FIGURE 1: DECADAL MEDIAN PDSI AND PHDI IN ARIZONA: 1908-2018

FIGURE 2: DECADAL MEDIAN PDSI AND PHDI IN CALIFORNIA: 1908-2018

FIGURE 3: DECADAL MEDIAN PDSI AND PHDI IN COLORADO: 1908-2018

FIGURE 4: DECADAL MEDIAN PDSI AND PHDI IN IDAHO: 1908-2018

FIGURE 5: DECADAL MEDIAN PDSI AND PHDI IN MONTANA: 1908-2018

FIGURE 6: DECADAL MEDIAN PDSI AND PHDI IN NEVADA: 1908-2018

FIGURE 7: DECADAL MEDIAN PDSI AND PHDI IN NEW MEXICO: 1908-2018

FIGURE 8: DECADAL MEDIAN PDSI AND PHDI IN OREGON: 1908-2018

FIGURE 9: DECADAL MEDIAN PDSI AND PHDI IN TEXAS: 1908-2018

FIGURE 10: DECADAL MEDIAN PDSI AND PHDI IN UTAH: 1908-2018

FIGURE 11: DECADAL MEDIAN PDSI AND PHDI IN WYOMING: 1908-2018

FIGURE 12: MODERATE DROUGHT EVENTS PDSI / PHDI < -2

FIGURE 13: SEVERE DROUGHT EVENTS PDSI / PHDI < -3

FIGURE 14: LONG TERM TRENDS IN SEVERE DROUGHT EVENTS PDSI / PHDI < -3

FIGURE 15: THE TEXAS SHARPSHOOTER FALLACY

[LIST OF POSTS ON THIS SITE]

