Climate alarmists contend that rising global temperatures lead to more severe and longer-lasting droughts on the basis of projections of global climate change produced by mathematical models that are primarily driven by increases in the atmosphere's CO2 concentration; and with respect to the western United States, there has been growing interest in understanding drought in that part of the country in light of the pronounced impact it has had there in recent decades. Therefore, many scientists have conducted research to attempt to better understand the characteristics of historic hydrologic variability in this important region, so that a more proper evaluation can be made of how unusual, unnatural or unprecedented droughts of the recent past have been, which droughts climate alarmists typically claim have been made worse - or even been entirely caused - by CO2-induced global warming.
In the following pages, this claim is evaluated by reviewing what scientists have learned in this regard from various studies that have examined historic droughts across the western United States, organizing our review into several sub-domains within the overall region of study.
Knapp et al. (2002) created a 500-year history of severe single-year Pacific Northwest droughts from a study of eighteen western juniper tree-ring chronologies that they used to identify what they call extreme Climatic Pointer Years or CPYs, which are indicative of severe single-year droughts. As they describe it, this procedure revealed that "widespread and extreme CPYs were concentrated in the 16th and early part of the 17th centuries," while "both the 18th and 19th centuries were largely characterized by a paucity of drought events that were severe and widespread." Thereafter, however, they say that "CPYs became more numerous during the 20th century," although the number of 20th century extreme CPYs (26) was still substantially less than the mean of the number of 16th and 17th century extreme CPYs (38), when the planet was considerably colder. Such data, therefore, fail to support the climate-alarmist claim that global warming increases the frequency of severe droughts.
In another paper, Gedalof et al. (2004) used a network of 32 drought-sensitive tree-ring chronologies to reconstruct mean water-year flow on the Columbia River at The Dales in Oregon since 1750. This study of the second largest drainage basin in the United States is stated by them to have been done "for the purpose of assessing the representativeness of recent observations, especially with respect to low frequency changes and extreme events." When finished, it revealed, in their words, that "persistent low flows during the 1840s were probably the most severe of the past 250 years," and that "the drought of the 1930s is probably the second most severe."
More recent droughts, in the words of the researchers, "have led to conflicts among uses (e.g., hydroelectric production versus protecting salmon runs), increased costs to end users (notably municipal power users), and in some cases the total loss of access to water (in particular junior water rights holders in the agricultural sector)." Nevertheless, they say that "these recent droughts were not exceptional in the context of the last 250 years and were of shorter duration than many past events." In fact, they say that "the period from 1950 to 1987 is anomalous in the context of this record for having no notable multiyear drought events," once again demonstrating the fact that Pacific Northwest droughts have not become more severe or long-lasting as temperatures have risen over the course of the 20th century.
Writing as background for her study, Wise (2010) states that "the 1667 km Snake River is one of the largest rivers in the United States, draining a semiarid region that covers 283,000 km2 [that] includes most of Idaho, as well as portions of Wyoming, Utah, Nevada, Oregon, and Washington," and she says that the river's water has been "historically allocated almost entirely for agricultural irrigation." She also notes that the Snake River is "the largest tributary of the Columbia River (based on both discharge and watershed size)," which makes it "also important for users further downstream." In addition, Wise reports that "the 20th century was an abnormally wet period in this region (Gray and McCabe, 2010)," but she says that an early 21st century drought "has raised questions about whether these dry conditions should be considered an extreme event or if this drought is within the range of natural variability." In addressing such questions in her study, Wise utilized tree ring samples collected near the headwaters of the Snake River in Wyoming that were augmented with pre-existing tree ring chronologies to extend the short (1911-2006) instrumental water supply record of the region, thereby providing the first multi-century (1591-2005) record of the river's water supply variability, which record could then provide context for the early 21st century drought in this region.
Results indicated that "individual low-flow years in 1977 and 2001 and the longer-term 1930s Dust Bowl drought meet or exceed the magnitude of dry periods in the extended reconstructed period." However, she writes that in terms of overall severity, "the instrumental record does not contain a drought of the extent seen in the mid-1600s." Enlarging on this finding, Wise further writes that "twenty-four of 34 years in the 1626-1659 time period had below-average flow, including periods of six and seven consecutive below-mean years (1626-1632 and 1642-1647)," and that "during the most severe period from 1626 to 1647, 17 of 22 years (77%) were below-normal flow." Hence, she concludes that "this type of event could represent a new 'worst-case scenario' for water planning in the upper Snake River."
