Climate Change and...

Annotated Bibliography

Climate Variability

Tree-Ring Studies

P. A. Knapp, P. T. Soulé, H. D. Grissino-Mayer (2004). Occurrence of sustained droughts in the Interior Pacific Northwest (A.D. 1733–1980) inferred from tree-ring data. Journal of Climate 17 (1): 140-150

ABSTRACT: The occurrence of moderate and severe sustained droughts in the interior Pacific Northwest (PNW) from 1733 to 1980 was mapped using 18 western juniper (Juniperus occidentalis var. occidentalis Hook.) tree-ring chronologies. The frequency and duration of both moderate and severe sustained droughts are substantially greater in the northwest region of the interior PNW. Thus, this area is identified as a drought core region. These droughts are chiefly attributed to the presence of a Pacific blocking high off the NW coast that is associated with significant reductions in cool season precipitation. Specifically, the impacted northwest region lies within a transition zone between the fluxes of marine airflow during the cool season months and interior air during the warm season months. The waxing and waning of the boundaries of this transition zone particularly affect western juniper trees growing in this region. During years in which a blocking high is present, marine airflow is substantially reduced, exposing the trees in the transition zone to substantially drier springtime conditions that limit soil moisture and reduce radial growth. Although the most severe and persistent droughts were concentrated in the northwest region, four large-scale droughts also impacted the entire interior PNW during the study period. These droughts occurred principally during PDO warm phases, suggesting a regional-scale linkage to this climatic oscillation.

R. D. D'Arrigo, G. C. Jacoby (1999). Northern North American tree-ring evidence for regional temperature changes after major volcanic events. Climatic Change 41 (1): 1-15

ABSTRACT: Maximum latewood density data from trees at thirteen temperature-sensitive sites along the northern treeline of North America are used to evaluate the spatial patterns of response to four known volcanic events just prior to the period of modern observations: in 1640, 1783, 1815 and 1835. A previously unknown event is also postulated for 1699. This tree-ring density parameter is used because it shows a stronger and more consistent short-term, temperature-related volcanic signal than ring width. Normalized density departures following these events vary in sign and magnitude from region to region, with the coldest summer conditions inferred for the Northwest Territories in 1641, Alaska in 1783, Quebec and Labrador in 1816 and the Northwest Territories in 1836. For these as well as other events, low density values are often among the most extreme on record at their respective locations. We suggest that these regional variations in tree growth reflect cooling induced by volcanism and the distribution of cooling influenced by atmospheric circulation patterns.

D'Arrigo, R., Wiles, G., Jacoby, G., Villalba, R. (1999). North Pacific sea surface temperatures: Past variations inferred from tree rings. Geophysical Research Letters 26 (17): 2757-2760

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.

Hidalgo, H. G. (2004). Climate precursors of multidecadal drought variability in the western United States. Water Resources Research 40 (12): W12504

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.

Alftine, K. J., G.P. Malanson, D.B. Fagre (2003). Feedback-driven response to multidecadal climatic variability at an alpine treeline. Physical Geography 24 (6): 520-535

ABSTRACT: The Pacific Decadal Oscillation (PDO) has significant climatological and ecological effects in northwestern North America. Its possible effects and their modification by feedbacks are examined in the forest-tundra ecotone in Glacier National Park, Montana, USA. Tree ring samples were collected to estimate establishment dates in 10 quadrats. Age-diameter regressions were used to estimate the ages of uncored trees. The temporal pattern of establishment and survival was compared to the pattern of the PDO. A wave of establishment began in the mid-1940s, rose to a peak rate in the mid-1970s, and dropped precipitously beginning ca. 1980 to near zero for the 1990s. The period of establishment primarily coincided with the negative phase of the PDO, but the establishment and survival pattern is not correlated with the PDO index. The pattern indicates a period during which establishment was possible and was augmented by positive feedback from surviving trees. Snow may be the most important factor in the feedback, but studies indicate that its effects vary locally. Spatially differentiated analyses of decadal or longer periodicity may elucidate responses to climatic variation.