1. The context for this study is the prediction by climate models that anthropogenic global warming due to fossil fuel emissions will cause widespread drought conditions in the Southwest of the United States that is expected to include a gradual decline in the flow of the Colorado River, an important source of water in the region. This study is a presentation of the data for two Drought Indexes, the Palmer Drought Severity Index (PDSI) and the Palmer hydrological drought index (PHDI) for eleven states in West and Southwest of the United States where climate models have predicted drought and wildfire effects of fossil fuel emissions.
2. These data are available for the long time period from 1895 to 2018 and are provided by the National Climate Data Center (NCDC) of the National Oceanic and Atmospheric Agency (NOAA) and made available at the [/pub/data/cirs/climdiv] data archive as mean values for each state of the United States. Statewide mean values are derived from area-weighted averages of 5km by 5km grid-point estimates interpolated from station data. The drought indexes are combinations of temperature, precipitation, and humidity data that are thought to discriminate drought conditions from non-drought conditions. The data are provided as monthly mean values and are studied separately for each calendar month as there significant differences among the calendar months in the behavior of drought indexes. The Standardized Precipitation Index (SPI) is a new index thought to be better than the Palmer indexes (see Bibliography below) but is not included in this study.
3. The Palmer Drought Severity Index (PDSI) indicates the severity
   of a wet and dry spells. It is based on the principles of a balance between moisture supply and moisture demand. The index values usually range from -6 to +6, with negative values denoting dry spells and positive values indicating wet spells. The index is interpreted as follows: values in the range PDSI=[0 to -.5] indicate normal conditions with PDSI < –0.5 indicating various degrees of drought conditions described as Incipient Drought [-1.0 < PDSI < -0.5], Mild Drought [-2 < PDSI < -1], Moderate Drought [-3 < PDSI < -2], and Severe Drought [PDSI < -3]. The additional category of “extreme drought” for PDSI < -4 and the interpretation of positive values of the index in terms of wetness are not considered in this study. The Palmer Hydrological Drought Index (PHDI) is similar but with an emphasis on long-term moisture supply. The index values have the same interpretation as the PDSI. Both PDSI and PHDI values are reported in this study and their behavior is found to be similar for the states studied.
4. Eleven states of the United States are selected for study. They are, alphabetically, Arizona (AZ), California (CA), Colorado (CO), Idaho (ID), Oregon (OR), Montana (MT), Nevada (NV), New Mexico (NM), Texas (TX), Utah (UT), and Wyoming (WY). Some of these states are known to be drought prone (AZ, CA, NV, NM, TX, WY) while others are not (ID, OR) and the comparison of their drought indexes serve as the ability of the index to discriminate between them.
5. Figure 1 to Figure 11 present the PDSI and PHDI data for the eleven states in two side by side frames with PDSI in the left frame and PHDI in the right frame. Each figure is a GIF animation that cycles through the twelve calendar months presenting the data for one calendar month at a time from January to December. The range of values displayed are limited to PDSI/PHDI < +2 since the focus of the study is on detecting drought conditions that lie in the region of negative values. Three different drought categories are demarcated by color coded horizontal lines. The YELLOW line drawn at [PDSI/PHDI = -1] demarcates drought conditions below it from non-drought conditions above. The ORANGE line drawn at [PDSI/PHDI = -2] demarcates moderate drought conditions below it from mild drought; and finally the RED LINE drawn at [PDSI/PHDI = -3] demarcates severe drought below it from moderate drought. The “extreme drought” condition found in the PDSI literature is not included in this work. Drought periods are seen mostly in the summer months in the 1930s, again in the 1950s, and in the most recent period since 2003 for many of the states. The data shown are not the source data but their moving 10-year medians. Decadal smoothing was made necessary here by the extreme volatility of the source data and because of some extremely high positive values, the median rather than simple average was used.
6. Figures 12&13 summarize the drought patterns observed in the GIF animations of Figures 1 to 11. Figure 12 is a tabulation of the number of moderate drought or more severe (PDSI/PHDI<-2) events for each state and each calendar month using the PDSI as the criterion in the top panel and the PHDI in the bottom panel. In the tabulation each row represents one of the eleven states and each column represents one of twelve calendar months. The numbers in the tabulation are counts of the number of moderate or more extreme drought events for each state in each calendar month. At the end of each row and at the bottom of each column, these counts are summed to yield the total number of such drought events for each state and for each calendar month. The sum of these totals is the grand total of all such drought events for all eleven states in all twelve calendar months. The total for each calendar month and for each state are converted into a percentage with division by the grand total. These percentages are then used to discriminate among the calendar months and among the states in terms of relative drought frequency. This procedure is carried out for both the PDSI and the PHDI.
7. The procedure is repeated exactly in Figure 13 where the number of severe or greater drought events (PDSI/PHDI<-3) for each state and each calendar month are tabulated. A comparison of the Figure 12 percentages and Figure 13 percentages  shows that the severe drought category has  greater discriminating power in terms of a clear distinction between summer months (when droughts are more common) and non-summer months and between drought prone states and non-drought-prone states. Using this criterion, it is found that Figure 13 shows a greater discriminating power of the Severe Drought category for both PDSI and PHDI measures of drought severity. Here the summer months are clearly demarcated as are the differences between drought prone states and non-drought prone states.
8. A curious result in Figure 13 is that the state of California, though thought to be drought prone and currently (as of this writing in November 2018) considered a casualty of climate change in terms of drought driven wildfires, neither the  PDSI nor PHDI tabulation in Figure 13 places California in the same category as the four clearly drought prone states indicated by the percent scores and marked in red.
9. Figure 14 displays the timeline of these drought events as the total number of severe or greater drought events in distinct 5-year windows that moves 5-years at a time across the time span of the data from 1908 to 2018. The counts in the left panel use the PDSI as the criterion while the right panel shows the counts for the PHDI criterion. Here we find little evidence of drought until the 1930s where it rises to a peak of 25 PDSI drought events and 40 PHDI drought events per 5-year period by 1943 and then drops back to the zero line by the end of the 1940s. It then rises precipitously to more than 52 PDSI events and 55 PHDI events in the year 1958 before falling back to zero in 1968. Thereafter, no drought event is found in either index for 30 years from 1968 to 1998.
10. At the end of this drought hiatus, a steep rise is seen from 2003 to 2008, with 83 PDSI events and 95 PHDI events. It is this sharp increase in drought events in the West and Southwest USA that led their attribution to global warming in terms of climate model simulations that confirmed a devastating drought effect of global warming in terms of drying soils and heat waves (see bibliography below). The idea of the so called “desertification of the Southwest” due to human caused global warming entered into the climate change lexicon after these events and that helped to cultivate a sequence of climate model simulation papers that confirmed the attribution (see bibliography below).
11. Although a linear regression line through the curves in Figure 14 will show a rising trend, the actual pattern seen does not support that view. Rather, the the attribution of these events to human cause and the climate model predictions of drought in the South therefrom, appear to fit a pattern of circular reasoning and the so called “Texas sharpshooter fallacy” possibly by virtue of a strong confirmation bias in climate science that has shaped prior research in the so called “Event Attribution Science”. A related post at this site on confirmation bias is relevant in this regard  [LINK].

