Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
The 20th Century
ABSTRACT: A long-term, homogeneous set of daily maximum and minimum temperature data representing a subset of daily U.S. Historical Climatology Network stations is used to analyze trends in extreme temperature occurrence across the contiguous United States. Time series of various lengths are analyzed, with the longest spanning the period 1900–96. Trends in the annual occurrence of extreme maximum and minimum temperatures (e.g., values greater than the 90th, 95th, or 99th percentile) are strongly influenced by high exceedence counts during drought periods in the 1930s and 1950s. Peaks in exceedences during these years result in predominantly decreasing warm exceedence trends across the country during the 1930–96 period. This is uncharacteristic of recent years (1960–96) in which a large majority of stations show increases in warm extreme temperature exceedences. Significant increases in warm minimum temperature exceedences are found at nearly one-third of the stations during this period. Multiday warm temperature exceedence runs also show strong increases during this more recent period. The most rapid increases in high maximum and minimum temperature extremes occur at stations classified as urban, by satellite land use information.
Trends in the annual occurrence of extremely cold maximum and minimum temperatures display an analogous decrease during the 1960–96 period. Here again, there is a distinct shift in the number of decreasing trends between the 1950–96 and 1960–96 periods. Based on starting decades prior to 1960, there is not a strong tendency for either increasing or decreasing trends. The period 1910–96 is an exception, with almost all stations exhibiting decreasing cold extreme occurrence trends. The extreme cold exceedence trends during the 1960–96 period are also influenced by urbanization, but to a lesser degree than the warm extremes.
ABSTRACT: The spatial patterns and temporal trends of temperature and precipitation for northern North America (Alaska Canada and western Greenland) have been analyzed. Over approximately the past hundred years, three temperature regimes are identified that correspond roughly to similar climatic regimes identified in separate studies for the contiguous United States. Through 1980, warming is evident only from around the mid-1920s to about the early 1960s. No recent trends are present in winter or fall. Some cooling is evident during summer while spring shows cooling from 1963 to 1976 and warming thereafter.
Spatially, the largest changes occur in areas where variations in the amplitude of the long waves result in large advective differences; these areas are also sensitive to fluctuations in the mean position of the arctic front. Changes from one temperature regime to another occur quite abruptly and last for several years to a few decades.
There are two areas where well-defined precipitation changes coincide with temperature changes: the southern Canadian Plains near the 100°W meridian; and from the Great Lakes to James Bay northeastward toward Labrador. The location of these areas within a principal storm corridor suggests that the changes are associated with southward and northward shifts in the storm track that runs from the upper Midwest/Great Lakes region along the St. Lawrence River Valley toward the North Atlantic.
ABSTRACT: The relation between changes in modern glaciers, not including the ice sheets of Greenland and Antarctica, and their climatic environment is investigated to shed light on paleoglacier evidence of past climate change and for projecting the effects of future climate warming on cold regions of the world. Loss of glacier volume has been more or less continuous since the 19th century, but it is not a simple adjustment to the end of an "anomalous" Little Ice Age. We address the 1961-1997 period, which provides the most observational data on volume changes. These data show trends that are highly variable with time as well as within and between regions; trends in the Arctic are consistent with global averages but are quantitatively smaller. The averaged annual volume loss is 147 mm yr-1 in water equivalent, totaling 3.7 x 103 km3 over 37 yr. The time series shows a shift during the mid-1970s, followed by more rapid loss of ice volume and further acceleration in the last decade; this is consistent with climatologic data. Perhaps most significant is an increase in annual accumulation along with an increase in melting; these produce a marked increase in the annual turnover or amplitude. The rise in air temperature suggested by the temperature sensitivities of glaciers in cold regions is somewhat greater than the global average temperature rise derived largely from low altitude gauges, and the warming is accelerating.
ABSTRACT: Climatological annual and seasonal dewpoint, specific humidity, and relative humidity maps for the United States are presented using hourly data from 188 first-order weather stations for the period 1961–90. Separate climatologies were calculated for daytime (three observations per day between 0800 and 1600 LST), nighttime (three observations per day between 2000 and 0400 LST), and the full day (eight observations per day, every 3 h).
With extended datasets for the period 1961–95, trends in these same variables and temperature are calculated for each of 170 stations and for eight regions of the country. The data show increases in specific humidity of several percent per decade, and increases in dewpoint of several tenths of a degree per decade, over most of the country in winter, spring, and summer. Nighttime humidity trends are larger than daytime trends. The specific humidity increases are consistent with upward temperature trends. The upward temperature and humidity trends are also consistent with upward trends in apparent temperature, a measure of human comfort based on temperature and humidity. Relative humidity trends are weaker than the specific humidity trends, but they do show evidence of increases, especially in winter and spring.
The possibility that the detected trends may be artifacts of changes in instrumentation was examined, but several lines of reasoning suggest that they are not. Anthropogenic water vapor produced from fossil fuel consumption, both locally and globally, is too small a source to explain the observed trends.
ABSTRACT: An analysis of extreme precipitation events indicates that there has been a sizable increase in their frequency since the 1920s/1930s in the U.S. There has been no discernible trend in the frequency of the most extreme events in Canada, but the frequency of less extreme events has increased in some parts of Canada, notably in the Arctic. In the U.S., frequencies in the late 1800s/early 1900s were about as high as in the 1980s/1990s. This suggests that natural variability of the climate system could be the cause of the recent increase, although anthropogenic forcing due to increasing greenhouse gas concentrations cannot be discounted as another cause. It is likely that anthropogenic forcing will eventually cause global increases in extreme precipitation, primarily because of probable increases in atmospheric water vapor content and destabilization of the atmosphere. However, the location, timing, and magnitude of local and regional changes remain unknown because of uncertainties about future changes in the frequency/intensity of meteorological systems that cause extreme precipitation.
FIRST PARAGRAPH: “Climate” is simply defined as the statistics of weather over a specified period of time, and is often expressed as long-term monthly averaged temperature and precipitation, or the frequency for cloudy days or river flooding in any given month, season or year. As a general rule, important elements of any region’s climate vary from year to year – one year is warmer or cooler than another, or maybe one summer sees many more (or less) sunny days than the next. While the vagaries of climate have often seemed random and unpredictable, recent advances in climate science point to a handful of regularly occurring hemispheric scale patterns that impose some order in the climate system. The El Niño/Southern Oscillation (ENSO), for instance, is the best-known “natural pattern,” or mode, of Earth’s climate (Rasmussen and Wallace 1983).
ABSTRACT: Climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) on the whole reproduce the observed seasonal cycle and 20th century warming trend of 0.8°C (1.5°F) in the Pacific Northwest, and point to much greater warming for the next century. These models project increases in annual temperature of, on average, 1.2°C (2.2°F) by the 2020s, 2.0°C (3.5°F) by the 2040s, and 3.3°C (5.9°F) by the 2080s, compared to 1970 to1999, averaged across all climate models. Rates of warming range from 0.1 to 0.6°C (0.2° to 1.0°F) per decade. Projected changes in annual precipitation, averaged over all models, are small (+1 to +2%), but some models project an enhanced seasonal cycle with changes toward wetter autumns and winters and drier summers.