Briffa, K. R. (2000). Annual climate variability in the Holocene: interpreting the message of ancient trees. Quaternary Science Reviews 19 (1-5): 87-105

ABSTRACT: Over vast areas of the world's landmasses, where climate beats out a strong seasonal rhythm, tree growth keeps unerring time. In their rings, trees record many climate melodies, played in different places and different eras. Recent years have seen a consolidation and expansion of tree-ring sample collections across the traditional research areas of North America and Europe, and the start of major developments in many new areas of Eurasia, South America and Australasia. From such collections are produced networks of precisely dated chronologies; records of various aspects of tree growth, registered continuously, year by year across many centuries. Their sensitivities to different climate parameters are now translated into ever more detailed histories of temperature and moisture variability across expanding dimensions of time and space. With their extensive coverage, high temporal resolution and rigid dating control, dendroclimatic reconstructions contribute significantly to our knowledge of late Holocene climates, most importantly on timescales ranging from 1 to 100 years. In special areas of the world, where trees live for thousands of years or where subfossil remnants of long dead specimens are preserved, work building chronologies covering many millennia continues apace. Very recently, trees have provided important new information about major modes of general circulation dynamics linked to the El Niño/Southern Oscillation and the North Atlantic Oscillation, and about the effect of large volcanic eruptions. As for assessing the significance of 20th century global warming, the evidence from dendroclimatology in general, supports the notion that the last 100 years have been unusually warm, at least within a context of the last two millennia. However, this evidence should not be considered equivocal. The activities of humans may well be impacting on the ‘natural’ growth of trees in different ways, making the task of isolating a clear climate message subtly difficult.

Brown, P.M., C.H. Sieg (1999). Historical variability in fire at the ponderosa pine - Northern Great Plains prairie ecotone, southeastern Black Hills, South Dakota. Ecoscience 6 (4): 539-547

ABSTRACT: Ecotones are boundaries between plant assemblages that can represent a physiological or competitive limit of species’ local distributions, usually through one or more biotic or abiotic constraints on species’ resource requirements. However, ecotones also result from the effects of chronic or episodic disturbances, and changes in disturbance regimes may have profound effects on vegetation patterns in transitional areas. In this study, centuries-long chronologies of surface fire events were reconstructed from fire-scarred ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees in three sites at the ecotone between ponderosa pine forest and Northern Great Plains mixed grass prairie in the southeastern Black Hills of South Dakota. The fire chronologies provide baseline data to assess the possible role of fire in this transitional area and to document historical variability in fire regimes in this region of the Northern Great Plains. Regular fire events were recorded at all three sites from the beginning of the fire chronologies in the 1500s up to the late 1800s or early 1900s, at which time spreading fires ceased. Fire frequencies derived from the fire chronologies were compared to each other and to four sites from interior ponderosa pine forest in the south-central Black Hills. Mean fire intervals at the savanna sites were between 10 to 12 years, whereas Weibull median probability intervals were one year shorter. Fire frequency at the savanna sites was twice as high as at the interior forest sites, and most likely was due to spatial extent of fires on the mixed-grass prairie coupled with warmer and drier climate regime. Post-settlement shifts in the ponderosa pine savanna during the twentieth century in this area may be largely attributed to lack of fire occurrences, although grazing and other factors also likely contributed to observed changes in forest and grassland margins.

Blasing, T.J., Fritts, H.C. (1976). Reconstructing past climatic anomalies in the north Pacific and western North America from tree-ring data. Quaternary Research 6 (4): 563-579

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.

Graumlich, L.J. (1987). Precipitation variation in the Pacific Northwest (1675-1975) as reconstructed from tree rings. Annals of the Association of American Geographers 77 (1): 19-29

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).

S.T. Gray, L.J. Graumlich, J. L. Betancourt (2007). Annual precipitation in the Yellowstone National Park region since AD 1173. Quaternary Research 68 (1): 18-27

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.

Meko, D.M., M.D. Therrell, C.H. Baisan, M.K. Hughes (2001). Sacramento River flow reconstructed to AD 869 from tree rings. Journal of the American Water Resources Association 37 (4): 1029-1040

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.

Swetnam, T.W. (1993). Fire history and climate change in giant sequoia groves. Science 262 (5135): 885-889

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.

Grissino-Mayer, H. D., Swetnam, T.W. (2000). Century-scale climate forcing of fire regimes in the American Southwest. The Holocene 10 (2): 213-220

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.

S.T. Gray, Julio L. Betancourt, S.T. Jackson, R.G. Eddy (2006). Role of multidecadal climate variability in a range extension of pinyon pine. Ecology 87 (5): 1124-1130

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.

Gedalof, Z., D.L. Peterson, N.J. Mantua (2004). Columbia River flow and drought since 1750. Journal of the American Water Resources Association 40 (6): 1579-1592

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.