BIBLIOGRAPHY 

Featured Authors: Thomas Swetnam, Ed Fredirickson, Philip Dennison, Virginia Dale, Anthony Westerling, & Richard Seager

1. 1990: Swetnam, Thomas W., and Julio L. Betancourt. “Fire-southern oscillation relations in the southwestern United States.” Science 249.4972 (1990): 1017-1020. Fire scar and tree growth chronologies (1700 to 1905) and fire statistics (since 1905) from Arizona and New Mexico show that small areas burn after wet springs associated with the low phase of the Southern Oscillation (SO), whereas large areas burn after dry springs associated with the high phase of the SO. Through its synergistic influence on spring weather and fuel conditions, climatic variability in the tropical Pacific significantly influences vegetation dynamics in the southwestern United States. Synchrony of fire-free and severe fire years across diverse southwestern forests implies that climate forces fire regimes on a subcontinental scale; it also underscores the importance of exogenous factors in ecosystem dynamics.
2. 1996: Hayes, Michael J., et al. “Monitoring the 1996 drought using the standardized precipitation index.” Bulletin of the American meteorological society 80.3 (1999): 429-438. Droughts are difficult to detect and monitor. Drought indices, most commonly the Palmer Drought Severity Index (PDSI), have been used with limited success as operational drought monitoring tools and triggers for policy responses. Recently, a new index, the Standardized Precipitation Index (SPI), was developed to improve drought detection and monitoring capabilities. The SPI has several characteristics that are an improvement over previous indices, including its simplicity and temporal flexibility, that allow its application for water resources on all timescales. In this article, the 1996 drought in the southern plains and southwestern United States is examined using the SPI. A series of maps are used to illustrate how the SPI would have assisted in being able to detect the onset of the drought and monitor its progression. A case study investigating the drought in greater detail for Texas is also given. The SPI demonstrated that it is a tool that should be used operationally as part of a state, regional, or national drought watch system in the United States. During the 1996 drought, the SPI detected the onset of the drought at least 1 month in advance of the PDSI. This timeliness will be invaluable for improving mitigation and response actions of state and federal government to drought-affected regions in the future.
3. 1997: Edwards, Daniel C. Characteristics of 20th century drought in the United States at multiple time scales. No. AFIT-97-051. AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH, 1997. The purpose of this study is to define the occurrence and variability of drought in the United States in order to furnish climatologists and drought mitigation planners with information on how to put current drought into historical perspective. The opposite of drought is a period of anomalously wet conditions. Analyses of both drought and wet periods on national and regional scales are provided. Analysis of drought and wet periods in terms of areal coverage, intensity, duration, and variability at these different space and time scales provides valuable insight not only into the historical perspective of anomalously dry and wet conditions, but also into the long-term variation of climate in the United States.
4. 1998: Guttman, Nathaniel B. “Comparing the palmer drought index and the standardized precipitation index 1.” JAWRA Journal of the American Water Resources Association 34.1 (1998): 113-121. The Palmer Drought Index (PDI) is used as an indicator of drought severity, and a particular index value is often the signal to begin or discontinue elements of a drought contingency plan. The Standardized Precipitation Index (SPI) was recently developed to quantify a precipitation deficit for different time scales. It was designed to be an indicator of drought that recognizes the importance of time scales in the analysis of water availability and water use. This study compares historical time series of the PDI with time series of the corresponding SPI through spectral analysis. Results show that the spectral characteristics of the PDI vary from site to site throughout the U.S., while those of the SPI do not vary from site to site. They also show that the PDI has a complex structure with an exceptionally long memory, while the SPI is an easily interpreted, simple moving average process.