Changes in nearshore sea surface temperatures, though smaller than on land, are likely to substantially exceed interannual variability, but coastal upwelling changes little. Rates of 21st century sea level rise will depend on poorly known factors like ice sheet instability in Greenland and Antarctica, and could be as low as 20th century values (20cm, 8”) or as large as 1.3m (50”).
ABSTRACT: The poor relationship between what climatologists, hydrologists, and other physical scientists call floods, and those floods that actually cause damage to life or property, has limited what can be reliably said about the causes of observed trends in damaging floods. It further limits what can be said about future impacts of floods on society based on predicted changes in the global hydrological cycle. This paper presents a conceptual framework for the systematic assessment of the factors that condition observed trends in flood damage. Using the framework, it assesses the role that variability in precipitation has in damaging flooding in the United States at national and regional levels. Three different measures of flood damage—absolute, per capita, and per unit wealth—each lead to different conclusions about the nature of the flood problem. At a national level, of the 10 precipitation measures examined in this study, the ones most closely related to flood damage are the number of 2-day heavy rainfall events and the number of wet days. Heavy rainfall events are defined relative to a measure of average rainfall in each area, not as absolute thresholds. The study indicates that the growth in recent decades in total damage is related to both climate factors and societal factors: increased damage is associated with increased precipitation and with increasing population and wealth. At the regional level, this study reports a stronger relationship between precipitation measures and flood damage, and indicates that different measures of precipitation are most closely related to damage in different regions. This study suggests that climate plays an important, but by no means determining, role in the growth in damaging floods in the United States in recent decades.
G. A. Meehl, W. M. Washington, C. M. Ammann, J. M. Arblaster, T. M. L. Wigley, C. Tebaldi (2004). Combinations of natural and anthropogenic forcings in Twentieth-century climate. Journal of Climate 17 (19): 3721-3727
ABSTRACT: Ensemble simulations are run with a global coupled climate model employing five forcing agents that influence the time evolution of globally averaged surface air temperature during the twentieth century. Two are natural (volcanoes and solar) and the others are anthropogenic [e.g., greenhouse gases (GHGs), ozone (stratospheric and tropospheric), and direct effect of sulfate aerosols]. In addition to the five individual forcing experiments, an additional eight sets are performed with the forcings in various combinations. The late-twentieth-century warming can only be reproduced in the model with anthropogenic forcing (mainly GHGs), while the early twentieth-century warming is mainly caused by natural forcing in the model (mainly solar). However, the signature of globally averaged temperature at any time in the twentieth century is a direct consequence of the sum of the forcings. The similarity of the response to the forcings on decadal and interannual time scales is tested by performing a principal component analysis of the 13 ensemble mean globally averaged temperature time series. A significant portion of the variance of the reconstructed time series can be retained in residual calculations compared to the original single and combined forcing runs. This demonstrates that the statistics of the variances for decadal and interannual time-scale variability in the forced simulations are similar to the response from a residual calculation. That is, the variance statistics of the response of globally averaged temperatures in the forced runs are additive since they can be reproduced in the responses calculated as a residual from other combined forcing runs.
ABSTRACT: Direct observations of sunspot numbers are available for the past four centuries but longer time series are required, for example, for the identification of a possible solar influence on climate and for testing models of the solar dynamo. Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendrochronologically dated radiocarbon concentrations. We combine physics-based models for each of the processes connecting the radiocarbon concentration with sunspot number. According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago. We find that during the past 11,400 years the Sun spent only of the order of 10% of the time at a similarly high level of magnetic activity and almost all of the earlier high-activity periods were shorter than the present episode. Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades.
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: An analysis of ocean surface temperature records show that low frequency changes of tropical Pacific temperature lead global surface air temperature changes by about 4 years. Anomalies of tropical Pacific surface temperature are in turn preceded by subsurface temperature anomalies in the southern tropical Pacific by approximately 7 years. The results suggest that much of the decade to decade variations in global air temperature may be attributed to tropical Pacific decadal variability. The results also suggest that subsurface temperature anomalies in the southern tropical Pacific can be used as a predictor for decadal variations of global surface air temperature. Since the southern tropical Pacific temperature shows a distinct cooling over the last 8 years, the possibility exists that the warming trend in global surface air temperature observed since the late 1970's may soon weaken.
ABSTRACT: A number of recent studies have reported an ENSO-like EOF mode in the global sea surface temperature (SST) field, whose time variability is marked by an abrupt change toward a warmer tropical eastern Pacific and a colder extratropical central North Pacific in 1976–77. The present study compares this pattern with the structure of the interannual variability associated with the ENSO cycle and documents its time history back to 1900. The analysis is primarily based on the leading EOFs of the SST anomaly and “anomaly deviation” fields in various domains and the associated expansion coefficient (or principal component) time series, which are used to construct global regression maps of SST, sea level pressure (SLP), and a number of related variables. The use of “anomaly deviations” (i.e., departures of local SST anomalies from the concurrent global-mean SST anomaly) reduces the influence of global-mean SST trends upon the structure of the EOFs and their expansion coefficient time series. An important auxiliary time series used in this study is a “Southern Oscillation index” based on marine surface observations.
By means of several different analysis techniques, the time variability of the leading EOF of the global SST field is separated into two components: one identified with the “ENSO cycle-related” variability on the interannual timescale, and the other a linearly independent “residual” comprising all the interdecadal variability in the record. The two components exhibit rather similar spatial signatures in the global SST, SLP, and wind stress fields. The SST signature in the residual variability is less equatorially confined in the eastern Pacific and it is relatively more prominent over the extratropical North Pacific. The corresponding SLP signature is also stronger over the extratropical North Pacific, and its counterpart in the cold season 500-mb height field more closely resembles the PNA pattern. The amplitude time series of the ENSO-like pattern in the residual variability reflects the above-mentioned shift in 1976–77, as well as a number of other prominent features, including a shift of opposite polarity during the 1940s.
Hamlet, A. F., P. W. Mote, M. Clark, D. P. Lettenmaier (2005). Effects of temperature and precipitation variability on snowpack trends in the western United States. Journal of Climate 18 (21): 4545-4561
ABSTRACT: Recent studies have shown substantial declines in snow water equivalent (SWE) over much of the western US in the last half century, as well as trends towards earlier spring snowmelt and peak spring streamflows. These trends are influenced both by interannual and decadal scale climate variability, and also by temperature trends at longer time scales that are generally consistent with observations of global warming over the 20th century.
In this study we examine linear trends in April 1 snow water equivalent (SWE) over the western US as simulated by the Variable Infiltration Capacity hydrologic model implemented at 1/8 degree latitude-longitude spatial resolution, and driven by a carefully quality controlled gridded daily precipitation and temperature data set for the period 1915-2003. The long simulations of snowpack are used as surrogates for observations, and are the basis for an analysis of regional trends in snowpack over the western U.S. and southern British Columbia.
By isolating the trends due to temperature and precipitation in separate simulations, the influence of temperature and precipitation variability on the overall trends in SWE is evaluated. Downward trends in April 1 SWE over the western U.S. from 1916 to 2003, 1947-2003, and for a time series constructed using two warm Pacific Decadal Oscillation (PDO) epochs concatenated together, are shown to be primarily due to widespread warming. These temperature-related trends are not well explained by decadal climate variability associated with the PDO. Trends in SWE associated with precipitation trends, however, are very different in different time periods and are apparently largely controlled by decadal variability rather than longer term trends in climate.