Working in the Bighorn Basin of north-central Wyoming and south-central Montana, Gray et al. (2004) used cores and cross sections from 79 Douglas fir and limber pine trees at four different sites to develop a proxy for annual precipitation spanning the period AD 1260-1998. This reconstruction, in their words, "exhibits considerable nonstationarity, and the instrumental era (post-1900) in particular fails to capture the full range of precipitation variability experienced in the past ~750 years." More specifically, they say that "both single-year and decadal-scale dry events were more severe before 1900," and that "dry spells in the late thirteenth and sixteenth centuries surpass both [the] magnitude and duration of any droughts in the Bighorn Basin after 1900." In fact, they say that "single- and multi-year droughts regularly [italics added] surpassed the severity and magnitude of the 'worst-case scenarios' presented by the 1930s and 1950s droughts." It would thus appear, that if 20th-century global warming had any effect at all on Bighorn Basin precipitation, it was to make it less extreme rather than more extreme, in striking contradiction of claims to the contrary by the world's climate alarmists.
In a study covering a much longer period of time, Persico and Meyer (2009) studied "beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park," which feat was accomplished by comparing "the distribution of beaver-pond deposit ages to paleoclimatic proxy records in the Yellowstone region." In doing so, the pair of researchers found that "gaps in the beaver-pond deposit record from 2200-1800 and 700-1000 cal yr BP are contemporaneous with increased charcoal accumulation rates in Yellowstone lakes and peaks in fire-related debris-flow activity, inferred to reflect severe drought and warmer temperatures (Meyer et al., 1995)." In addition, they note that "the lack of evidence for beaver activity 700-1000 cal yr BP is concurrent with the Medieval Climatic Anomaly, a time of widespread multi-decadal droughts and high climatic variability in Yellowstone National Park (Meyer et al., 1995) and the western USA (Cook et al., 2004; Stine, 1998; Whitlock et al., 2003)," while it should also be noted that the lack of evidence for beaver activity 2200-1800 cal yr BP is concurrent with the Roman Warm Period. And in both of these instances, the two researchers concluded that the severe droughts of these periods "likely caused low to ephemeral discharges in smaller streams, as in modern severe drought [italics added]," implying that the Medieval and Roman Warm Periods were likely just as dry and warm as it is today.
These findings suggest that there is nothing unusual, unnatural or unprecedented about the degree of warmth and drought of the Current Warm Period. And the fact that there was more than 100 ppm less CO2 in the air of those earlier warm and dry periods than there is today suggests that something other than CO2 could well be responsible for the equivalent warmth and dryness of today, while the regular recurrence of such conditions suggests that their cause is a cyclical phenomenon of nature that is independent of the activities of the planet's human population.
Moving further south, Benson et al. (2002) developed continuous high-resolution δ18O records from cored sediments of Pyramid Lake, Nevada, which they used to help construct a 7600-year history of droughts throughout the surrounding region. Oscillations in the hydrologic balance that were evident in this record occurred, on average, about every 150 years, but with significant variability. Over the most recent 2740 years, for example, intervals between droughts ranged from 80 to 230 years; while drought durations ranged from 20 to 100 years, with some of the larger ones forcing mass migrations of indigenous peoples from lands that could no longer support them. In contrast, historical droughts typically have lasted less than a decade.
In another study based on sediment cores extracted from Pyramid Lake, Nevada, Mensing et al. (2004) analyzed pollen and algal microfossils deposited there over the prior 7630 years that allowed them to infer the hydrologic history of the area over that time period. Their results indicated that "sometime after 3430 but before 2750 cal yr B.P., climate became cool and wet," but, paradoxically, that "the past 2500 yr have been marked by recurring persistent droughts." The longest of these droughts, according to them, "occurred between 2500 and 2000 cal yr B.P.," while others occurred "between 1500 and 1250, 800 and 725, and 600 and 450 cal yr B.P," with none recorded in more recent warmer times.
The researchers also note that "the timing and magnitude of droughts identified in the pollen record compare favorably with previously published δ18O data from Pyramid Lake" and with "the ages of submerged rooted stumps in the Eastern Sierra Nevada and woodrat midden data from central Nevada." And last of all, noting that Bond et al. (2001) "found that over the past 12,000 yr, decreases in [North Atlantic] drift ice abundance corresponded to increased solar output," they report that when they "compared the pollen record of droughts from Pyramid Lake with the stacked petrologic record of North Atlantic drift ice ... nearly every occurrence of a shift from ice maxima (reduced solar output) to ice minima (increased solar output) corresponded with a period of prolonged drought in the Pyramid Lake record." As a result, Mensing et al. concluded that "changes in solar irradiance may be a possible mechanism influencing century-scale drought in the western Great Basin [of the United States]." In fact, it would appear from their analysis that variable solar activity may well have been the major factor in determining the hydrologic condition of the region.