5. 1998: Fredrickson, Ed, et al. “Perspectives on desertification: south-western United States.” Journal of Arid Environments 39.2 (1998): 191-207. Several climatic changes occurred in the northern Chihuahuan Desert and other parts of the south-west United States during the last 12,000 years leading to a markedly warmer and drier climate. Vegetation changed in response to this climatic shift. Generally, this transition was from coniferous woodland to grasslands and eventually to the present day desert scrub. PreColumbian inhabitants of this region adapted by changing from hunter-gatherer to primarily agrarian economics. European immigration into the south-west U.S. beginning in the mid 1500s greatly affected this region. The greatest impact occurred after the U.S. Civil War in the 1860s. Before that time land use tended to be localized near small agricultural areas, mines, and military installations. The post-war range livestock industry expanded dramatically, especially during the 1880s — a period of general abuse of arid lands in the region. Recognition of this abuse and the deteriorating productivity of the land led to greater government involvement, including establishment of experimental stations and eventually management of the public domain by governmental agencies. Fire suppression, mismanaged grazing, changing climatic conditions, loss of soil and increasing atmospheric CO2 concentrations, mainly due to the burning of fossil fuels, are among the probable causes of continued desertification trends. Urban and rural populations, presently technologically isolated from their environment, need to better understand the dynamic nature of their environment. A greater degree of co-operation among diverse entities will be crucial. [FULL TEXT]
6. 1999: Guttman, Nathaniel B. “Accepting the standardized precipitation index: A calculation algorithm1.” JAWRA Journal of the American Water Resources Association 35.2 (1999): 311-322. The Palmer Drought Severity Index (PDSI) has been calculated for about 30 years as a means of providing a single measure of meteorological drought severity. It was intended to retrospectively look at wet and dry conditions using water balance techniques. The Standardized Precipitation Index (SPI) is a probability index that was developed to give a better representation of abnormal wetness and dryness than the Palmer indices. Before the user community will accept the SPI as an alternative to the Palmer indices, a standard method must be developed for computing the index. Standardization is necessary so that all users of the index will have a common basis for both spatial and temporal comparison of index values. If different probability distributions and models are used to describe an observed series of precipitation, then different SPI values may be obtained. This article describes the effect on the SPI values computed from different probability models as well as the effects on dry event characteristics. It is concluded that the Pearson Type III distribution is the “best” universal model, and that the reliability of the SPI is sample size dependent. It is also concluded that because of data limitations, SPIs with time scales longer than 24 months may be unreliable. An internet link is provided that will allow users to access Fortran 77 source code for calculating the SPI.
7. 2001: Dale, Virginia H., et al. “Climate change and forest disturbances: climate change can affect forests by altering the frequency, intensity, duration, and timing of fire, drought, introduced species, insect and pathogen outbreaks, hurricanes, windstorms, ice storms, or landslides.” AIBS Bulletin 51.9 (2001): 723-734. Over geologic time, changes in disturbance regimes are a natural part of all ecosystems. Even so, as a consequence of climate change, forests may soon face rapid alterations in the timing, intensity, frequency, and extent of disturbances. The number and complexity of climate variables related to forest disturbance make integrated research an awesome challenge. Even if changes cannot always be predicted, it is important to consider ways in which impacts to forest systems can be mitigated under likely changes in disturbance regimes. The task for the next decade is to understand better how climate affects disturbances and how forests respond to them. Improved monitoring programs and analytic tools are needed to develop this understanding. Ultimately, this knowledge should lead to better ways to predict and cope with disturbance-induced changes in forests.  [FULL TEXT]
8. 2002: Cole, Julia E., Jonathan T. Overpeck, and Edward R. Cook. “Multiyear La Niña events and persistent drought in the contiguous United States.” Geophysical Research Letters29.13 (2002): 25-1.  La Niña events typically bring dry conditions to the southwestern United States. Recent La Niñas rarely exceed 2 years duration, but a new record of ENSO from a central Pacific coral reveals much longer La Niña anomalies in the 1800s. A La Niña event between 1855–63 coincides with prolonged drought across the western U.S. The spatial pattern of this drought correlates with that expected from La Niña during most of the La Niña event; land‐surface feedbacks are implied by drought persistence and expansion. Earlier periods also show persistent La Niña‐like drought patterns, further implicating Pacific anomalies and surface feedbacks in driving prolonged drought. An extended index of the Pacific Decadal Oscillation suggests that extratropical influences would have reinforced drought in the 1860s and 1890s but weakened it during the La Niña of the 1880s. [FULL TEXT]
   