Cook, E.R., Meko, D.M., Stahle, D.W., Cleaveland, M.K. (1999). Drought reconstructions for the continental United States. Journal of Climate 12 (4): 1145-1162

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.

Wiles, G. C., D'Arrigo, R.D., Jacoby, G.C. (1996). Temperature changes along the Gulf of Alaska and the Pacific Northwest coast. Canadian Journal of Forest Research 26 (3): 474-481

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.

C. J. Earle (1993). Asynchronous droughts in California streamflow as reconstructed from tree rings. Quaternary Research 39 (3): 290-299

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.

J. E. Kutzbach (1976). The nature of climate and climatic variations. Quaternary Research 6 (4): 471-480

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.

M.K. Hughes, G. Funkhouser (2003). Frequency-dependent climate signal in upper and lower forest Border tree rings in the mountains of the Great Basin. Climatic Change 59 (1): 233-244

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.

Esper, J., E.R. Cook, F.H. Schweingruber (2002). Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253

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.

Woodhouse, C. A., S.T. Gray, D. M. Meko (2006). Updated streamflow reconstructions for the Upper Colorado River Basin. Water Resources Research 42: W05415

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.

Meko, D., D. A. Graybill (1995). Tree-ring reconstruction of upper Gila River discharge. Water Resources Bulletin 31 (4): 605-616

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.

A. Schimmelmann, M. Zhao, C. C. Harvey, C. B. Lange (1998). A large California flood and correlative global climatic events 400 years ago. Quaternary Research 49 (1): 51-61

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.

Woodhouse, C. A. (2001). A tree-ring reconstruction of streamflow for the Colorado Front Range. Journal of the American Water Resources Association 37 (3): 561-569

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.

T. W. Swetnam, J. L. Betancourt (1998). Mesoscale disturbance and ecological response to decadal climatic variability in the American southwest. Journal of Climate 11 (12): 3128-3147

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.

Fall, P. L. (1997). Timberline fluctuations and late Quaternary paleoclimates in the Southern Rocky Mountains, Colorado. Geological Society of America Bulletin 109 (10): 1306-1320

ABSTRACT: Pollen and plant macrofossils from eight sedimentary basins on the west slope of the Colorado Rocky Mountains document fluctuations in upper and lower timberline since the latest Pleistocene. By tracking climatically sensitive forest boundaries, the moisture-controlled lower timberline and the temperature-controlled upper timberline, paleoclimatic estimates can be derived from modern temperature and precipitation lapse rates. Pollen data suggest that prior to 11 000 yr B.P., a subalpine forest dominated byPicea (spruce) andPinus (pine) grew 300–700 m below its modern limit. The inferred climate was 2–5 °C cooler and had 7–16 cm greater precipitation than today.Abies (fir) increased in abundance in the subalpine forest around 11 000 yr B.P., probably in response to cooler conditions with increased winter snow. Pollen and plant macrofossil data demonstrate that from 9000 to 4000 yr B.P. the subalpine forest occupied a greater elevational range than it does today. Upper timberline was 270 m above its modern limit, suggesting that mean annual and mean July temperatures were 1–2 °C warmer than today. Intensification of the summer monsoon, coupled with increased summer radiation between 9000 and 6000 yr B.P., raised mean annual precipitation by 8–11 cm and allowed the lower limit of the subalpine and montane forests to descend to lower elevations. The lower forest border began to retreat upslope between 6000 and 4000 yr B.P. in response to drier conditions, and the upper timberline descended after 4000 yr B.P., when temperatures cooled to about 1 °C warmer than today. The modern climatic regime was established about 2000 yr B.P., when the summer precipitation maxima of the early and middle Holocene were balanced by increased winter precipitation.

C. Whitlock (2001). Variations in Holocene fire frequency: a view from the western United States. Proceedings of the Royal Irish Academy 101B (1-2): 65-77

ABSTRACT: Fire history data provide information on the role of fire as a long-term ecological process and the changes in fire activity that have occurred in response to variations in climate, vegetation and human activity. Fire reconstructions are derived from dendrochronological records that span the last few centuries and charcoal records obtained from the sediments of lakes and wetlands that span millennia. Several advances in the study of charcoal records have improved their usefulness for palaeoecological research. For example, calibration studies of charcoal inputs during and following recent fires indicate that macroscopic charcoal particles are not transported far from the source area. In addition, high-resolution charcoal time series are now interpreted in terms of two components—a peaks component that registers past fire events and a background component that represents long-term variations in charcoal production and transport. This paper presents an examination of charcoal and pollen records from the western United States that illustrates the potential of this type of research for reconstructing past environments.