Studying drought over a much shorter interval of time in Utah, Gray et al. (2004a) used samples from 107 piņon pines at four different sites to develop a proxy record of annual precipitation spanning the AD 1226-2001 interval for the Uinta Basin watershed in the northeastern portion of the state. This effort revealed, in their words, that "single-year dry events before the instrumental period tended to be more severe than those after 1900," and that decadal-scale dry events were longer and more severe prior to 1900 as well. In particular, they found that "dry events in the late 13th, 16th, and 18th Centuries surpass the magnitude and duration of droughts seen in the Uinta Basin after 1900."
At the other end of the moisture spectrum, Gray et al. report that the 20th century was host to two of the strongest wet intervals (1938-1952 and 1965-1987), although these two periods were only the seventh and second most intense wet regimes, respectively, of the entire record. Hence, it would appear that in conjunction with 20th-century global warming, precipitation extremes (both high and low) within northeastern Utah's Uinta Basin have become more attenuated as opposed to more amplified, in yet another refutation of the climate-alarmist contention that both droughts and floods will become more severe and last longer in a significantly warmer world.
In a study published three years later focusing on the same region, MacDonald and Tingstad (2007) examined instrumental climate records to outline historical spatiotemporal patterns of precipitation variability in the Uinta Mountains, after which they "used tree-ring width chronologies from Pinus edulis Engelm. (two-needle pinyon pine) trees growing near the northern and southern flanks of the mountains to produce an ~600-year reconstruction (AD 1405-2001) of Palmer Drought Severity Index [PDSI] for Utah Climate Division 5," which they say "allows for the placement of 20th century droughts within the longer context of natural drought variability and also allows for the detection of long-term trends in drought."
Similar to the findings of Gray et al., MacDonal and Tingstad (2007) report that "in the context of prolonged severe droughts," the 20th century "has been relatively moist compared to preceding centuries," and they say that their PDSI reconstruction and the Uinta Basin precipitation reconstruction indicate that "the early to mid 17th century in particular, and portions of the 18th and 19th centuries, experienced prolonged (>10 years) dry conditions that would be unusually severe by 20th century standards," noting that "the most striking example of widespread extended drought occurred during a ~45-year period between 1625 and 1670 when PDSI only rarely rose above negative values."
Working in California, Malamud-Roam et al. (2006) conducted an extensive review of "the variety of paleoclimatic resources for the San Francisco Bay and watershed in order to identify major climate variations in the pre-industrial past, and to compare the records from the larger watershed region with the Bay records in order to determine the linkages between climate experienced over the larger watershed region and conditions in the San Francisco Bay." In doing so they found that "intermittent mega-droughts of the Medieval Climate Anomaly (ca. AD 900-1350) coincided with a period of anomalously warm coastal ocean temperatures in the California Current," and that "oxygen isotope compositions of mussel shells from archaeological sites along the central coast also indicate that sea surface temperatures were slightly warmer than present." In contrast, they note that "the Little Ice Age (ca. AD 1450-1800) brought unusually cool and wet conditions to much of the watershed," and that "notably stable conditions have prevailed over the instrumental period, i.e., after ca. AD 1850, even including the severe, short-term anomalies experienced during this period," namely, "the severe droughts of the 1930s and the mid-1970s." In considering their findings, the four researchers note that when longer paleoclimate records are considered, "current drought conditions experienced in the US Southwest do not appear out of the range of natural variability." However, as they opine, "warmer temperatures associated with anthropogenic global warming may exacerbate such conditions," which also suggests that, even now, it is still not as warm as it was back in the Medieval Warm Period.
In introducing their study of perfect drought in Southern California (USA), MacDonald et al. (2008) define the term as "a prolonged drought that affects southern California, the Sacramento River basin and the upper Colorado River basin simultaneously." They note that the instrumental record indicates the occurrence of such droughts throughout the past century, but that they "generally persist for less than five years." That they have occurred at all, however, suggests the possibility of even longer perfect droughts, which could well prove catastrophic for the region. Thus, the three researchers explored the likelihood of such droughts occurring in the future, based on dendrochronological reconstructions of the winter Palmer Drought Severity Index (PDSI) in southern California over the past thousand years, plus the concomitant annual discharges of the Sacramento and Colorado Rivers, under the logical assumption that what has occurred before may well occur again. So what did they find?
MacDonald et al. report finding that "prolonged perfect droughts (~30-60 years), which produced arid conditions in all three regions simultaneously, developed in the mid-11th century and the mid-12th century during the period of the so-called 'Medieval Climate Anomaly'," which is also widely known as the Medieval Warm Period, leading them to conclude that "prolonged perfect droughts due to natural or anthropogenic changes in radiative forcing, are a clear possibility for the near future."