ABSTRACT: This paper explores the effects of the interaction of anthropogenic trends and climate cycles on salmon declines in the Columbia and Snake river basins. A basic population model, including anthropogenic and environmental factors, is discussed and literature relating decadal scale climate patterns and the response of the North Pacific ecosystem is reviewed. From this background a ratchet-like decline in Columbia and Snake river salmon production resulted from the interactions of human activities and climatic regime shifts. These interactions are illustrated using hundred year patterns in spring chinook salmon (Oncorhynchus tshawytscha ) catch, the Columbia River hydroelectric generating capacity, and a climate index characterizing the shifts between a cool/wet regime favorable to West Coast salmon and a warm/dry regime unfavorable to West Coast salmon. A half century correlation of the climate index and chinook catch suggest that a favorable climate regime counteracted detrimental impacts of hydrosystem development between 1945 and 1977, while an unfavorable climate regime negated beneficial effects of salmon mitigation efforts after 1977. This hypothesis is elaborated by a comparison of changes in the climate index relative to changes in Snake River salmon survival indicators.
Proposed Snake River salmon restoration plans are considered in terms of this counteractive effects hypothesis. The recent declines of salmon stocks have led a number of groups to propose plans that discontinue the present recovery actions, especially transportation of juveniles salmon around the dams. This paper hypothesizes that salmon recovery efforts, in part, have been limited by recent poor climate/ocean conditions. If this hypothesis is true, then eliminating the transportation program could be detrimental to fish. If the hypothesis is false, then eliminating transportation may be a viable recovery measure. In either case resolving the issue of counteracting processes is essential prior to making major changes to the hydrosystem operations.
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.
ABSTRACT: Ongoing drought in the Colorado River Basin, unprecedented urban growth in the watershed, and numerical model simulations showing higher temperatures and lower precipitation totals in the future have all combined to heighten interest in drought in this region. In this investigation, we use principal components analysis (PCA) to independently assess the influence of various teleconnections on Basin-wide and sub-regional winter season Palmer Hydrological Drought Index (PHDI) and precipitation variations in the Basin. We find that the Pacific Decadal Oscillation (PDO) explains more variance in PHDI than El Niño-Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO), and the planetary temperature combined for the Basin as a whole. When rotated PCA is used to separate the Basin into two regions, the lower portion of the Basin is similar to the Basin as a whole while the upper portion, which contains the high-elevation locations important to hydrologic yield for the watershed, demonstrates poorly defined relationships with the teleconnections. The PHDI for the two portions of the Basin are shown to have been out of synch for much of the twentieth century. In general, teleconnection indices account for 19% of the variance in PHDI leaving large uncertainties in drought forecasting.
ABSTRACT: Documenting long-term trends or persistent shifts in temperature and precipitation is important for understanding present and future changes in flora and fauna. Carefully adjusted datasets for climate records in the USA and Canada are combined and used here to describe the spatial and seasonal variation in trends in the maritime, central, and Rocky Mountain climatic zones of the Pacific Northwest. Trends during the 20th century in annually averaged temperature (0.7 degrees C - 0.9 degrees C) and precipitation (13%-38%) exceed the global averages. Largest warming rates occurred in the maritime zone and in winter and at lower elevations in all zones, and smallest warming rates occurred in autumn and in the Rockies. Largest increases in precipitation (upwards of 60% per century) were observed in the dry areas in northeast Washington and south central British Columbia. Increases in precipitation were largest in spring, but were also large in summer in the central and Rocky Mountain climatic zones. These trends have already had profound impacts on streamflow and on certain plant species in the region (Cayan et al. 2001), and other important impacts remain to be discovered. The warming observed in winter and spring can be attributed partially to climatic variations over the Pacific Ocean, and the buildup of greenhouse gases probably also plays an important role.
ABSTRACT: Winter precipitation anomaly patterns between 1911 and 1977 over the southwestern United States are investigated by empirical orthogonal function (EOF) analysis. The first four EOFs explain 74% of the total variance in the original data assemblages. The first three were statistically significant at the 95% confidence level. The spatial distribution of the coefficients and the time plots of the amplitudes of the first two EOFs indicate that the first three decades of the study period, identified as encompassing the most anamolously warm decades in the western United States this century, was a period when negative precipitation anomalies predominated in the Great Basin and in the upper Colorado and Green River basins, but positive anomalies predominated in the lower Colorado and the Colorado Plateau region of Arizona. In the early 1950s not only was there a hiatus in winter precipitation distribution, but the earlier anomaly patterns reversed to a predominantly wet northern section and a predominantly drier south. This hiatus in anomaly patterns coincided with the change to increased meridionality in upper tropospheric circulation patterns. These results, when viewed in the light of those from Euler et al. (1979), Wigley, Jones and Kelly (1980), Williams (1980) and Jager and Kellogg (1983), support a strong case for an anomalously warm Southwest during strong hemispheric and hence Arctic warming and therefore predominance of the anomaly patterns exemplified by the first two EOFs. The third EOF depicts the influence of physiography and the nature and trajectory of precipitation-inducing systems in winter on the variance pattern. The spatial distributions of the EOFs raise questions about the inferential use of surrogate data for environmental reconstruction in the presence of anomaly patterns that lead to nonsynchroneity in the response of samples in time and space. Power spectra of the amplitudes of the first three EOFs show the biennial oscillation and oscillations with periodicities greater than 30 years to be significant at the 95% confidence level on a white noise continuum.
ABSTRACT: This paper reviews selected evidence of environmental changes in the central Canadian Rockies during the 20th century. The instrumental climate record shows that mean annual temperatures have risen ca. 1.4°C over the last 100 years but seasonal patterns of change are complex. The greatest increases have been in winter temperatures (3.2°C/century). Precipitation data, though limited, show variable patterns of change on decadel scales with generally higher levels of precipitation in the mid-20th century . The longest streamflow record shows considerable variability, with highest flows in the 1950s. A tree-ring-based temperature reconstruction indicates summer and spring temperatures in the last half of the 20th century are higher than any equivalent period over the last 900 years. Although no accurate regional estimates exist, glaciers have probably lost ca. 25% of their area in the last 100 years and may be smaller now than they have been at any time in the last 3000 yeas. These two lines of evidence suggest that the climate of the late 20th century is exceptional in the context of the last 1000 to 3000 years. Small but significant vegetation changes are taking place at the upper treeline ecotone in response to climate changes over the 20th century (e.g., seedling establishment). However, the most significant landscape changes in the last 100 years in the transformation of the character of the montane forest due to a reduction in forest fire frequency, largely due to an active policy of fire suppression.
ABSTRACT: The value of using climate indices such as ENSO or PDO in water resources predictions is dependent on understanding the local relationship between these indices and streamflow over time. This study identifies long term seasonal and spatial variations in the strength of El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) correlations with timing and magnitude of discharge in snowmelt streams in Oregon. ENSO is best correlated with variability in annual discharge, and PDO is best correlated with spring snowmelt timing and magnitude and timing of annual floods. Streams in the Cascades and Wallowa mountains show the strongest correlations, while the southernmost stream is not correlated with ENSO or PDO. ENSO correlations are weaker from 1920 to 1950 and vary significantly depending on whether Southern Oscillation Index (SOI) or Niño 3.4 is used. PDO correlations are strong from 1920 to 1950 and weak or insignificant other years. Although there are not consistent increasing or decreasing trends in annual discharge or spring snowmelt timing, there are significant increases in fractional winter runoff that are independent of precipitation, PDO, or ENSO and may indicate monotonic winter warming.
