Andreadis and Lettenmaier (2006) examined 20th-century trends in soil moisture, runoff and drought over the conterminous United States with a hydro-climatological model forced by real-world measurements of precipitation, air temperature and wind speed over the period 1915-2003. This work revealed, in their words, that "droughts have, for the most part, become shorter, less frequent, less severe, and cover a smaller portion of the country over the last century," in an absolutely stunning rebuke of climate-alarmist claims concerning global warming and its effects on drought. What makes this rebuke even more impressive is that climate alarmists have long claimed earth's temperature in the latter part of the 20th century was unprecedented over the past millennium, and that NASA's James Hansen is now claiming the planet's current mean temperature is within less than one degree Centigrade of the warmest temperature of the past million years. Clearly, either the temperature-drought relationship they promote is totally wrong or their claims of current unprecedented warmth are way off base. Or, as we shall see at the conclusion of this Summary, perhaps each claim is but moderately off the mark.

Using the self-calibrating Palmer (1965) drought severity index (SCPDSI), as described by Wells et al. (2004), Van der Schrier et al. (2006) constructed maps of summer moisture availability across a large portion of North America (20-50°N, 130-60°W) for the period 1901-2002 with a spatial latitude/longitude resolution of 0.5° x 0.5°. This operation revealed, in their words, that over the area as a whole, "the 1930s and 1950s stand out as times of persistent and exceptionally dry conditions, whereas the 1970s and the 1990s were generally wet." However, they say that "no statistically significant trend was found in the mean summer SCPDSI over the 1901-2002 period, nor in the area percentage with moderate or severe moisture excess or deficit." In fact, they could not find a single coherent area within the SCPDSI maps that "showed a statistically significant trend over the 1901-2002 period," once again demonstrating that one of the major calamitous predictions of the world's climate alarmists (that more dramatic droughts accompany global warming) is found to be totally unsupported by real-world data over a vast area of North America, including the conterminous United States.

Going back considerably further in time, Fye et al. (2003) developed gridded reconstructions of the summer (June-August) basic Palmer Drought Severity Index over the continental United States, based on "annual proxies of drought and wetness provided by 426 climatically sensitive tree-ring chronologies." This work revealed that the greatest 20th-century moisture anomalies across the United States were the 13-year pluvial over the West in the early part of the century, and the epic droughts of the 1930s (the Dust Bowl years) and 1950s, which lasted 12 and 11 years, respectively; and in comparing these events to earlier wet and dry periods, they made the following points.

The 13-year pluvial from 1905 to 1917 had three earlier analogs: an extended 16-year pluvial from 1825 to 1840, a prolonged 21-year wet period from 1602 to 1622, and a 10-year pluvial from 1549 to 1558. The 11-year drought from 1946 to 1956, on the other hand, had at least twelve earlier analogs in terms of location, intensity and duration; but the Dust Bowl drought was greater than all of them ... except for a sixteenth-century "megadrought," which lasted some 18 years and was, in the words of Fye et al., "the most severe sustained drought to impact North America in the past 500 to perhaps 1000 years."

In another long-term study, Stahle et al. (2000) developed a long-term history of North American drought from reconstructions of the Palmer Drought Severity Index based on analyses of many lengthy tree-ring records. This history also revealed that the 1930s Dust Bowl drought in the United States was eclipsed in all three of these categories by the 16th-century megadrought. This incredible period of dryness, as they describe it, persisted "from the 1540s to 1580s in Mexico, from the 1550s to 1590s over the [U.S.] Southwest, and from the 1570s to 1600s over Wyoming and Montana." In addition, it "extended across most of the continental United States during the 1560s," and it recurred with greater intensity over the Southeast during the 1580s to 1590s. So horrendous were its myriad impacts, Stahle et al. unequivocally state that "the 'megadrought' of the 16th century far exceeded any drought of the 20th century." In fact, they state that a "precipitation reconstruction for western New Mexico suggests that the 16th-century drought was the most extreme prolonged drought in the past 2000 years."

Last of all, we come to the intriguing study of Herweijer et al. (2006), who begin the report of their work by noting that "drought is a recurring major natural hazard that has dogged civilizations through time and remains the 'world's costliest natural disaster'." With respect to the 20th century, for example, they report that the "major long-lasting droughts of the 1930s and 1950s covered large areas of the interior and southern states and have long served as paradigms for the social and economic cost of sustained drought in the USA." However, they add that "these events are not unique to the twentieth century," and they go on to describe three periods of widespread and persistent drought in the latter half of the nineteenth century - 1856-1865 (the "Civil War" drought), 1870-1877 and 1890-1896 - based on evidence obtained from proxy, historical and instrumental data.