G.C. Jacoby, R.D. D'Arrigo (1997). Tree rings, carbon dioxide, and climatic change. Proceedings of the National Academy of Sciences 94 (16): 8350-8353

ABSTRACT: Tree rings have been used in various applications to reconstruct past climates as well as to assess the effects of recent climatic and environmental change on tree growth. In this paper we briefly review two ways that tree rings provide information about climate change and CO2 : (i) in determining whether recent warming during the period of instrumental observations is unusual relative to prior centuries to millennia, and thus might be related to increasing greenhouse gases; and (ii) in evaluating whether enhanced radial growth has taken place in recent decades that appears to be unexplained by climate and might instead be due to increasing atmospheric CO2 or other nutrient fertilization. It is found that a number of tree-ring studies from temperature-sensitive settings indicate unusual recent warming, although there are also exceptions at certain sites. The present tree-ring evidence for a possible CO2 fertilization effect under natural environmental conditions appears to be very limited.

Jacoby, G.C., D'Arrigo, R.D. (1995). Tree ring width and density evidence of climatic and potential forest change in Alaska. Global Biogeochemical Cycles 9 (2): 227-234

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.

T. W. Swetnam, C.D. Allen, J. L. Betancourt (1999). Applied historical ecology: using the past to manage for the future. Ecological Applications 9 (4): 1189-1206

ABSTRACT: Applied historical ecology is the use of historical knowledge in the management of ecosystems. Historical perspectives increase our understanding of the dynamic nature of landscapes and provide a frame of reference for assessing modern patterns and processes. Historical records, however, are often too brief or fragmentary to be useful, or they are not obtainable for the process or structure of interest. Even where long historical time series can be assembled, selection of appropriate reference conditions may be complicated by the past influence of humans and the many potential reference conditions encompassed by nonequilibrium dynamics. These complications, however, do not lessen the value of history; rather they underscore the need for multiple, comparative histories from many locations for evaluating both cultural and natural causes of variability, as well as for characterizing the overall dynamical properties of ecosystems. Historical knowledge may not simplify the task of setting management goals and making decisions, but 20th century trends, such as increasingly severe wildfires, suggest that disregarding history can be perilous.

We describe examples from our research in the southwestern United States to illustrate some of the values and limitations of applied historical ecology. Paleoecological data from packrat middens and other natural archives have been useful for defining baseline conditions of vegetation communities, determining histories and rates of species range expansions and contractions, and discriminating between natural and cultural causes of environmental change. We describe a montane grassland restoration project in northern New Mexico that was justified and guided by an historical sequence of aerial photographs showing progressive tree invasion during the 20th century. Likewise, fire scar chronologies have been widely used to justify and guide fuel reduction and natural fire reintroduction in forests. A southwestern network of fire histories illustrates the power of aggregating historical time series across spatial scales. Regional fire patterns evident in these aggregations point to the key role of interannual lags in responses of fuels and fire regimes to the El Niño–Southern Oscillation (wet/dry cycles), with important implications for long-range fire hazard forecasting. These examples of applied historical ecology emphasize that detection and explanation of historical trends and variability are essential to informed management.

D. M. Meko, C. H. Baisan (2001). Pilot study of latewood-width of conifers as an indicator of variability of summer rainfall in the North American monsoon region. International Journal of Climatology 21 (6): 697-708

ABSTRACT: The variability of the North American Monsoon System (NAMS) is important to the precipitation climatology of Mexico and the southwestern United States. Tree-ring studies have been widely applied to climatic reconstruction in western North America, but as yet, have not addressed the NAMS. One reason is the need for highly resolved seasonal dendroclimatic information. Latewood-width, the portion of the annual tree ring laid down late in the growing season, can potentially yield such information. This paper describes a pilot study of the regional summer precipitation signal in latewood-width from a network of fivePseudotsuga menziesii chronologies developed in the heart of the region of NAMS influence in Arizona, USA. Exploratory data analysis reveals that the summer precipitation signal in latewood is strong, but not equally so over the full range of summer precipitation. Scatter in the relationship increases toward higher levels of precipitation. Adjustment for removal of inter-correlation with earlywood-width appears to strengthen the summer precipitation signal in latewood-width. To demonstrate a possible application to climatic reconstruction, the latewood precipitation signal is modelled using a nonlinear model - a binary recursive classification tree (CT) that attempts to classify summers as dry or not dry from threshold values of latewood-width. The model identifies narrow latewood-width as an effective predictor of a dry summer. Of 14 summers classified dry by latewood-width for the period 1868-1992, 13 are actually dry by the instrumental precipitation record. The results suggest that geographical expansion of coverage by latewood-width chronologies and further development of statistical methods may lead to successful reconstruction of variability of the NAMS on century time scales.