9. 2006: Westerling, Anthony L., et al. “Warming and earlier spring increase western US forest wildfire activity.” science 313.5789 (2006): 940-943. Western United States forest wildfire activity is widely thought to have increased in recent decades, yet neither the extent of recent changes nor the degree to which climate may be driving regional changes in wildfire has been systematically documented. Much of the public and scientific discussion of changes in western United States wildfire has focused instead on the effects of 19th- and 20th-century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it with hydroclimatic and land-surface data. Here, we show that large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt. Wildfires have consumed increasing areas of western U.S. forests in recent years, and fire-fighting expenditures by federal land-management agencies now regularly exceed US$1 billion/year. Hundreds of homes are burned annually by wildfires, and damages to natural resources are sometimes extreme and irreversible. Media reports of recent, very large wildfires (>100,000 ha) burning in western forests have garnered widespread public attention, and a recurrent perception of crisis has galvanized legislative and administrative action (1–3). Extensive discussions within the fire-management and scientific communities and the media seek to explain these phenomena, focusing on either land-use history or climate as primary causes. If increased wildfire risks are driven primarily by land-use history, then ecological restoration and fuels management are potential solutions. However, if increased risks are largely due to changes in climate during recent decades, then restoration and fuels treatments may be relatively ineffective in reversing current wildfire trends (4, 5). We investigated 34 years of western U.S. (hereafter, “western”) wildfire history together with hydroclimatic data to determine where the largest increases in wildfire have occurred and to evaluate how recent climatic trends may have been important causal factors. Competing explanations: Climate versus management. Land-use explanations for increased western wildfire note that extensive livestock grazing and increasingly effective fire suppression began in the late 19th and early 20th centuries, reducing the frequency of large surface fires (6–8). Forest regrowth after extensive logging beginning in the late 19th century, combined with an absence of extensive fires, promoted forest structure changes and biomass accumulation, which now reduce the effectiveness of fire suppression and increase the size of wildfires and total area burned (3, 5, 9). The effects of land-use history on forest structure and biomass accumulation are, however, highly dependent upon the “natural fire regime” for any particular forest type. For example, the effects of fire exclusion are thought to be profound in forests that previously sustained frequent, low-intensity surface fires [such as Southwestern ponderosa pine and Sierra Nevada mixed conifer (2, 3, 10, 11)], but of little or no consequence in forests that previously sustained only very infrequent, high-severity crown fires (such as Northern Rockies lodgepole pine or spruce-fir (1, 5, 12)]. In contrast, climatic explanations posit that increasing variability in moisture conditions (wet/dry oscillations promoting biomass growth, then burning), and/or a trend of increasing drought frequency, and/or warming temperatures have led to increased wildfire activity (13, 14). Documentary records and proxy reconstructions (primarily from tree rings) of fire history and climate provide evidence that western forest wildfire risks are strongly positively associated with drought concurrent with the summer fire season and (particularly in ponderosa pine–dominant forests) positively associated to a lesser extent with moist conditions in antecedent years (13–18). Variability in western climate related to the Pacific Decadal Oscillation and intense El Niño/La Niña events in recent decades along with severe droughts in 2000 and 2002 may have promoted greater forest wildfire risks in areas such as the Southwest, where precipitation anomalies are significantly influenced by patterns in Pacific sea surface temperature (19–22). Although corresponding decadal-scale variations and trends in climate and wildfire have been identified in paleo studies, there is a paucity of evidence for such associations in the 20th century. We describe land-use history versus climate as competing explanations, but they may be complementary in some ways. In some forest types, past land uses have probably increased the sensitivity of current forest wildfire regimes to climatic variability through effects on the quantity, arrangement, and continuity of fuels. Hence, an increased incidence of large, high-severity fires may be due to a combination of extreme droughts and overabundant fuels in some forests. Climate, however, may still be the primary driver of forest wildfire risks on interannual to decadal scales. On decadal scales, climatic means and variability shape the character of the vegetation [e.g., species populations and their drought tolerance (23) and biomass (fuel) continuity (24), thus also affecting fire regime responses to shorter term climate variability]. On interannual