With respect to the first of these impressive mid- to late-19th-century droughts, Herweijer et al. say it "is likely to have had a profound ecological and cultural impact on the interior USA, with the persistence and severity of drought conditions in the Plains surpassing those of the infamous 1930s Dust Bowl drought." In addition, they report that "drought conditions during the Civil War, 1870s and 1890s droughts were not restricted to the summer months, but existed year round, with a large signal in the winter and spring months."

Taking a still longer look back in time, the three researchers cite the work of Cook and Krusic (2004), who constructed a North American Drought Atlas using hundreds of tree-ring records. This atlas reveals what Herweijer et al. describe as "a 'Mediaeval Megadrought' that occurred from AD 900 to AD 1300," along with "an abrupt shift to wetter conditions after AD 1300, coinciding with the 'Little Ice Age', a time of globally cooler temperatures" that ultimately gave way to "a return to more drought-prone conditions beginning in the nineteenth century."

The broad picture that emerges from these observations is one where the most severe North American droughts of the past millennium were associated with the globally-warmer temperatures of the Medieval Warm Period plus the initial stage of the globally-warmer Current Warm Period. Superimposed upon this low-frequency behavior, however, Herweijer et al. find evidence for a "linkage between a colder eastern equatorial Pacific and persistent North American drought over the last 1000 years," noting further that "Rosby wave propagation from the cooler equatorial Pacific amplifies dry conditions over the USA." In addition, they report that after using "published coral data for the last millennium to reconstruct a NINO 3.4 history," they applied "the modern-day relationship between NINO 3.4 and North American drought ... to recreate two of the severest Mediaeval 'drought epochs' in the western USA."

But how is it that simultaneous global-scale warmth and regional-scale cold combine to produce the most severe North American droughts? The answer fits nicely with our view of what drives the millennial-scale oscillation of climate that produced the global Medieval Warm Period, Little Ice Age and Current Warm Period, i.e., variable solar activity. When solar activity is in an ascending mode, the globe as a whole warms; but at the same time, to quote from Herweijer et al.'s concluding sentence, increased irradiance typically "corresponds to a colder eastern equatorial Pacific and, by extension, increased drought occurrence in North America and other mid-latitude continental regions."

An important implication of these observations is that the most severe North American droughts should occur during major multi-centennial global warm periods, as has in fact been observed to be the case; and since the greatest such droughts of the Current Warm Period have not yet approached the severity of those that occurred during the Medieval Warm Period, it seems a good bet that the global temperature of the Current Warm Period is not yet as high as the global temperature that prevailed throughout the Medieval Warm Period.

And this implication, of course, begets one further implication: since the Medieval Warm Period was likely warmer than the Current Warm Period has been to date (see also, in this regard, our Medieval Warm Period Project), and since it achieved such warmth when there was approximately 25% less CO2 in the air than there is today, there is no need to believe that the historical and still-ongoing rise in the air's CO2 content has had much at all to do with the creation of our current warmth.

References
Andreadis, K.M. and Lettenmaier, D.P. 2006. Trends in 20th century drought over the continental United States. Geophysical Research Letters 33: 10.1029/2006GL025711.

Cook, E.R. and Krusic, P.J. 2004. North American Summer PDSI Reconstructions. IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series # 2004-045. NOAA/NGDC Paleoclimatology Program.

Fye, F.K., Stahle, D.W. and Cook, E.R. 2003. Paleoclimatic analogs to twentieth-century moisture regimes across the United States. Bulletin of the American Meteorological Society 84: 901-909.

Herweijer, C., Seager, R. and Cook, E.R. 2006. North American droughts of the mid to late nineteenth century: a history, simulation and implication for Mediaeval drought. The Holocene 16: 159-171.

Palmer, W.C. 1965. Meteorological Drought. Office of Climatology Research Paper 45. U.S. Weather Bureau, Washington, DC, USA.

Stahle, D.W., Cook, E.R., Cleaveland, M.K, Therrell, M.D., Meko, D.M., Grissino-Mayer, H.D., Watson, E. and Luckman, B.H. 2000. Tree-ring data document 16th century megadrought over North America. EOS, Transactions, American Geophysical Union 81: 121, 125.

Van der Schrier, G., Briffa, K.R., Osborn, T.J. and Cook, E.R. 2006. Summer moisture availability across North America. Journal of Geophysical Research 111: 10.1029/2005JD006745.

Wells, N., Goddard, S. and Hayes, M.J. 2004. A self-calibrating Palmer drought severity index. Journal of Climate 17: 2335-2351.