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Drought (North America - United States: Central) -- Summary
The United States' Northern Great Plains is an important agricultural region of North America, providing a significant source of grain both locally and internationally. Because of its location, it is also susceptible to extreme droughts that tend to persist longer than in any other region of the country (Karl et al., 1987; Soule, 1992); and because of this fact, it is a good place to review the history of drought to determine if the region is presently experiencing a manifestation of the climate-alarmist claim (Gore, 2006; Mann and Kump, 2008) that global warming will usher in a period of more frequent and intense drought.

Focusing on the recent past, Mauget (2004) looked for what he called "initial clues" to the commencement of the great drying of the U.S. Heartland that had been predicted to occur in response to CO2-induced global warming by Manabe and Wetherald (1987), Rind et al. (1990), Rosenzweig and Hillel (1993), and Manabe et al. (2004), which Mauget reasoned would be apparent in the observational streamflow record of the region. In this endeavor, he thus employed data he obtained from the archives of the U.S. Geological Survey's Hydro-Climatic Data Network, which come from 42 stations covering the central third of the United States that stretches from the Canadian border on the north to the Gulf of Mexico on the south, with the most dense coverage being found within the U.S. Corn Belt. So what did Mauget learn?

He reports finding "an overall pattern of low flow periods before 1972, and high flow periods occurring over time windows beginning after 1969." Of the 42 stations' high flow periods, he says that "34 occur during 1969-1998, with 25 of those periods ending in either 1997 or 1998," and that "of those 25 stations 21 are situated in the key agricultural region known as the Corn Belt." He also reports that "among most of the stations in the western portions of the Corn Belt during the 1980s and 1990s there is an unprecedented tendency toward extended periods of daily high flow conditions, which lead to marked increases in the mean annual frequency of hydrological surplus conditions relative to previous years." Furthermore, he notes that "in 15 of the 18 Corn Belt gage stations considered here at daily resolution, a more than 50% reduction in the mean annual incidence of hydrological drought conditions is evident during those periods." Last of all, Mauget reports that "the gage station associated with the largest watershed area - the Mississippi at Vicksburg - shows more than a doubling of the mean annual frequency of hydrological surplus days during its 1973-1998 high flow period relative to previous years, and more than a 50% reduction in the mean annual incidence of hydrological drought condition."

In summarizing his findings, Mauget states that the overall pattern of climate variation "is that of a reduced tendency to hydrological drought and an increased incidence of hydrological surplus over the Corn Belt and most of the Mississippi River basin during the closing decades of the 20th century," noting further that "some of the most striking evidence of a transition to wetter conditions in the streamflow analyses is found among streams and rivers situated along the Corn Belt's climatologically drier western edge."

But do such findings represent the early stages of real-world climate change? Mauget states that the streamflow data do indeed "suggest a fundamental climate shift, as the most significant incidence of high ranked annual flow was found over relatively long time scales at the end of the data record." Yet that shift is away from the droughty conditions long predicted to result from CO2-induced global warming in this important agricultural region of the United States.

Taking a longer view of the topic was Shapley et al. (2005), who developed a 1000-year hydroclimate reconstruction from local bur oak tree-ring records and various lake sediment cores in the U.S. Northern Great Plains. Based on this record, they determined that prior to 1800, "droughts tended towards greater persistence [italics added] than during the past two centuries," suggesting that droughts of the region became shorter-lived as opposed to longer-lasting as the Earth gradually recovered from the cold temperatures of the Little Ice Age.

In another study covering the same time period, but examining all of the U.S. Great Plains, Daniels and Knox (2005) analyzed the alluvial stratigraphic evidence for an episode of major channel incision in tributaries of the upper Republican River that occurred between 1100 and 800 years ago, after which they compared their findings with proxy drought records from 28 other locations throughout the Great Plains and surrounding regions. This work revealed that channel incision in the Republican River between about AD 900 and 1200 was well correlated with a multi-centennial episode of widespread drought, which in the words of Daniels and Knox, "coincides with the globally recognized Medieval Warm Period." Of great interest, however, is the fact that modern 20th-century warming has not led to a repeat of those widespread drought conditions.