and shorter time scales, climate variability affects the flammability of live and dead forest vegetation (13–19, 25).High-quality time series are essential for evaluating wildfire risks, but for various reasons (26), previous works have not rigorously documented changes in large-wildfire frequency for western forests. Likewise, detailed fire-climate analyses for the region have not been conducted to evaluate what hydroclimatic variations may be associated with recent increased wildfire activity, and the spatial variations in these patterns. We compiled a comprehensive time series of 1166 large (>400 ha) forest wildfires for 1970 to 2003 from federal land-management units containing 61% of western forested areas (and 80% above 1370 m) (26) (fig. S1). We compared these data with corresponding hydroclimatic and land surface variables (26–34) to address where and why the frequency of large forest wildfire has changed. Increased forest wildfire activity. We found that the incidence of large wildfires in western forests increased in the mid-1980s (Fig. 1) [hereafter, “wildfires” refers to large-fire events (>400 ha) within forested areas only (26)]. Subsequently, wildfire frequency was nearly four times the average of 1970 to 1986, and the total area burned by these fires was more than six and a half times its previous level. Interannual variability in wildfire frequency is strongly associated with regional spring and summer temperature (Spearman’s correlation of 0.76, P < 0.001, n = 34). A second-order polynomial fit to the regional temperature signal alone explains 66% of the variance in the annual incidence of these fires, with many more wildfires burning in hotter than in cooler years. [FULL TEXT]
10. 2010: Woodhouse, Connie A., et al. “A 1,200-year perspective of 21st century drought in southwestern North America.” Proceedings of the National Academy of Sciences 107.50 (2010): 21283-21288. A key feature of anticipated 21st century droughts in Southwest North America is the concurrence of elevated temperatures and increased aridity. Instrumental records and paleoclimatic evidence for past prolonged drought in the Southwest that coincide with elevated temperatures can be assessed to provide insights on temperature-drought relations and to develop worst-case scenarios for the future. In particular, during the medieval period, ∼AD 900–1300, the Northern Hemisphere experienced temperatures warmer than all but the most recent decades. Paleoclimatic and model data indicate increased temperatures in western North America of approximately 1 °C over the long-term mean. This was a period of extensive and persistent aridity over western North America. Paleoclimatic evidence suggests drought in the mid-12th century far exceeded the severity, duration, and extent of subsequent droughts. The driest decade of this drought was anomalously warm, though not as warm as the late 20th and early 21st centuries. The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future. The severity, extent, and persistence of the 12th century drought that occurred under natural climate variability, have important implications for water resource management. The causes of past and future drought will not be identical but warm droughts, inferred from paleoclimatic records, demonstrate the plausibility of extensive, severe droughts, provide a long-term perspective on the ongoing drought conditions in the Southwest, and suggest the need for regional sustainability planning for the future.
11. 2010: Seager, Richard, and Gabriel A. Vecchi. “Greenhouse warming and the 21st century hydroclimate of southwestern North America.” Proceedings of the National Academy of Sciences 107.50 (2010): 21277-21282.  Climate models robustly predict that the climate of southwestern North America, defined as the area from the western Great Plains to the Pacific Ocean and from the Oregon border to southern Mexico, will dry throughout the current century as a consequence of rising greenhouse gases. This regional drying is part of a general drying of the subtropics and poleward expansion of the subtropical dry zones. Through an analysis of 15 coupled climate models it is shown here that the drying is driven by a reduction of winter season precipitation associated with increased moisture divergence by the mean flow and reduced moisture convergence by transient eddies. Due to the presence of large amplitude decadal variations of presumed natural origin, observations to date cannot confirm that this transition to a drier climate is already underway, but it is anticipated that the anthropogenic drying will reach the amplitude of natural decadal variability by midcentury. In addition to this drop in total precipitation, warming is already causing a decline in mountain snow mass and an advance in the timing of spring snow melt disrupting the natural water storage systems that are part of the region’s water supply system. Uncertainties in how radiative forcing will impact the tropical Pacific climate system create uncertainties in the amplitude of drying in southwest North America with a La Niña-like response creating a worst case scenario of greater drying.