R. A. Case, G. M. MacDonald (2003). Tree ring reconstructions of streamflow for three Canadian prairie rivers. Journal of the American Water Resources Association 39 (3): 703-716

ABSTRACT: Information regarding long term hydrological variability is critical for the effective management of surface water resources. In the Canadian Prairie region, growing dependence on major river systems for irrigation and other consumptive uses has resulted in an increasing vulnerability to hydrological drought and growing interprovincial tension. This study presents the first dendrochronological records of streamflow for Canadian Prairie rivers. We present 1,113-year, 522-year, and 325-year reconstructions of total water year (October to September) streamflow for the North Saskatchewan, South Saskatchewan, and Saskatchewan Rivers, respectively. The reconstructions indicate relatively high flows during the 20th Century and provide evidence of past prolonged droughts. Low flows during the 1840s correspond with aridity that extended over much of the western United States. Similarly, an exceptional period of prolonged low flow conditions, approximately 900 A.D. to 1300 A.D., is coincident with evidence of sustained drought across central and western North America. The 16th Century megadrought of the western United States and Mexico, however, does not appear to have had a major impact on the Canadian rivers. The dendrohydrological records illustrate the risks involved if future water policy and infrastructure development in the Canadian Prairies are based solely on records of streamflow variability over the historical record.

Garfin, G. M., Hughes, M.K. (1996). Eastern Oregon PDSI and precipitation reconstructions. NOAA/NGDC Paleoclimatology Program USDA Forest Service Cooperative Agreement PNW 90-174

ABSTRACT: Information regarding long term hydrological variability is critical for the effective management of surface water resources. In the Canadian Prairie region, growing dependence on major river systems for irrigation and other consumptive uses has resulted in an increasing vulnerability to hydrological drought and growing interprovincial tension. This study presents the first dendrochronological records of streamflow for Canadian Prairie rivers. We present 1,113-year, 522-year, and 325-year reconstructions of total water year (October to September) streamflow for the North Saskatchewan, South Saskatchewan, and Saskatchewan Rivers, respectively. The reconstructions indicate relatively high flows during the 20th Century and provide evidence of past prolonged droughts. Low flows during the 1840s correspond with aridity that extended over much of the western United States. Similarly, an exceptional period of prolonged low flow conditions, approximately 900 A.D. to 1300 A.D., is coincident with evidence of sustained drought across central and western North America. The 16th Century megadrought of the western United States and Mexico, however, does not appear to have had a major impact on the Canadian rivers. The dendrohydrological records illustrate the risks involved if future water policy and infrastructure development in the Canadian Prairies are based solely on records of streamflow variability over the historical record.

D. M. Meko, C. A. Woodhouse (2005). Tree-ring footprint of joint hydrologic drought in Sacramento and upper Colorado river basins, western USA. Journal of Hydrology 308 (1-4): 196-213

ABSTRACT: Growing and changing demands on water supply, along with natural climate variability and possible anthropogenically induced climate change, make water resource management and planning increasingly challenging, particularly in arid regions. Instrumental climate and gaged streamflow records provide just a snapshot of recent natural hydrologic variability. In this paper, we use tree-ring-based annual streamflow reconstructions for the Sacramento River in California and the Blue River in western Colorado to analyze the temporal and spatial variability of widespread drought simultaneously affecting both basins over the past five centuries. Stability of joint-drought episodes and the covariation of reconstructed flows in the two basins are analyzed with sliding correlations, spectral analysis and a hypergeometric test. Year-to-year spatial patterns of moisture anomalies in a singular joint-drought episode in the late-1500s are mapped with a network of tree-ring data. Climatological aspects of joint droughts of the 20th century are investigated with 500-mb geopotential height data and climatic indices.