ABSTRACT: We present global fields of decadal annual surface temperature anomalies, referred to the period 1951–1980, for each decade from 1881–1890 to 1981–1990 and for 1984–1993. In addition, we show decadal calendar-seasonal anomaly fields for the warm decades 1936–1945 and 1981–1990. The fields are based on sea surface temperature (SST) and land surface air temperature data. The SSTs are corrected for the pre-World War II use of uninsulated sea temperature buckets and incorporate adjusted satellite-based SSTs from 1982 onward. Our results extend those published in the 1990 Intergovernmental Panel on Climate Change Scientific Assessment and its 1992 supplement. We assess the impact of various sources of error in the fields. Despite poor data coverage initially and around the two World Wars the generally cold end of the nineteenth century and start to the twentieth century are confirmed, together with the substantial warming between about 1920 and 1940. Slight cooling of the northern hemisphere took place between the 1950s and the mid-1970s, although slight warming continued south of the equator. Recent warmth has been most marked over the northern continents in winter and spring, but the 1980s were warm almost everywhere apart from Greenland, the northwestern Atlantic and the midlatitude North Pacific. Parts of the middle- to high-latitude southern ocean may also have been cool in the 1980s, but in this area the 1951–1980 climatology is unreliable.
The impact of the satellite data is reduced because the record of blended satellite and in situ SST is still too short to yield a climatology from which to calculate representative anomalies reflecting climatic change in the southern ocean. However, we propose a method of using existing satellite data in a step toward this target. The maps are condensed into global and hemispheric decadal surface temperature anomalies. We show the sensitivity of these estimated anomalies to alternative methods of compositing the spatially incomplete fields. Renning decadal zonal means and annual global and hemispheric time series are also shown. Finally, we discuss some salient features in terms of observed atmospheric circulation changes and of the results of climate model integrations with increasing atmospheric greenhouse gases.
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.
ABSTRACT: Winter and summer mass balance measurements from four French glaciers have been used to assess the sensitivity of mass balance to climatic fluctuations. The sensitivity of summer ablation to temperature is maximum in low-elevation zones (1.4 m water equivalent (w.e.) °C-1 at 1800 m above sea level (asl)) and decreases with altitude (0.5 m w.e. °C-1 at 2900 m asl). As a consequence, the sensitivity of equilibrium line altitude to temperature is 60–70 m °C-1 . This is half the value previously reported in the literature, implying that alpine glacier retreat scenarios for the 21st Century have been largely overestimated. Winter accumulation can be as high as 3 times the amount of precipitation recorded downvalley. These relationships between mass balance and meteorological data were then used to reconstruct the mass balances of these four glaciers back to 1907 using old maps and photogrammetric measurements. Model sensitivity analysis shows that a 25–30% increase in precipitation would compensate a 1°C temperature rise for the mass balances of glaciers. From these results the 20th Century may be divided into four periods: two steady state periods, 1907–1941 and 1954–1981, during which the mass of glaciers remained almost constant, and two deficit periods, 1942–1953 and 1982–1999, marked by a sharp reduction in glacier mass. Regarding mean ablation at 2800 m asl, a 22 W m-2 increase in energy balance is required to explain the ablation difference between the two most recent periods, 1954–1981 and 1982–1999. According to the energy balance analysis the increase in air temperature explains more than 60% of this ablation rise.
P.Y. Groisman, R.W. Knight, T. R. Karl (2001). Heavy precipitation and high streamflow in the contiguous United States: trends in the Twentieth Century. Bulletin of the American Meteorological Society 82 (2): 219-246
ABSTRACT: Changes in several components of the hydrological cycle over the contiguous United States have been documented during the twentieth century: an increase of precipitation, especially heavy and very heavy precipitation, and a significant retreat in spring snow cover extent over western regions during the last few decades.
These changes have affected streamflow, including the probability of high flow.
In the eastern half of the United States a significant relationship is found between the frequency of heavy precipitation and high streamflow events both annually and during the months of maximum streamflow. Two factors contributed to finding such a relation: 1) the relatively small contribution of snowmelt to heavy runoff in the eastern United States (compared to the west), and 2) the presence of a sufficiently dense network of streamflow and precipitation gauges available for analysis. An increase of spring heavy precipitation events over the eastern United States indicates with high probability that during the twentieth century an increase of high streamflow conditions has also occurred. In the West, a statistically significant reduction of snow cover extent has complicated the relation between heavy precipitation and streamflow. Increases in peak stream flow have not been observed here, despite increases in heavy precipitation events, and less extensive snow cover is the likely cause.
ABSTRACT: Trend analysis is used frequently in climate studies, but it is vulnerable to a number of conceptual shortcomings. This analysis of U.S. climate division data uses an alternate approach. The method used here subjects time series of annual average temperature and total precipitation to tests of Mann–Whitney U statistics over moving sampling windows of intra- to multidecadal (IMD) duration. In applying this method to time series of nationally averaged annual rainfall, a highly significant incidence of wet years is found after the early 1970s. When applied to individual climate divisions this test provides the basis for a climate survey method that is more robust than linear trend analysis, and capable of objectively isolating the timing and location of major IMD climate events over the United States. From this survey, four such periods emerge between 1932 and 1999: the droughts of the 1930s and 1950s, a cool 1964–79 period, and wet–warm time windows at the end of the century. More circumstantial consideration was also given here to the state of ENSO, the Pacific decadal oscillation (PDO), the winter state of the North Atlantic Oscillation, and mean annual Northern Hemisphere surface temperature during those periods. Anecdotal evidence presented here suggests that wet years associated with warm-phase ENSO conditions and the positive phase of the PDO may have played a role in ending the drought periods of the 1930s and 1950s. Conversely, the La Niña–like climate impacts found here during the late 1940s to mid-1950s, and the increased incidence of cold phase ENSO and negative phase PDO conditions during that time, suggests connections between that ocean state and severe drought. Significant late-century warmth was found mainly in the western United States after the mid-1980s, but no evidence of a cooling trend was evident in the southeast, as reported elsewhere. The late-century wet regime appears to have occurred in two phases, with wetness confined to the east during 1972–79, and more concentrated in the southwest and central United States during 1982–99.
ABSTRACT: Persistent, widespread wet conditions in the western United States in the early twentieth century have been noted in a number of studies. Here, we investigate the character of this pluvial, which covered a roughly 9-state region and lasted about 13 years. Paleoclimatic data used to evaluate the period in a long-term context indicate that the twentieth-century pluvial is an extremely rare event, as previous studies have suggested, even when assessed in the context of a 1186-year reconstruction of regional drought. An analysis of twentieth-century climate data, characterizing precipitation seasonality, intensity, and frequency, shows that the pluvial was primarily a result of winter season, heavy to moderately heavy precipitation events, during a handful of extremely wet winters. Temperatures were also anomalously cool. The combination of duration, intensity, and spatial extent make this an unusual event, not only in twentieth century, but in the past 12 centuries.