In one final study from California, Kleppe et al. (2011) reconstructed the duration and magnitude of extreme droughts in the northern Sierra Nevada region based on dendrochronology, geomorphic analysis, and hydrologic modeling of the Fallen Leaf Lake (California) watershed in order to estimate paleo-precipitation near the headwaters of the Truckee River-Pyramid Lake watershed of eastern California and northwestern Nevada. In describing their findings, the six scientists say that "submerged Medieval trees and geomorphic evidence for lower shoreline corroborate a prolonged Medieval drought near the headwaters of the Truckee River-Pyramid Lake watershed," when and where water-balance calculations independently indicate precipitation to have been "less than 60% normal." And they note that these findings "demonstrate how prolonged changes of Fallen Leaf's shoreline allowed the growth and preservation of Medieval trees far below the modern shoreline." In addition, they add that age groupings of such trees suggest that similar mega-droughts "occurred every 600-1050 years during the late Holocene.".
Considered in their entirety, the several findings of Kleppe et al. (and many others who they cite) suggest that the Medieval Warm Period experienced far less precipitation and far longer and more severe drought than what has been experienced to date in the Current Warm Period. In addition, their data suggest that such dry conditions have occurred regularly, in cyclical fashion, "every 650-1150 years during the mid- and late-Holocene." And all of these observations suggest that there is nothing unusual, unnatural or unprecedented about the nature of drought during the Current Warm Period in the western United States.
Colorado/Colorado River Basin
In examining drought in the Upper Colorado River Basin well over a decade ago, Hidalgo et al. (2000) used a new form of principal components analysis to reconstruct a history of streamflow for the region based on information obtained from tree-ring data, after which they compared their results to those of Stockton and Jacoby (1976). In doing so, they found the two approaches yielded similar results, except that Hidalgo et al.'s approach responded with more intensity to periods of below-average streamflow or regional drought. Thus, it was easier for them to determine there has been "a near-centennial return period of extreme drought events in this region," going all the way back to the early 1500s.
Six years later, Woodhouse et al. (2006) also generated proxy reconstructions of water-year streamflow for the Upper Colorado River Basin, based on four key gauges (Green River at Green River, Utah; Colorado near Cisco, Utah; San Juan near Bluff, Utah; and Colorado at Lees Ferry, Arizona) and using an expanded tree-ring network and longer calibration records than in previous efforts. The results of this program indicated that the major drought of 2000-2004, "as measured by 5-year running means of water-year total flow at Lees Ferry ... is not without precedence in the tree ring record," and that "average reconstructed annual flow for the period 1844-1848 was lower." They also report that "two additional periods, in the early 1500s and early 1600s, have a 25% or greater chance of being as dry as 1999-2004," and that six other periods "have a 10% or greater chance of being drier." In addition, their work revealed that "longer duration droughts have occurred in the past," and that "the Lees Ferry reconstruction contains one sequence each of six, eight, and eleven consecutive years with flows below the 1906-1995 average."
"Overall," in the words of the three researchers, "these analyses demonstrate that severe, sustained droughts are a defining feature of Upper Colorado River hydroclimate." In fact, Woodhouse et al. conclude from their work that "droughts more severe than any 20th to 21st century event occurred in the past," meaning the preceding few centuries; and, of course, this finding is the opposite of what the climate models project, i.e., that global warming promotes longer lasting droughts of greater severity. In stark contrast to this claim, the real-world record of Upper Colorado River Basin droughts suggests that such devastating climatic conditions are more strongly associated with the much colder temperatures that characterized the Little Ice Age.
Expanding on the work of Woodhouse et al. (2006), in a contemporaneous study Woodhouse and Lukas (2006) went on to develop "a network of 14 annual streamflow reconstructions, 300-600 years long, for gages in the Upper Colorado and South Platte River basins in Colorado generated from new and existing tree-ring chronologies." In the words of the two authors, their expanded streamflow reconstructions indicated that "the 20th century gage record does not fully represent the range of streamflow characteristics seen in the prior two to five centuries." Of greatest significance, in this regard, was probably the fact that "multi-year drought events more severe than the 1950s drought have occurred," and that "the greatest frequency of extreme low flow events occurred in the 19th century," with a "clustering of extreme event years in the 1840s and 1850s."
One year closer to the present, Meko et al. (2007) used a newly developed network of tree-ring sites located within the Upper Colorado River Basin (UCRB) -- which consists of tree-ring samples from living trees, augmented by similar samples obtained from logs and dead standing trees (remnant wood) -- to extend the record of reconstructed annual flows of the Colorado River at Lees Ferry, Arizona, into the Medieval Warm Period, during which period they say that "epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America." So what did their analysis reveal?
