Starting at the U.S.-Canadian border and working our way south, we begin with the study of Fritz et al. (2000), who utilized data derived from sediment cores retrieved from three North Dakota lakes to reconstruct a 2000-year history of drought in this portion of the Northern Great Plains. This work suggested, in their words, "that droughts equal or greater in magnitude to those of the Dust Bowl period were a common occurrence [our italics] during the last 2000 years." Hence, their real-world data demonstrate that, if anything, the modest warming that released the planet from the global chill of the Little Ice Age may have initiated a period of less frequent and severe droughts.

Also working in the Northern Great Plains, but extending down into South Dakota, Shapley et al. (2005) developed a 1000-year hydroclimate reconstruction from local bur oak tree-ring records and various lake sediment cores. Based on this record, they determined that prior to 1800, "droughts tended towards greater persistence [our italics] 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.

The above observations are significant because the United States' Northern Great Plains is an important agricultural region, providing a significant source of grain for both local and international consumption. However, the region is susceptible to periodic extreme droughts that tend to persist longer than those in any other part of the country (Karl et al., 1987; Soule, 1992); and because of this fact, 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.

Continuing our southward trek, we encounter the work of Forman et al. (2005), who 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 [our italics], and signify droughts of greater severity and persistence [our italics] than thus far instrumentally recorded." Consequently, their study 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.

In a study that covered the entirety 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.

Working in pretty much the same area some seven years earlier, Woodhouse and Overpeck (1998) reviewed what we know 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, we might add, irrespective of whatever the air's CO2 concentration or temperature might be doing in future years.

In concluding this mini-review, we focus on the work of Mauget (2004), who 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 rightly 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 stretch 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." What is more, 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."

Do these 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." And that shift, as we hardly need to emphasize, is away from the droughty conditions long predicted to result from CO2-induced global warming in this important agricultural region of the United States.

What more can we say? Climate alarmists have it all wrong, as do the models upon which they base their errant claims. Modern global warming, if anything, has tended to lessen drought conditions throughout the central third of the United States.

References
Daniels, J.M. and Knox, J.C. 2005. Alluvial stratigraphic evidence for channel incision during the Mediaeval Warm Period on the central Great plains, USA. The Holocene 15: 736-747.

Forman, S.L., Marin, L., Pierson, J., Gomez, J., Miller, G.H. and Webb, R.S. 2005. Aeolian sand depositional records from western Nebraska: landscape response to droughts in the past 1500 years. The Holocene 15: 973-981.

Fritz, S.C., Ito, E., Yu, Z., Laird, K.R. and Engstrom, D.R. 2000. Hydrologic variation in the Northern Great Plains during the last two millennia. Quaternary Research 53: 175-184.

Karl, T., Quinlan, F. and Ezell, D.S. 1987. Drought termination and amelioration: its climatological probability. Journal of Climate and Applied Meteorology 26: 1198-1209.

Laird, K.R., Fritz, S.C. and Cumming, B.F. 1998. A diatom-based reconstruction of drought intensity, duration, and frequency from Moon Lake, North Dakota: a sub-decadal record of the last 2300 years. Journal of Paleolimnology 19: 161-179.

Manabe, S., Milly, P.C.D. and Wetherald, R. 2004. Simulated long-term changes in river discharge and soil moisture due to global warming. Hydrological Sciences Journal 49: 625-642.

Manabe, S. and Wetherald, R.T. 1987. Large-scale changes of soil wetness induced by an increase in atmospheric carbon dioxide. Journal of the Atmospheric Sciences 44: 1211-1235.

Mauget, S.A. 2004. Low frequency streamflow regimes over the central United States: 1939-1998. Climatic Change 63: 121-144.

Murray, A.S. and Wintle, A.G. 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32: 57-73.

Rind, D., Goldberg, R., Hansen, J., Rosenzweig, C. and Ruedy, R. 1990. Potential evapotranspiration and the likelihood of future drought. Journal of Geophysical Research 95: 9983-10004.

Rosenzweig, C. and Hillel, D. 1993. The Dust Bowl of the 1930's: Analog of greenhouse effect in the Great Plains? Journal of Environmental Quality 22: 9-22.

Shapley, M.D., Johnson, W.C., Engstrom, D.R. and Osterkamp, W.R. 2005. Late-Holocene flooding and drought in the Northern Great Plains, USA, reconstructed from tree rings, lake sediments and ancient shorelines. The Holocene 15: 29-41.

Soule, P.T. 1992. Spatial patterns of drought frequency and duration in the contiguous USA based on multiple drought event definitions. International Journal of Climatology 12: 11-24.

Woodhouse, C.A. and Overpeck, J.T. 1998. 2000 years of drought variability in the central United States. Bulletin of the American Meteorological Society 79: 2693-2714.