Although flow in the two rivers is only very weakly correlated over the full 538-yr reconstruction period, more years of joint drought occur than would be expected by chance alone. Covariation in reconstructed flows is stronger in the late 1500s and mid-1700s than at any time since 1800. The late 1500s period of drought is not characterized as a decades-long unbroken drought, but as a series of drought impulses broken by wet years, with widespread moisture deficits in joint dry years. Periods of high inter-basin correlation in reconstructed flow are characterized by coherency at frequencies within the ENSO band. However, joint droughts in instrumental gage records do not display any consistent relationship with ENSO or the Pacific Decadal Oscillation (PDO), and so it is difficult to infer either as a causal mechanism for joint droughts in the past.

Tarboton, D.G. (1994). The source hydrology of severe sustained drought in the southwestern United States. Journal of Hydrology 161 (1-4): 31-69

ABSTRACT: This paper considers the risk of drought and develops drought scenarios for use in the study of severe sustained drought in the southwestern United States. The focus is on the Colorado River basin and regions to which Colorado River water is exported, especially southern California, which depends on water from the Colorado River as well as the four major rivers in northern California. Drought scenarios are developed using estimates of unimpaired historic streamflow as well as reconstructions of streamflow based on tree ring widths. Drought scenarios in the Colorado River are defined on the basis of annual flow at Lees Ferry. Possible spatial manifestations of the Colorado River drought scenarios for input into a Colorado River system simulation model are developed by disaggregating the Lees Ferry flow to monthly flows at 29 source locations required by the model. The risk, in terms of return period, of the drought scenarios developed, is assessed using stochastic models applied to both the Colorado River basin and the combined flow in four major California rivers. The risk of severe sustained drought occurring concurrently in the Colorado River basin and California is also assessed.

Pierce, J. L., G.A. Meyer, A.J. Timothy Jull (2004). Fire-induced erosion and millennial-scale climate change in northern ponderosa pine forests. Nature 432 (7013): 87-90

ABSTRACT: Western US ponderosa pine forests have recently suffered extensive stand-replacing fires followed by hillslope erosion and sedimentation. These fires are usually attributed to increased stand density as a result of fire suppression, grazing and other land use, and are often considered uncharacteristic or unprecedented. Tree-ring records from the past 500 years indicate that before Euro-American settlement, frequent, low-severity fires maintained open stands. However, the pre-settlement period between about AD 1500 and AD 1900 was also generally colder than present, raising the possibility that rapid twentieth-century warming promoted recent catastrophic fires. Here we date fire-related sediment deposits in alluvial fans in central Idaho to reconstruct Holocene fire history in xeric ponderosa pine forests and examine links to climate. We find that colder periods experienced frequent low-severity fires, probably fuelled by increased understory growth. Warmer periods experienced severe droughts, stand-replacing fires and large debris-flow events that comprise a large component of long-term erosion and coincide with similar events in sub-alpine forests of Yellowstone National Park. Our results suggest that given the powerful influence of climate, restoration of processes typical of pre-settlement times may be difficult in a warmer future that promotes severe fires.

Kochel, R.C., D.F. Ritter, J.R. Miller (1997). Geomorphic responses to minor cyclic changes, San Diego County, California. Geomorphology 19 (3-4): 277-302

ABSTRACT: Short-term episodic cycles of wet and dry patterns of climate are common in southern California. Wet intervals, like the one in 1978-83, are often characterized by more than double the average annual precipitation. The impact of these episodic climatic fluctuations on landforms and surficial processes has not been well documented for areas inland of the coast. The response to these cycles may be significant in the evolution of hillslopes and fluvial landforms, and may have significant implications for geologic hazards in this rapidly developing region.

Using aerial photographs and field investigations we found little response to the 1978-83 wet interval on upland hillslopes, but documented significant response on alluvial fans and in channels in desert piedmont areas. These observations may lend support to the Langbein-Schumm (1958) model relating sediment yield to precipitation. A variety of techniques, including dendrogeomorphology, studies of the weathering of clasts, soil stratigraphy, and aerial photo mapping were used to discern at least six units on alluvial fans ranging from Late Pleistocene to present. Terraces along active fan channels and the San Felipe River record a geomorphic record of the most recent wet intervals (ca. 1940 and 1980) as a significant depositional event. Geomorphic responses to the wet interval along the San Felipe River were complex, varying locally according to controls on sediment storage and downstream transfer through a recently integrated drainage system. Additional complex responses to the wet period were experienced in selected sites where antecedence and response times may be measured in months or even years.

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