"The most prominent feature of the smoothed long-term reconstruction," in the words of Meko et al., "is the major period of low flow in the mid-1100s," which "25-year running mean occurred in AD 1130-1154." For this level of smoothing, they say that "conditions in the mid-1100s in the UCRB were even drier than during the extremely widespread late-1500s North American mega-drought (e.g., Stahle et al., 2000)." For comparison, for example, they state that "if 'normal' is defined as the observed mean annual flow for 1906-2004, the anomalous flow for AD 1130-1154 was less than 84% of normal," whereas "the lowest 25-year mean of observed flows (1953-1977) was 87% of normal," noting further that the 80% confidence band of their data "suggests a greater than 10% probability that the true mean for AD 1130-1154 was as low as 79% of normal." Additionally, the seven scientists report that "a detailed view of the time series of annual reconstructed flow reveals that the mid-1100s is characterized by a series of multi-year low-flow pluses imbedded in a generally dry 62-year period (1118-1179)," and that "the key drought signature is a stretch of 13 consecutive years of below normal flow (1143-1155)," noting that "in no other period of the reconstruction was flow below normal for more than 10 consecutive years, and the longest stretch of consecutive dry years in the reconstruction for the modern instrumental period (post 1905) was just 5 years."
In prefacing their study, authors Gray et al. (2011) write that "over the past decade severe drought conditions in the western United States have driven a growing interest in the range of natural hydrologic variability that has occurred over past centuries to millennia," as have "concerns related to the detection and prediction of anthropogenic climate-change impacts," for in order to know how unusual or unprecedented certain aspects of climate may have been recently, one has to know how they varied over past centuries to millennia, when man's influence on them was minimal to non-existent. Against this backdrop, the three U.S. researchers derived millennial-length records of water year (October-September) streamflow for three key Upper Colorado River tributaries -- the White, Yampa and Little Snake Rivers -- based on tree-ring data they obtained from seventy-five preexistent chronologies for a number of sites scattered throughout the region, where each chronology was derived from average annual ring-widths of at least 15 and as many as 80 trees per site.
The results indicated that "as in previous studies focused on the Upper Colorado River system as a whole (e.g., Meko et al., 2007)," the sub-basin reconstructions "show severe drought years and extended dry periods well outside the range of observed flows." Although they note that 1902 and 2002 "were among the most severe in the last ~1,000 years," they state that "pre-instrumental dry events often lasted a decade or longer with some extended low-flow regimes persisting for 30 years or more." In addition, they indicate that their research "shows anomalous wetness in the 20th century, a finding that has been well documented in the Colorado River basin and surrounding areas (Gray et al., 2004b, 2007; Woodhouse et al., 2006; Watson et al., 2009)."
In one additional study from the Colorado Plateau region, but with implications for drought in the entire western U.S., Routson et al. (2011) write that "many southwestern United States high-resolution proxy records show numerous droughts over the past millennium, including droughts far more severe than we have experienced during the historical period (e.g., Woodhouse and Overpeck, 1998; Cook et al., 2004, 2010; Meko et al., 2007)," adding that (1) "the medieval interval (ca. AD 900 to 1400), a period with relatively warm Northern Hemisphere temperatures, has been highlighted as a period in western North America with increased drought severity, duration and extent (e.g., Stine, 1994; Cook et al., 2004, 2010; Meko et al., 2007; Woodhouse et al., 2010)," and that (2) "the mid-12th century drought associated with dramatic decreases in Colorado River flow (Meko et al., 2007), and the 'Great Drought' associated with the abandonment of Ancient Pueblo civilization in the Colorado Plateau region (Douglass, 1929), all occurred during the medieval period," which observations would appear to suggest that significant Northern Hemispheric warmth tends to produce western North America megadroughts.
As their contribution to the topic, Routson et al. used a new tree-ring record derived from living and remnant bristlecone pine wood from the headwaters region of the Rio Grande River in Colorado (USA), along with other regional records, to evaluate what they describe as "periods of unusually severe drought over the past two millennia (268 BC to AD 2009)." In doing so the three researchers report that the record they derived "reveals two periods of enhanced drought frequency and severity relative to the rest of the record," and that "the later period, AD ~1050-1330, corresponds with medieval aridity well documented in other records," while "the earlier period is more persistent (AD ~1-400), and includes the most pronounced event in the ... chronology: a multi-decadal-length drought during the 2nd century," which "includes the unsmoothed record's driest 25-year interval (AD 148-173) as well as a longer 51-year period, AD 122-172, that has only two years with ring width slightly above the long-term mean," and where "the smoothed chronology shows the periods AD 77-282 and AD 301-400 are the longest (206 and 100 years, respectively, below the long-term average) droughts of the entire 2276-year record." And they note that this 2nd-century drought "impacted a region that extends from southern New Mexico north and west into Idaho."