12. 2010: Allen, Craig D., et al. “A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests.” Forest ecology and management 259.4 (2010): 660-684. Greenhouse gas emissions have significantly altered global climate, and will continue to do so in the future. Increases in the frequency, duration, and/or severity of drought and heat stress associated with climate change could fundamentally alter the composition, structure, and biogeography of forests in many regions. Of particular concern are potential increases in tree mortality associated with climate-induced physiological stress and interactions with other climate-mediated processes such as insect outbreaks and wildfire. Despite this risk, existing projections of tree mortality are based on models that lack functionally realistic mortality mechanisms, and there has been no attempt to track observations of climate-driven tree mortality globally. Here we present the first global assessment of recent tree mortality attributed to drought and heat stress. Although episodic mortality occurs in the absence of climate change, studies compiled here suggest that at least some of the world’s forested ecosystems already may be responding to climate change and raise concern that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, even in environments that are not normally considered water-limited. This further suggests risks to ecosystem services, including the loss of sequestered forest carbon and associated atmospheric feedbacks. Our review also identifies key information gaps and scientific uncertainties that currently hinder our ability to predict tree mortality in response to climate change and emphasizes the need for a globally coordinated observation system. Overall, our review reveals the potential for amplified tree mortality due to drought and heat in forests worldwide.
13. 2010: Cayan, Daniel R., et al. “Future dryness in the southwest US and the hydrology of the early 21st century drought.” Proceedings of the National Academy of Sciences 107.50 (2010): 21271-21276. Recently the Southwest has experienced a spate of dryness, which presents a challenge to the sustainability of current water use by human and natural systems in the region. In the Colorado River Basin, the early 21st century drought has been the most extreme in over a century of Colorado River flows, and might occur in any given century with probability of only 60%. However, hydrological model runs from downscaled Intergovernmental Panel on Climate Change Fourth Assessment climate change simulations suggest that the region is likely to become drier and experience more severe droughts than this. In the latter half of the 21st century the models produced considerably greater drought activity, particularly in the Colorado River Basin, as judged from soil moisture anomalies and other hydrological measures. As in the historical record, most of the simulated extreme droughts build up and persist over many years. Durations of depleted soil moisture over the historical record ranged from 4 to 10 years, but in the 21st century simulations, some of the dry events persisted for 12 years or more. Summers during the observed early 21st century drought were remarkably warm, a feature also evident in many simulated droughts of the 21st century. These severe future droughts are aggravated by enhanced, globally warmed temperatures that reduce spring snowpack and late spring and summer soil moisture. As the climate continues to warm and soil moisture deficits accumulate beyond historical levels, the model simulations suggest that sustaining water supplies in parts of the Southwest will be a challenge.
14. 2013: Seager, Richard, et al. “Projections of declining surface-water availability for the southwestern United States.” Nature Climate Change 3.5 (2013): 482. Global warming driven by rising greenhouse-gas concentrations is expected to cause wet regions of the tropics and mid to high latitudes to get wetter and subtropical dry regions to get drier and expand polewards^1,2,3,4. Over southwest North America, models project a steady drop in precipitation minus evapotranspiration, P−E, the net flux of water at the land surface^5,6,7, leading to, for example, a decline in Colorado River flow^8,9,10,11. This would cause widespread and important social and ecological consequences^12,13,14. Here, using new simulations from the Coupled Model Intercomparison Project Five, to be assessed in Intergovernmental Panel on Climate Change Assessment Report Five, we extend previous work by examining changes in P, E, runoff and soil moisture by season and for three different water resource regions. Focusing on the near future, 2021–2040, the new simulations project declines in surface-water availability across the southwest that translate into reduced soil moisture and runoff in California and Nevada, the Colorado River headwaters and Texas.