ABSTRACT: Intra- to multidecadal variation in annual streamflow, precipitation, and temperature over the continental United States are evaluated here through the calculation of Mann–Whitney U statistics over running-time windows of 6–30-yr duration. When this method is demonstrated on time series of nationally averaged annual precipitation and mean temperature during 1896–2001, it reveals that 8 of the 10 wettest years occurred during the last 29 yr of that 106-yr period, and 6 of the 10 warmest years during the last 16. Both of these results indicate highly significant departures from long-term stationarity in U.S. climate at the end of the twentieth century. The effects of increased wetness are primarily evident in the central and eastern United States, while evidence of warmth is found throughout the Rocky Mountain region and in the West. Analysis of annual streamflow records across the United States during 1939–98 shows broadly consistent effects. Initial evidence of the recent wet regime is most apparent in eastern streamflow, which shows a clear pattern of high-ranked mean annual values during the 1970s. Over the midwestern states, a coherent pattern of high-ranked annual flow is found during multidecadal periods beginning during the late 1960s and early 1970s and ending in either 1997 or 1998. During the late 1980s and early 1990s, a significant incidence of low-ranked annual flow conditions throughout the West was roughly coincident with the onset of western warmth during the mid-1980s. Evidence of highly significant transitions to wetter and warmer conditions nationally, and consistent variation in streamflow analyses, suggests that increased hydrological surplus in the central and eastern United States and increased hydrological deficit in the West may be representative of the initial stages of climate change over the continental United States.
ABSTRACT: The annual timing of river flows is a good indicator of climate-related changes, or lack of changes, for rivers with long-term data that drain unregulated basins with stable land use. Changes in the timing of annual winter/spring (January 1 to May 31) and fall (October 1 to December 31) center of volume dates were analyzed for 27 rural, unregulated river gaging stations in New England, USA with an average of 68 years of record. The center of volume date is the date by which half of the total volume of water for a given period of time flows past a river gaging station, and is a measure of the timing of the bulk of flow within the time period. Winter/spring center of volume (WSCV) dates have become significantly earlier (p<0.1) at all 11 river gaging stations in areas of New England where snowmelt runoff has the most effect on spring river flows. Most of this change has occurred in the last 30 years with dates advancing by 1–2 weeks. WSCV dates were correlated with March through April air temperatures (r=-0.72) and with January precipitation (r=-0.37). Three of 16 river gaging stations in the remainder of New England had significantly earlier WSCV dates. Four out of 27 river gaging stations had significantly earlier fall center of volume dates in New England. Changes in the timing of winter/spring and fall peak flow dates were consistent with the changes in the respective center of volume dates, given the greater variability in the peak flow dates. Changes in the WSCV dates over the last 30 years are consistent with previous studies of New England last-frost dates, lilac bloom dates, lake ice-out dates, and spring air temperatures. This suggests that these New England spring geophysical and biological changes all were caused by a common mechanism, temperature increases.
ABSTRACT: Spatial patterns in trends of four monthly variables: average temperature, precipitation, streamflow, and average of the daily temperature range were examined for the continental United States for the period 1948–88. The data used are a subset of the Historical Climatology Network (1036 stations) and a stream gage network of 1009 stations. Trend significance was determined using the nonparametric seasonal Kendall's test on a monthly and annual basis, and a robust slope estimator was used for determination of trend magnitudes. A bivariate test was used for evaluation of relative changes in the variables, specifically, streamflow relative to precipitation, streamflow relative to temperature, and precipitation relative to temperature.
Strong trends were found in all of the variables at many more stations than would be expected due to chance. There is a strong spatial and seasonal structure in the trend results. For instance, although annual temperature increases were found at many stations, mostly in the North and West, there were almost as many downtrends, especially in the South and East. Among the most important trend patterns are (a) increases in March temperature at almost half of the stations; (b) increases in precipitation from September through December at as many as 25 percent of the stations, mostly in the central part of the country; (c) strong increases in streamflow in the period November–April at a maximum of almost half of the stations, with the largest trend magnitudes in the north-central states; (d) changes in the temperature range (mostly downward) at a large number of stations beginning in late spring and continuing through winter, affecting as many as over half of the stations. The observed trends in streamflow are not entirely consistent with the changes in the climatic variables and may be due to a combination of climatic and water management effects.
ABSTRACT: Secular trends in streamflow are evaluated for 395 climate-sensitive streamgaging stations in the conterminous United States using the non-parametric Mann-Kendall test. Trends are calculated for selected quantiles of discharge, from the 0th to the 100th percentile, to evaluate differences between low-, medium-, and high-flow regimes during the twentieth century. Two general patterns emerge; trends are most prevalent in the annual minimum (Q0 ) to median (Q50 ) flow categories and least prevalent in the annual maximum (Q100 ) category; and, at all but the highest quantiles, streamflow has increased across broad sections of the United States. Decreases appear only in parts of the Pacific Northwest and the Southeast. Systematic patterns are less apparent in the Q100 flow. Hydrologically, these results indicate that the conterminous U.S. is getting wetter, but less extreme.
ABSTRACT: Long-term streamflow series in the western United States were examined for evidence of secular changes related to climate. Streamflow series contained appreciable low-frequency variation related to the combined influence of temperature and precipitation. Evidence of nonstationarity was found in selected records for the Pacific Northwest and the Upper Colorado Basins: mean annual streamflow increased significantly (0.05 level) from the first to last half of the 1914–80 period in the Pacific Northwest, and decreased significantly over the same period in the Upper Colorado region. Correlation analyses and examination of drought years revealed a strong tendency for anomalies of opposite sign in the Pacific Northwest and the Southwest. Drought in the Upper Colorado Basin was statistically independent of drought in the Pacific Northwest. Under exceptional meteorological conditions (e.g., water-year 1976–77), however, low flows occurred over a vast area from the Northwest coast to the mountains of central Arizona.
ABSTRACT: April–September streamflow volume data from 141 unregulated basins in the western United States were analyzed for trends in year-to-year variability and persistence. Decadal time-scale changes in streamflow variability and lag-1-yr autocorrelation (persistence) were observed. The significance of the variability trends was tested using a jackknife procedure involving the random resampling of seasonal flows from the historical record. The 1930s–50s was a period of low variability and high persistence, the 1950s–70s was a period of low variability and antipersistence, and the period after 1980 was highly variable and highly persistent. In particular, regions from California and Nevada to southern Idaho, Utah, and Colorado have recently experienced an unprecedented sequence of consecutive wet years along with multiyear extreme droughts.
ABSTRACT: Freshwater discharge to high-latitude oceans in 64 Canadian rivers is investigated. The mean annual discharge rate attains 1252 km3 yr-1 for an area of 5.6 × 106 km2 , equating to a sink of 225 mm yr-1 in the surface water budget of northern Canada (excluding the Arctic Archipelago where insufficient data exist). Application of the Mann-Kendall test to the data reveals a 10% decrease (-125 km3 yr-1 or -22 mm yr-1 ) in the total annual river discharge to the Arctic and North Atlantic Oceans from 1964 to 2003. This trend in river runoff is consistent with a 21 mm yr-1 decline in observed precipitation over northern Canada between 1964 and 2000. We find evidence of statistically-significant links between the Arctic Oscillation, El Niño/Southern Oscillation, and the Pacific Decadal Oscillation to the total annual freshwater discharge in northern Canada's rivers at interannual-to-decadal timescales.