In commenting about their findings, Routson et al. note that "reconstructed Colorado Plateau temperature suggests warmer than average temperature could have influenced both 2nd century and medieval drought severity," and that "available data also suggest that the Northern Hemisphere may have been warm during both intervals." As such, Routson et al. go on to suggest that the southwestern United States could well experience similar or even more severe megadroughts in the future, as they suspect it will continue to warm in response to continued anthropogenic CO2 emissions. However, such a view of the future may well be premature, for it is important to note that studies from all around the globe -- which depict both a Medieval Warm Period and a Roman Warm Period that were equally as warm or even warmer than the Current Warm Period has been to date, and at times when there was way less CO2 in the atmosphere than there is today -- suggest that there is nothing unusual, unnatural or unprecedented about Earth's current level of warmth, and, if global warming is in fact the major cause of western USA drought, then it must be significantly cooler now than it was during those two prior multi-century warm periods, since we have not yet experienced droughts of anywhere near the severity or duration of those that were experienced in the Roman and Medieval Warm Periods, which further suggests that the planet's current level of warmth is likely not a result of historical anthropogenic CO2 emissions, but rather a result of a milder expression of whatever was the cause of those two earlier stellar warm periods.
Moving to the southern portion of the region under study, Ni et al. (2002) developed a 1000-year history of cool-season (November-April) precipitation for each climate division of Arizona and New Mexico from a network of 19 tree-ring chronologies. As a result of their efforts, they determined that "sustained dry periods comparable to the 1950s drought" occurred in "the late 1000s, the mid 1100s, 1570-97, 1664-70, the 1740s, the 1770s, and the late 1800s." They also note that although the 1950s drought was large in both scale and severity, "it only lasted from approximately 1950 to 1956," whereas the 16th-century mega-drought lasted more than four times longer.
Working solely in New Mexico, Rasmussen et al. (2006) derived a record of regional relative moisture from variations in the annual band thickness and mineralogy of two columnar stalagmites collected from Carlsbad Cavern and Hidden Cave in the Guadalupe Mountains near the New Mexico/Texas border. From this work they discovered that both records "suggest periods of dramatic precipitation variability over the last 3000 years, exhibiting large shifts unlike anything seen in the modern record [italics added]," revealing that significant droughts and floods of recent times are certainly not unprecedented over the past millennium or more.
Several studies have examined historical drought trends across multiple states in the western United States. Gray et al. (2003), for example, examined fifteen tree ring-width chronologies that had been used in previous reconstructions of drought for evidence of low-frequency variations in five regional composite precipitation histories in the central and southern Rocky Mountains,. In doing so, they found that "strong multidecadal phasing of moisture variation was present in all regions during the late 16th-century megadrought," and that "oscillatory modes in the 30-70 year domain persisted until the mid-19th century in two regions, and wet-dry cycles were apparently synchronous at some sites until the 1950s drought." They thus speculate that "severe drought conditions across consecutive seasons and years in the central and southern Rockies may ensue from coupling of the cold phase Pacific Decadal Oscillation with the warm phase Atlantic Multidecadal Oscillation," which is something they envision as having happened in both the severe 1950s drought and the late 16th-century megadrought. Thus, there is reason to believe that episodes of extreme dryness in this part of the country may be driven in part by naturally-recurring climate "regime shifts" in the Pacific and Atlantic Oceans.
Also suggesting that ocean oscillations might bear a good deal of the blame for large-scale drought in the western U.S. was Seager (2007), who studied the global context of the drought that affected nearly the entire United States, northern Mexico and the Canadian Prairies -- but most particularly the American West -- between 1998 and 2004. Based on atmospheric reanalysis data and ensembles of climate model simulations forced by global or tropical Pacific sea surface temperatures over the period January 1856 to April 2005, Seager compared the climatic circumstances of the recent drought with those of the five prior great droughts of North America: (1) the Civil War drought of 1856-65, (2) the 1870s drought, (3) the 1890s drought, (4) the great Dust Bowl drought, and (5) the 1950s drought. In doing so, Seager reports that the 1998-2002 period of the recent drought "was most likely caused by multiyear variability of the tropical Pacific Ocean," noting that the recent drought "was the latest in a series of six persistent global hydroclimate regimes, involving a persistent La Niņa-like state in the tropical Pacific and dry conditions across the midlatitudes of each hemisphere."
Of additional note, there was no aspect of Seager's study that implicates global warming, either CO2-induced or otherwise, as a cause of -- or contributor to -- the great turn-of-the-20th-century drought that affected large portions of North America. Seager notes, for example, that "although the Indian Ocean has steadily warmed over the last half century, this is not implicated as a cause of the turn of the century North American drought because the five prior droughts were associated with cool Indian Ocean sea surface temperatures [italics added]." In addition, the five earlier great droughts occurred during periods when the mean global temperature was also significantly cooler than what it was during the last great drought.
