CLIVAR Intersection Panel meeting that took place in June 2006. The review is in three parts: the principal high-resolution proxy disciplines (trees, corals, ice cores and documentary evidence), emphasizing current issues in their use for climate reconstruction; the various approaches that have been adopted to combine multiple climate proxy records to provide estimates of past annual-to-decadal timescale Northern Hemisphere surface temperatures and other climate variables, such as large-scale circulation indices; and the forcing histories used in climate model simulations of the past millennium. We discuss the need to develop a framework through which current and new approaches to interpreting these proxy data may be rigorously assessed using pseudo-proxies derived from climate model runs, where the `answer' is known. The article concludes with a list of recommendations. First, more raw proxy data are required from the diverse disciplines and from more locations, as well as replication, for all proxy sources, of the basic raw measurements to improve absolute dating, and to better distinguish the proxy climate signal from noise. Second, more effort is required to improve the understanding of what individual proxies respond to, supported by more site measurements and process studies. These activities should also be mindful of the correlation structure of instrumental data, indicating which adjacent proxy records ought to be in agreement and which not. Third, large-scale climate reconstructions should be attempted using a wide variety of techniques, emphasizing those for which quantified errors can be estimated at specified timescales. Fourth, a greater use of climate model simulations is needed to guide the choice of reconstruction techniques (the pseudo-proxy concept) and possibly help determine where, given limited resources, future sampling should be concentrated.