ABSTRACT: Trends in flood and low flows in the US were evaluated using a regional average Kendall's S trend test at two spatial scales and over two timeframes. Field significance was assessed using a bootstrap methodology to account for the observed regional cross-correlation of streamflows. Using a 5% significance level, we found no evidence of trends in flood flows but did find evidence of upward trends in low flows at the larger scale in the Midwest and at the smaller scale in the Ohio, the north central and the upper Midwest regions. A dramatically different interpretation would have been achieved if regional cross-correlation had been ignored. In that case, statistically significant trends would have been found in all but two of the low flow analyses and in two-thirds of the flood flow analyses. We show that the cross-correlation of flow records dramatically reduces the effective number of samples available for trend assessment. We also found that low flow time series exhibit significant temporal persistence. Even when the serial correlation was removed from the time series, significant trends in low flow series were apparent, though the number of significant trends decreased.
ABSTRACT: We used a simulated data set of hydro-climatological variables to examine for 20th century trends in soil moisture, runoff, and drought characteristics over the conterminous United States (U.S.). An increasing trend is apparent in both model soil moisture and runoff over much of the U.S., with a few decreasing trends in parts of the Southwest. The trend patterns were qualitatively similar to those found in streamflow records observed at a station network minimally affected by anthropogenic activities. This wetting trend is consistent with the general increase in precipitation in the latter half of the 20th century. Droughts have, for the most part, become shorter, less frequent, and cover a smaller portion of the country over the last century. The main exception is the Southwest and parts of the interior of the West, where, notwithstanding increased precipitation (and in some cases increased soil moisture and runoff), increased temperature has led to trends in drought characteristics that are mostly opposite to those for the rest of the country especially in the case of drought duration and severity, which have increased.
A. F. Hamlet, P. W. Mote, M. P. Clark, D. P. Lettenmaier (2007). Twentieth-century trends in runoff, evapotranspiration, and soil moisture in the western United States. Journal of Climate 20 (8): 1468-1486
ABSTRACT: A physically based hydrology model is used to produce time series for the period 1916–2003 of evapotranspiration (ET), runoff, and soil moisture (SM) over the western United States from which long-term trends are evaluated. The results show that trends in ET in spring and summer are determined primarily by trends in precipitation and snowmelt that determine water availability. From April to June, ET trends are mostly positive due primarily to earlier snowmelt and earlier emergence of snow-free ground, and secondarily to increasing trends in spring precipitation. From July to September trends in ET are more strongly influenced by precipitation trends, with the exception of areas (most notably California) that receive little summer precipitation and have experienced large changes in snowmelt timing. Trends in the seasonal timing of ET are modest, but during the period 1947–2003 when temperature trends are large, they reflect a shift of ET from midsummer to early summer and late spring. As in other studies, it is found that runoff is occurring earlier in spring, a trend that is related primarily to increasing temperature, and is most apparent during 1947–2003. Trends in the annual runoff ratio, a variable critical to western water management, are determined primarily by trends in cool season precipitation, rather than changes in the timing of runoff or ET. It was found that the signature of temperature-related trends in runoff and SM is strongly keyed to mean midwinter [December–February (DJF)] temperatures. Areas with warmer winter temperatures show increasing trends in the runoff fraction as early as February, and colder areas as late as June. Trends toward earlier spring SM recharge are apparent and increasing trends in SM on 1 April are evident over much of the region. The 1 July SM trends are less affected by snowmelt changes and are controlled more by precipitation trends.
C. I. Millar, R. D. Westfall, D. L. Delany, J. C. King, L.J. Graumlich (2004). Response of subalpine conifers in the Sierra Nevada, California, U.S.A., to 20th-century warming and decadal climate variability. Arctic, Antarctic, and Alpine Research 36 (2): 181-200
ABSTRACT: Four independent studies of conifer growth between 1880 and 2002 in upper elevation forests of the central Sierra Nevada, California, U.S.A., showed correlated multidecadal and century-long responses associated with climate. Using tree-ring and ecological plot analysis, we studied annual branch growth of krummholzPinus albicaulis ; invasion byP. albicaulis andPinus monticola into formerly persistent snowfields; dates of vertical branch emergence in krummholzP. albicaulis ; and invasion byPinus contorta into subalpine meadows. Mean annual branch growth at six treeline sites increased significantly over the 20th century (range 130–400%), with significant accelerations in rate from 1920 to 1945 and after 1980. Growth stabilized from 1945 to 1980. Similarly, invasion of six snowfield slopes began in the early 1900s and continued into snowfield centers throughout the 20th century, with significantly accelerated mean invasion from 1925 to 1940 and after 1980. Rate of snowfield invasion decreased between 1950 and 1975. Meadow invasion and vertical leader emergence showed synchronous, episodic responses.Pinus contorta invaded each of ten subalpine meadows in a distinct multidecadal pulse between 1945 and 1976 (87% of all trees) and vertical release in five krummholzP. albicaulis sites also occurred in one pulse between 1945 and 1976 (86% of all branches). These synchronies and lack of effect of local environments implicate regional climate control. Composite weather records indicated significant century-long increases in minimum monthly temperature and multidecadal variability in minimum temperature and precipitation. All ecological responses were significantly correlated with minimum temperature. Significant interactions among temperature, precipitation, Pacific Decadal Oscillation (PDO) indices, and multiyear variability in moisture availability further explained episodic ecological responses. Four multidecadal periods of the 20th century that are defined by ecological response (<1925; 1925–1944; 1945–1976; >1976) correlate with positive and negative PDO phases, as well as with steps in the rate of temperature increase. These diverse factors in spatially distributed upper-montane and treeline ecosystems respond directionally to century-long climate trends, and also exhibit abrupt and reversible effects as a consequence of interdecadal climate variability and complex interactions of temperature and moisture.
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.
Balling, R.C., Jr., S.G. Wells (1990). Historic rainfall patterns and arroyo activity within the Zuni River drainage basin, New Mexico. Annals of the Association of American Geographers 80 (4): 603-617
ABSTRACT: Climate change, grazing practices, timbering activities, and erosional thresholds have been proposed to explain wide-spread accelerated arroyo erosion near the turn of the century in the southwestern United States. We analyze the morphology and potential causes of arroyo activity in the Zuni River drainage basin of New Mexico; our analyses illustrate the linkage between arroyos and changes that occurred through time in local precipitation patterns. Substantial archival and geological evidence confirms dominant downcutting in the intermediate and small-size arroyos within the basin for a 20- to 30-year period beginning near 1905. Historical climatic records reveal a long and severe drought from 1898-1904; this drought ended abruptly with three years dominated by unusually frequent high-intensity summer rainfall events. Daily weather records show the following two to three decades had a high number of intense summer storms, large rainfall totals, and few precipitation days. The results add support for the climate change explanation of periods dominated by arroyo incision and infilling in the southwestern U.S.