Covering the whole of the western United States was Woodhouse (2004), who reported approximately a decade ago what is known about natural hydroclimatic variability throughout the entire region via descriptions of several major droughts that occurred there over the past three millennia, all but the last century of which had atmospheric CO2 concentrations that never varied by more than about 10 ppm from a mean value of 280 ppm.
For comparative purposes, Woodhouse begins by noting that "the most extensive U.S. droughts in the 20th century were the 1930s Dust Bowl and the 1950s droughts." The first of these droughts lasted "most of the decade of the 1930s" and "occurred in several waves," while the latter "also occurred in several waves over the years 1951-1956." Far more severe than either of these two droughts was the 16th-Century Megadrought, which lasted from 1580 to 1600 and included northwestern Mexico in addition to the southwestern United States and the western Great Plains. Then there was The Great Drought, which spanned the last quarter of the 13th century and was actually the last in a series of three 13th-century droughts, the first of which may have been even more severe than the last. In addition, Woodhouse notes there was a period of remarkably sustained drought in the second half of the 12th century.
It is evident from these observations, according to Woodhouse, that "the 20th century climate record contains only a subset of the range of natural climate variability in centuries-long and longer paleoclimatic records." It is also obvious that this subset, as it pertains to water shortage, does not even begin to approach the level of drought severity and duration experienced in prior centuries and millennia, which fact was confirmed in a separate paper published by Woodhouse with four coauthors six years later in 2010 (Woodhouse et al., 2010). This being the case, it is also clear it would take a drought much more extreme than the most extreme droughts of the 20th century to propel the western United States and adjacent portions of Canada and Mexico into a truly unprecedented state of dryness.
Adding a human element to historical drought occurrences, Benson et al. (2007) reviewed and discussed the possible impacts of early-11th-, middle-12th-, and late-13th-century droughts on three Native American cultures that occupied parts of the western United States (Anasazi, Fremont, Lovelock) plus another culture that occupied parts of southwestern Illinois (Cahokia). According to the group of authors, "population declines among the various Native American cultures were documented to have occurred either in the early-11th, middle-12th, or late-13th centuries" - AD 990-1060, 1135-1170, and 1276-1297, respectively - and that "really extensive droughts impacted the regions occupied by these prehistoric Native Americans during one or more of these three time periods." In particular, they say the middle-12th-century drought "had the strongest impact on the Anasazi and Mississippian Cahokia cultures," noting that "by AD 1150, the Anasazi had abandoned 85% of their great houses in the Four Corners region and most of their village sites, and the Cahokians had abandoned one or more of their agricultural support centers, including the large Richland farming complex." In addition, they write that "the sedentary Fremont appear to have abandoned many of their southern area habitation sites in the greater Uinta Basin area by AD 1150 as well as the eastern Great Basin and the Southern Colorado Plateau," so that "in some sense, the 13th century drought may simply have 'finished off' some cultures that were already in decline." Lastly, they state that these "major reductions in prehistoric Native American habitation sites/population" occurred during a period of "anomalously warm" climatic conditions, which characterized the Medieval Warm Period throughout much of the world at that particular time.
In considering the findings of Benson et al., logic suggests that the fact that the deadly North American droughts of the MWP have never been equaled throughout all the ensuing years argues strongly that what Benson et al. call the anomalous warmth of that period has also "never been equaled throughout all the ensuing years," which further suggests (since the air's CO2 content was so much less during the MWP than it is now) that the lesser warmth of today need not in any way be related to the much higher CO2 concentration of Earth's current atmosphere.
In concluding this discussion on western United States drought trends, we report the findings of two papers led by lead author E.R. Cook. In the first of such papers, Cook et al. (2004) developed a 1200-year history of drought for the western half of the country and adjacent parts of Canada and Mexico (hereafter the "West"), based on annually-resolved tree-ring records of summer-season Palmer Drought Severity Index that were derived for 103 points on a 2.5° x 2.5° grid, 68 of which grid points (66% of them) possessed data that extended back to AD 800. This reconstruction, in the words of the authors, revealed "some remarkable earlier increases in aridity that dwarf the comparatively short-duration current drought in the 'West'." Specifically, they report that "the four driest epochs, centered on AD 936, 1034, 1150 and 1253, all occur during a ~400 year interval of overall elevated aridity from AD 900 to 1300," which they say is "broadly consistent with the Medieval Warm Period."