In still another study of this time period, Stambaugh et al. (2011) "used a new long tree-ring chronology developed from the central U.S. to reconstruct annual drought and characterize past drought duration, frequency, and cycles in the agriculturally-important U.S. Corn Belt region during the last millennium," which chronology they calibrated and verified against monthly values of the instrumental Palmer Hydrologic Drought Index during the summer season of June, July and August. In doing so, the six scientists report that "20th century droughts, including the Dust Bowl, were relatively unremarkable when compared to drought durations prior to the instrumental record." They note, for example, that the 19th century was the driest of the past millennium, with major drought periods occurring from about 1816 to 1844 and 1849 to 1880, during what they describe as the transition out of the Little Ice Age. Prior to that, there had been 45 years of drought in the latter part of the 17th century that were coincident with the Maunder Minimum of solar activity, which is associated with the coldest period of the current interglacial. And going back further in time, there was an approximately 35-year drought in the mid- to late-15th century during "a period of decreased radiative forcing and northern hemisphere temperatures."

Eclipsing them all, however, Stambaugh et al. write that "the approximately 61-year drought in the late 12th century (ca. AD 1148-1208) appears to be the most significant drought of the entire reconstruction," noting that it "corresponds to the single greatest megadrought in North America during the last 2000 years (Cook et al., 2007), as well as "unmatched persistent low flows in western U.S. river basins (Meko et al., 2007)." And this drought, as they describe it, occurred during the middle of the Medieval Warm Period -- "an interval of warmer temperatures between approximately AD 800-1300 characterized by greater drought duration and frequency in the Northern Great plains compared to more modern times." Thus, it is abundantly clear from Stambaugh et al.'s findings that there is nothing unusual, unnatural or unprecedented about any 20th or 21st century droughts that may have occurred throughout the agricultural heartland of the United States. It is also clear that the much greater droughts of the past millennium occurred during periods of both relative cold and relative warmth, as well as the transitions between them.

Introducing their study of the subject, Forman et al. (2005) note that "periods of dune reactivation reflect sustained moisture deficits for years to decades and reflect broader environmental change with diminished surface- and ground-water resources," which observation prompted them to focus on "the largest dune system in North America, the Nebraska Sand Hills," where they utilized "recent advances in optically stimulated luminescence dating (Murray and Wintle, 2000) to improve chronologic control on the timing of dune reactivation," while linking landscape response to drought over the past 1500 years to tree-ring records of aridity.

In pursuing their goal, Forman et al. identified six major aeolian depositional events in the past 1500 years, all but one of which (the 1930s "Dust Bowl" drought) occurred prior to the 20th century. Moving backwards in time from the Dust Bowl, the next three major events occurred during the depths of the Little Ice Age, the next one near the Little Ice Age's inception, and the earliest one near the end of the Dark Ages Cold Period. As for how the earlier droughts compare with those of the past century, the researchers say the 1930s drought (the 20th century's worst depositional event) was less severe than the others, especially the one that has come to be known as the 16th-century megadrought. Forman et al. thus conclude that the aeolian landforms they studied "are clear indicators of climate variability beyond twentieth century norms [italics added], and signify droughts of greater severity and persistence [italics added] than thus far instrumentally recorded." Consequently, their study also reveals that post-Little Ice Age warming - which climate alarmists claim to be unprecedented over the past two millennia - has not produced similarly unprecedented droughts. In fact, in the U.S. Great Plains the increase in temperature appears to have done just the opposite.

Advancing the window of study back further in time, Woodhouse and Overpeck (1998) reviewed what is known about the frequency and severity of drought in the central United States over the last two thousand years based upon empirical evidence of drought from various proxy indicators. Their study indicated the presence of numerous "multidecadal- to century-scale droughts," leading them to conclude that "twentieth-century droughts are not representative of the full range of drought variability that has occurred over the last 2000 years." In addition, they noted that the 20th century was characterized by droughts of "moderate severity and comparatively short duration, relative to the full range of past drought variability."

With respect to the causes of drought, Woodhouse and Overpeck suggest a number of different possibilities that either directly or indirectly induce changes in atmospheric circulation and moisture transport. However, they caution that "the causes of droughts with durations of years (i.e., the 1930s) to decades or centuries (i.e., paleodroughts) are not well understood." Hence, they conclude that "the full range of past natural drought variability, deduced from a comprehensive review of the paleoclimatic literature, suggests that droughts more severe than those of the 1930s and 1950s are likely to occur in the future," and, it might be added, irrespective of whatever the air's CO2 concentration or temperature might be doing in future years.