Hereford, R. (1984). Climate and ephemeral stream processes: Twentieth century geomorphology and alluvial stratigraphy of the Little Colorado River, Arizona. Geological Society of America Bulletin 95 (6): 654-668
ABSTRACT: During the first 40 years of the twentieth century, erosion was the dominant geomorphic process affecting the morphology of the Little Colorado River channel. The discharge regimen was one of frequent large floods and high annual discharge that created a wide sandy channel free of vegetation. In the 1940s and early 1950s, average annual precipitation declined, reducing annual discharge to about 57% of that of the preceding period as well as reducing the frequency of large floods. The channel adjusted to the new hydrologic regimen by reducing its width. Parts of the channel were frequently dry, and riparian vegetation, primarily nonnative salt cedar, became established on the higher channel surfaces. Precipitation and discharge thereafter increased and aggradation by overbank deposition was the primary geomorphic process, as indicated by accretion of 2 to 5 m of flood-plain alluvium between 1952 and 1978. Events of 1980, however, suggest that the flood plain has ceased to accrete, although climate has not fluctuated. The flood plain has probably reached a critical height above the channel, beyond which further accretion is unlikely under the existing discharge regimen. The recent history of the Little Colorado broadly suggests that flood-plain development was initiated by climatically induced hydrologic fluctuations. Flood-plain deposits in the stratigraphic column of such ephemeral streams may record repeated adjustments to altered hydrologic conditions.
ABSTRACT: River systems in semiarid regions are susceptible to rapid and dramatic channel erosion and arroyo formation. Climate plays an important role in arroyo development through changes in precipitation intensity, seasonality, and variability. Here, trends in precipitation and streamflow at the annual, monthly, and daily timescales for the last 50 yr are analyzed for the Rio Puerco Basin in northwestern New Mexico, and connections with recent watershed and channel changes are examined. The increasing trend in annual precipitation in the basin is shown to be part of larger-scale climatic variability that affects the U.S. Southwest region, which is associated with climatic anomalies in the northern Pacific. Results of hydroclimatic data analyses point to a general increase in wetness in nonsummer months—an increase in the number of rainy days and in the frequency of flow days in the stream system is observed. There are substantial shifts in the distributions of both daily precipitation and streamflow. Rainfall with moderate intensity has been increasing, while the intensity of annual maximum rainfall events has remained largely unaffected. At the same time, the number of annual maximum runoff events in the basin has been steadily decreasing in the studied period. It is argued that recent watershed and arroyo changes that affect the rainfall–runoff relationship in the basin may be responsible for the decreasing trend in maximum runoff events. Field evidence of such changes in the Rio Puerco watershed and fluvial system is discussed.
ABSTRACT: Alpine glacier retreat resulting from global warming since the close of the Little Ice Age in the 19th and 20th centuries has increased the risk and incidence of some geologic and hydrologic hazards in mountainous alpine regions of North America. Abundant loose debris in recently deglaciated areas at the toe of alpine glaciers provides a ready source of sediment during rainstorms or outburst floods. This sediment can cause debris flows and sedimentation problems in downstream areas. Moraines built during the Little Ice Age can trap and store large volumes of water. These natural dams have no controlled outlets and can fail without warning. Many glacier-dammed lakes have grown in size, while ice dams have shrunk, resulting in greater risks of ice-dam failure. The retreat and thinning of glacier ice has left oversteepened, unstable valley walls and has led to increased incidence of rock and debris avalanches.
Menounos, B., Clague, J.J., Gilbert, R., Slaymaker, O. (2005). Environmental reconstruction from a varve network in the southern Coast Mountains, British Columbia, Canada. The Holocene 15 (8): 1163-1171
ABSTRACT: Cores of annually laminated sediments (varves) from five lakes in the southern Coast Mountains of British Columbia, Canada, document clastic sediment response to climate and geomorphic change over the past 120 years. Interannual varve thickness correlates with annual flood magnitude. Interdecadal trends in varve thickness are influenced by other environmental factors such as glacier recession. Despite major differences in the lakes and their contributing watersheds, substantial concordance is observed among the records. A pronounced change in the nature of lake sedimentation, accompanied by higher interannual variability, occurred in 1980. The change coincides with an increase in the magnitude of autumn flooding and major re-organization of the North Pacific climate system. These results highlight new directions for palaeoenvironmental research using varved sediment records, specifically to study the magnitude and spatial extent of past hydro-climatic events.
ABSTRACT: Snow course and SNOTEL measurements of spring snowpack, corroborated by a physically-based hydrologic model, are examined here for climate-driven fluctuations and trends during the period 1916-2002. Much of the mountain West has experienced declines in spring snowpack, especially since mid-century, and despite increases in winter precipitation in many places. Analysis and modeling shows that climatic trends are the dominant factor, not changes in land use, forest canopy, or other factors. The largest decreases have occurred where winter temperatures are mild, especially in the Cascade Mountains and Northern California. In most mountain ranges, relative declines grow from minimal at ridgetop to substantial at snowline. Taken together, these results emphasize that although the Pacific Decadal Oscillation has played some role in fluctuations in the region’s SWE, the West’s snow resources are already declining as Earth’s climate warms.
ABSTRACT: This paper describes the results of an analysis of trends in short duration (1–7 days) extreme precipitation events that have a recurrence interval of 1 yr or longer for stations in the United States and Canada. This definition of extreme precipitation was chosen because such events are highly correlated with hydrologic flooding in some U.S. regions. The dominant temporal characteristic of a national event composite index is significant low-frequency variability. There were lengthy periods of a below-average number of events in the 1930s and 1950s and an above-average number of events in the early 1940s, early 1980s, and 1990s. Regional variations often differ substantially from the national composite. A simple linear analysis indicates that the overall trend covering the period 1931–96 has been upward at a highly statistically significant rate over the southwest United States and in a broad region from the central Great Plains across the middle Mississippi River and southern Great Lakes basins. The national trend for the United States is upward at a rate of 3% decade-1 for the period 1931–96. While the annual trend for Canada is upward for the period 1951–93, it is not statistically significant. Although the high statistical significance of the results is partially a consequence of the low frequency during the 1930s and 1950s located in the first half of the record, the latter half of the record exhibits an upward trend nearly identical to the entire record. However, an analysis of a 101-yr record of midwestern stations shows that heavy precipitation event frequencies around the turn of the twentieth century (1896–1906) were higher than for other periods of comparable length, except for 1986–96. Although data were not available in digital form to extend the analysis back to 1896 for the entire United States, the midwestern analysis shows that interpretation of the recent upward trends must account for the possibility of significant natural forcing of variability on century timescales.
ABSTRACT: The sensitivity of streamflow to climate change was investigated in the American, Carson, and Truckee River Basins, California and Nevada. Nine gaging stations were used to represent streamflow in the basins. Annual models were developed by regressing 1961-1991 streamflow data on temperature and precipitation. Climate-change scenarios were used as inputs to the models to determine streamflow sensitivities. Climate-change scenarios were generated from historical time series by modifying mean temperatures by a range of +4°C to -4°C and total precipitation by a range of +25 percent to -25 percent. Results show that streamflow on the warmer, lower west side of the Sierra Nevada generally is more sensitive to temperature and precipitation changes than is streamflow on the colder, higher east side. A 2°C rise in temperature and a 25-percent decrease in precipitation results in streamflow decreases of 56 percent on the American River and 25 percent on the Carson River. A 2°C decline in temperature and a 25-percent increase in precipitation results in streamflow increases of 102 percent on the American River and 22 percent on the Carson River.