Commenting on the strength and severity of Medieval drought, the five scientists say "the overall coincidence between our megadrought epoch and the Medieval Warm Period suggests that anomalously warm climate conditions during that time may have contributed to the development of more frequent and persistent droughts in the 'West'," as well as the megadrought that was discovered by Rein et al. (2004) to have occurred in Peru at about the same time (AD 800-1250); and after citing nine other studies that provide independent evidence of drought during this time period for various sub-regions of the West, they warn that "any trend toward warmer temperatures in the future could lead to a serious long-term increase in aridity over western North America," noting that "future droughts in the 'West' of similar duration to those seen prior to AD 1300 would be disastrous."
However, it is important to note that such an unfortunate fate could well befall the western United States, even in the absence of (likely miniscule) CO2-induced global warming (a fact that climate alarmists are wont to ignore); for the millennial-scale oscillation of climate that brought the world the Medieval Warm Period (which was obviously not CO2-induced) could well be in process of repeating itself during the possibly still-ongoing development of the Current Warm Period. In addition, if the association between global warmth and drought in the western United States is indeed robust, it suggests that current world temperatures are still far below those experienced during large segments of the Medieval Warm Period, as no drought of Medieval magnitude has accompanied the modern rise in temperature.
In the second of the two papers, Cook et al. (2009) prefaced their analysis by writing that "IPCC Assessment Report 4 model projections suggest that the subtropical dry zones of the world will both dry and expand poleward in the future due to greenhouse warming," and that "the US southwest is particularly vulnerable in this regard and model projections indicate a progressive drying there out to the end of the 21st century." They then state that "the USA has been in a state of drought over much of the West for about 10 years now," and that "while severe, this turn of the century drought has not yet clearly exceeded the severity of two exceptional droughts in the 20th century," so that "while the coincidence between the turn of the century drought and projected drying in the Southwest is cause for concern, it is premature to claim that the model projections are correct."
This fact is understood when the "turn of the century drought" is compared with the two "exceptional droughts" that preceded it by a few decades. Based on gridded instrumental Palmer Drought Severity indices for tree ring reconstruction that extend back to 1900, for example, Cook et al. (2009) calculated that the turn-of-the-century drought had its greatest Drought Area Index value of 59% in the year 2002, while the Great Plains/Southwest drought covered 62% of the US in its peak year of 1954, and the Dust Bowl drought covered 77% of the US in 1934. In terms of drought duration, on the other hand, things are not quite as clear. Stahle et al. (2007) estimated that the first two droughts lasted for 12 and 14 years, respectively; Seager et al. (2005) estimated them to have lasted for 8 and 10 years; and Andreadis et al. (2005) estimated them to have lasted for 7 and 8 years, yielding means of 9 and 11 years for the two exceptional droughts, which durations are to be compared to 10 or so years for the turn-of-the-century drought, which again makes the latter drought not unprecedented compared to those that occurred earlier in the 20th century.
Real clarity, however, comes when the turn-of-the-century drought is compared to droughts of the prior millennium. Cook et al. (2009) write that "perhaps the most famous example is the 'Great Drouth' (sic) of AD 1276-1299 described by A.E. Douglass (1929, 1935)." Yet this 24-year drought was eclipsed by the 38-year drought that was found by Weakley (1965) to have occurred in Nebraska from AD 1276 to 1313, which the authors say "may have been a more prolonged northerly extension of the 'Great Drouth'." But even these multi-decade droughts truly pale in comparison to the "two extraordinary droughts discovered by Stine (1994) in California that lasted more than two centuries before AD 1112 and more than 140 years before AD 1350." And each of these megadroughts, as Cook et al. (2009) describe them, occurred, in their words, "in the so-called Medieval Warm Period." And they add that "all of this happened prior to the strong greenhouse gas warming that began with the Industrial Revolution [authors' italics]."
Given that the above-referenced medieval megadroughts "occurred without any need for enhanced radiative forcing due to anthropogenic greenhouse gas forcing" -- because, of course, there was none at that time - Cook et al. (2009) rightfully conclude that "there is no guarantee that the response of the climate system to greenhouse gas forcing will result in megadroughts of the kind experienced by North America in the past." And if the world's climate alarmists refuse to acknowledge this possibility and continue to claim that global warming will most assuredly trigger the occurrence of medieval-like megadroughts, they will also have to acknowledge that the Medieval Warm Period of a thousand years ago had to have been much warmer than the Current Warm Period has been to date. But this acknowledgement destroys yet another of their claims, i.e., that the Earth is currently warmer than it has been for one (Mann et al., 1999) to two (Mann and Jones, 2003) millennia.
So when consulting the real world on the matter, climate alarmists find themselves positioned squarely between the proverbial rock and a hard place, with nowhere to run, no place to hide, because their contentions of present and future drought in the western United States are simply untenable.
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