Two years after the publication of Woodhouse and Overpeck's work, Fritz et al. (2000) constructed three 2000-year histories of lake-water salinity at three sites in North Dakota -- Moon Lake, Coldwater Lake and Rice Lake -- to infer regional patterns of drought and to comment on its potential cause. So what did they learn?

"From the vantage point of the 20th century," in the words of Fritz et al., "the three North Dakota sites suggest that droughts equal or greater in magnitude to those of the Dust Bowl period were a common occurrence during the last 2000 years and that severe droughts may have been frequent for multiple decades within this period." In addition, they report that "studies from the northern Great Plains and western North America (Cook et al., 1997; Dean, 1997; Laird et al., 1996; Yu and Ito, 1999) have shown a correlation between solar forcing and centennial- and decadal-scale drought patterns." Thus, they conclude that "solar variability may influence the duration of dry periods through its impact on convective activity and circulation (Rind and Overpeck, 1993)."

In another study, Laird et al. (1998) examined the region's historical record of drought in an attempt to establish a baseline of natural drought variability that could help in attempts to determine if current and future droughts might be anthropogenically influenced. Working with a high-resolution sediment core obtained from Moon Lake, North Dakota, which provided a sub-decadal record of salinity (drought) over the past 2300 years, they discovered that the U.S. Northern Great Plains were relatively wet during the final 750 years of this period. In fact, throughout the 1550 prior years, Laird et al. determined that "recurring severe droughts were more the norm," and that they were "of much greater intensity and duration than any in the 20th century," including the great Dust Bowl event of the 1930s. Consequently, and in light of their finding that there were, as they put it, "no modern equivalents" to Northern Great Plains droughts experienced prior to AD 1200, it would appear that 20th-century global warming has had absolutely no effect on drought conditions in this part of the world.

More recently, Tian et al. (2006) derived a 31-century high-resolution δ18O record of aridity, which they obtained from sediments extracted from Steel Lake (46°58'N, 94°41'W) in north-central Minnesota, USA. Among their findings, they note that "the region was relatively dry during the Medieval Climate Anomaly (~1400-1100 AD) and relatively wet during the Little Ice Age (~1850-1500 AD), but that the moisture regime varied greatly within each of these two periods." Their most striking finding of all, however, was the fact that "drought variability was anomalously low during the 20th century." In fact, it was so depressed, as they describe it, that "~90% of the variability values during the last 3100 years were greater than the 20th-century average."

In one final paper covering nearly the entire Holocene period, Springer et al. (2008) derived a multi-decadal-scale record of drought in east-central North America based on Sr/Ca ratios and δ13C data obtained from stalagmite BCC-002 from Buckeye Creek Cave (BCC), West Virginia (USA) that "grew continuously from ~7000 years B.P. to ~800 years B.P." and then again "from ~800 years B.P. until its collection in 2002." In analyzing the record, they identified seven significant Mid- to Late-Holocene droughts, six of which "correlate with cooling of the Atlantic and Pacific Oceans as part of the North Atlantic Ocean ice-rafted debris [IRD] cycle, which has been linked to the solar irradiance cycle," as per Bond et al. (2001). In addition, they determined that the Sr/Ca and δ13C time series "display periodicities of ~200 and ~500 years and are coherent in those frequency bands." They also say "the ~200-year periodicity is consistent with the de Vries (Suess) solar irradiance cycle," and they "interpret the ~500-year periodicity to be a harmonic of the IRD oscillations." Noting further that "cross-spectral analysis of the Sr/Ca and IRD time series yields statistically significant coherencies at periodicities of 455 and 715 years," they go on to note that "these latter values are very similar to the second (725-years) and third (480-years) harmonics of the 1450 500-years IRD periodicity."

The five researchers thus conclude their report by saying their findings "corroborate works indicating that millennial-scale solar-forcing is responsible for droughts and ecosystem changes in central and eastern North America (Viau et al., 2002; Willard et al., 2005; Denniston et al., 2007)," adding that their high-resolution time series now provide even stronger evidence "in favor of solar-forcing of North American drought by yielding unambiguous spectral analysis results."

Given the above findings, it is clear that there is nothing unusual, unnatural, or unprecedented about recent droughts in the Central United States. Droughts of greater duration and intensity have occurred numerous times in the past, eclipsing anything that has been observed in the modern record. Claims of increasing future drought as a result of global warming are therefore not supported by real-world data, as modern global warming, if anything, has tended to lessen drought conditions throughout the central third of the United States.

References
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Last updated 13 February 2013