15. 2014: AghaKouchak, Amir, et al. “Global warming and changes in risk of concurrent climate extremes: Insights from the 2014 California drought.” Geophysical Research Letters 41.24 (2014): 8847-8852.  Global warming and the associated rise in extreme temperatures substantially increase the chance of concurrent droughts and heat waves. The 2014 California drought is an archetype of an event characterized by not only low precipitation but also extreme high temperatures. From the raging wildfires, to record low storage levels and snowpack conditions, the impacts of this event can be felt throughout California. Wintertime water shortages worry decision‐makers the most because it is the season to build up water supplies for the rest of the year. Here we show that the traditional univariate risk assessment methods based on precipitation condition may substantially underestimate the risk of extreme events such as the 2014 California drought because of ignoring the effects of temperature. We argue that a multivariate viewpoint is necessary for assessing risk of extreme events, especially in a warming climate. This study discusses a methodology for assessing the risk of concurrent extremes such as droughts and extreme temperatures.
16. 2014: Dennison, Philip E., et al. “Large wildfire trends in the western United States, 1984–2011.” Geophysical Research Letters41.8 (2014): 2928-2933. We used a database capturing large wildfires (> 405 ha) in the western U.S. to document regional trends in fire occurrence, total fire area, fire size, and day of year of ignition for 1984–2011. Over the western U.S. and in a majority of ecoregions, we found significant, increasing trends in the number of large fires and/or total large fire area per year. Trends were most significant for southern and mountain ecoregions, coinciding with trends toward increased drought severity. For all ecoregions combined, the number of large fires increased at a rate of seven fires per year, while total fire area increased at a rate of 355 km² per year. Continuing changes in climate, invasive species, and consequences of past fire management, added to the impacts of larger, more frequent fires, will drive further disruptions to fire regimes of the western U.S. and other fire‐prone regions of the world.  [FULL TEXT]
17. 2015: Diffenbaugh, Noah S., Daniel L. Swain, and Danielle Touma. “Anthropogenic warming has increased drought risk in California.” Proceedings of the National Academy of Sciences(2015): 201422385. California ranks first in the United States in population, economic activity, and agricultural value. The state is currently experiencing a record-setting drought, which has led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Our analyses show that California has historically been more likely to experience drought if precipitation deficits co-occur with warm conditions and that such confluences have increased in recent decades, leading to increases in the fraction of low-precipitation years that yield drought. In addition, we find that human emissions have increased the probability that low-precipitation years are also warm, suggesting that anthropogenic warming is increasing the probability of the co-occurring warm–dry conditions that have created the current California drought.
18. 2015: Cook, Benjamin I., Toby R. Ault, and Jason E. Smerdon. “Unprecedented 21st century drought risk in the American Southwest and Central Plains.” Science Advances 1.1 (2015): e1400082. In the Southwest and Central Plains of Western North America, climate change is expected to increase drought severity in the coming decades. These regions nevertheless experienced extended Medieval-era droughts that were more persistent than any historical event, providing crucial targets in the paleoclimate record for benchmarking the severity of future drought risks. We use an empirical drought reconstruction and three soil moisture metrics from 17 state-of-the-art general circulation models to show that these models project significantly drier conditions in the later half of the 21st century compared to the 20th century and earlier paleoclimatic intervals. This desiccation is consistent across most of the models and moisture balance variables, indicating a coherent and robust drying response to warming despite the diversity of models and metrics analyzed. Notably, future drought risk will likely exceed even the driest centuries of the Medieval Climate Anomaly (1100–1300 CE) in both moderate (RCP 4.5) and high (RCP 8.5) future emissions scenarios, leading to unprecedented drought conditions during the last millennium.
19. 2015: Williams, A. Park, et al. “Contribution of anthropogenic warming to California drought during 2012–2014.” Geophysical Research Letters 42.16 (2015): 6819-6828. A suite of climate data sets and multiple representations of atmospheric moisture demand are used to calculate many estimates of the self‐calibrated Palmer Drought Severity Index, a proxy for near‐surface soil moisture, across California from 1901 to 2014 at high spatial resolution. Based on the ensemble of calculations, California drought conditions were record breaking in 2014, but probably not record breaking in 2012–2014, contrary to prior findings. Regionally, the 2012–2014 drought was record breaking in the agriculturally important southern Central Valley and highly populated coastal areas. Contributions of individual climate variables to recent drought are also examined, including the temperature component associated with anthropogenic warming. Precipitation is the primary driver of drought variability but anthropogenic warming is estimated to have accounted for 8–27% of the observed drought anomaly in 2012–2014 and 5–18% in 2014. Although natural variability dominates, anthropogenic warming has substantially increased the overall likelihood of extreme California droughts.