ABSTRACT: The biases and large-scale inhomogeneities in the time series of measured precipitation and snowfall over the United States and Canada are discussed and analyzed. The spatial statistical characteristics of monthly and annual snowfall and total precipitation are investigated and parameterized. After adjustments and selection of the “best” network, reliable “first guess” estimates of North American snowfall and precipitation are obtained. Century-long time series of unbiased annual precipitation over the regions to the south of 55°N and 40-year time series of unbiased area-averaged annual precipitation and snowfall for all of North America are developed. The analysis of their trends shows the following.
1) During the last 100 years, annual precipitation has increased in southern Canada (south of 55°N) by 13% and in the contiguous United States by 4%; however, the main domain of this century-scale precipitation increase is eastern Canada and adjacent to it northern regions of the United States.
2) Up to a 20% increase has occurred in annual snowfall and rainfall during the last four decades in Canada north of 55°N.
The relationships between century-long precipitation time series over North America with Northern Hemisphere surface air temperature and the South Oscillation index (SOI) are investigated. It is shown that ENSO (negative anomaly of SOI) is usually accompanied by an increase of precipitation whenever it affects the United States (especially in the southwestern region of the country).
ABSTRACT: The map patterns of streamflow conditions in the U.S.A. and southern Canada are examined by means of a principal components analysis of the monthly flow records of 102 streams. The analysis reveals the basic anomaly patterns of streamflow, and also describes the variation of these patterns through time. The basic anomaly patterns are of large spatial scale, and vary slowly through time, reflecting the temporal and spatial scale of the controlling climatic anomalies. An extension of the analysis allows the gaging stations to be assigned to homogeneous hydrologic regions.
ABSTRACT: Annual minimum, median, and maximum daily streamflow for 400 sites in the conterminous United States (U.S.), measured during 1941–1999, were examined to identify the temporal and spatial character of changes in streamflow statistics. Results indicate a noticeable increase in annual minimum and median daily streamflow around 1970, and a less significant mixed pattern of increases and decreases in annual maximum daily streamflow. These changes in annual streamflow statistics primarily occurred in the eastern U.S. In addition, the streamflow increases appear as a step change rather than as a gradual trend and coincide with an increase in precipitation.
J. N. Moore, J. T. Harper, M. C. Greenwood (2007). Significance of trends toward earlier snowmelt runoff, Columbia and Missouri Basin headwaters, western United States. Geophysical Research Letters 34 (L16402)
ABSTRACT: We assess changes in runoff timing over the last 55 years at 21 gages unaffected by human influences, in the headwaters of the Columbia-Missouri Rivers. Linear regression models and tests for significance that control for “false discoveries” of many tests, combined with a conceptual runoff response model, were used to examine the detailed structure of spring runoff timing. We conclude that only about one third of the gages exhibit significant trends with time but over half of the gages tested show significant relationships with discharge. Therefore, runoff timing is more significantly correlated with annual discharge than with time. This result differs from previous studies of runoff in the western USA that equate linear time trends to a response to global warming. Our results imply that predicting future snowmelt runoff in the northern Rockies will require linking climate mechanisms controlling precipitation, rather than projecting response to simple linear increases in temperature.
ABSTRACT: This study presents trends computed for the past 30-50 years for 11 hydroclimatic variables obtained from the recently created Canadian Reference Hydrometric Basin Network database. It was found that annual mean streamflow has generally decreased during the periods, with significant decreases detected in the southern part of the country. Monthly mean streamflow for most months also decreased, with the greatest decreases occurring in August and September. The exceptions are March and April, when significant increases in streamflow were observed. Significant increases were identified in lower percentiles of the daily streamflow frequency distribution over northern British Columbia and the Yukon Territory. In southern Canada, significant decreases were observed in all percentiles of the daily streamflow distribution. Breakup of river ice and the ensuing spring freshet occur significantly earlier, especially in British Columbia. There is also evidence to suggest earlier freeze-up of rivers, particularly in eastern Canada. The trends observed in hydroclimatic variables are entirely consistent with those identified in climatic variables in other Canadian studies.
ABSTRACT: Analyses of streamflow, snow mass temperature, and precipitation in snowmelt-dominated river basins in the western United States indicate an advance in the timing of peak spring season flows over the past 50 years. Warm temperature spells in spring have occurred much earlier in recent years, which explains in part the trend in the timing of the spring peak flow. In addition, a decrease in snow water equivalent and a general increase in winter precipitation are evident for many stations in the western United States. It appears that in recent decades more of the precipitation is coming as rain rather than snow. The trends are strongest at lower elevations and in the Pacific Northwest region, where winter temperatures are closer to the melting point; it appears that in this region in particular, modest shifts in temperature are capable of forcing large shifts in basin hydrologic response. It is speculated that these trends could be potentially a manifestation of the general global warming trend in recent decades and also due to enhanced ENSO activity. The observed trends in hydroclimatology over the western United States can have significant impacts on water resources planning and management.
ABSTRACT: Principal component analysis reveals that Pacific salmon catches in Alaska have varied inversely with catches from the U.S. West Coast during the past 70 years. If variations in catch reflect variations in salmon production, then results of our analysis suggest that the spatial and temporal characteristics of this “inverse” catch/production pattern are related to climate forcing associated with the Pacific Decadal Oscillation, a recurring pattern of pan-Pacific atmosphere-ocean variability. Temporally, both the physical and biological variability are best characterized as alternating 20-to 30-year-long regimes punctuated by abrupt reversals. From 1977 to the early 1990s, ocean conditions have generally favored Alaska stocks and disfavored West Coast stocks. Unfavorable ocean conditions are likely confounding recent management efforts focused on increasing West Coast Pacific salmon production. Recovery of at-risk (threatened and endangered) stocks may await the next reversal of the Pacific Decadal Oscillation. Managers should continue to limit harvests, improve hatchery practices, and restore freshwater and estuarine habitats to protect these populations during periods of poor ocean productivity.
ABSTRACT: Successive surveys of the cross section of ephemeral channels in New Mexico over a period of 15 years 1960–1975, show that arroyos that were actively eroding early in the century have reversed the trend and are alluviating. This appears to be associated with the worldwide cooling trend that began about 1940. If the inference proves to be correct, it is significant hydrologically, for it provides some specific knowledge of the climatic conditions associated with the alternate periods of valley erosion and valley alluviation that created the widespread terraces of the semiarid West.
ABSTRACT: Climatic warming during the last 100-150 years has resulted in a significant glacier ice loss from mountainous areas of the world. Certain natural processes which pose hazards to people and development in these areas have accelerated as a result of this recent deglaciation. These include glacier avalanches, landslides and slope instability caused by glacier debuttressing, and outburst floods from moraine- and glacier-dammed lakes. In addition, changes in sediment and water supply induced by climatic warming and glacier retreat have altered channel and floodplain patterns of rivers draining high mountain ranges.
The perturbation of natural processes operating in mountain environments, caused by recent climatic warming, ranges from tens of decades for moraine-dam failures to hundreds of years or more for landslides. The recognition that climatic change as modest as that of the last century can perturb natural alpine processes has important implications for hazard assessment and future development in mountains. Even so, these effects are probably at least an order of magnitude smaller than those associated with late Pleistocene deglaciation ca. 15,000 to 10,000 years ago.