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A Brief History of North American Drought: Comparing the 20th Century with the Prior Millennium
Volume 9, Number 34: 23 August 2006

Climate alarmists typically claim that CO2-induced global warming will lead to more extreme meteorological phenomena, such as droughts, floods and storms; and, therefore, whenever one of these phenomena occurs, they are quick to attribute it to the ongoing rise in the air's CO2 content. But are these claims correct? To answer this question, one must know how these phenomena varied over past centuries, when the air's CO2 concentration was essentially invariant at about three-quarters of its current value. In this editorial we pursue this course with respect to North American droughts, as described Herweijer et al. (2006) in a recent study published in The Holocene.

The three Lamont-Doherty Earth Observatory scientists begin 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, 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 distinct 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 the huge mid- to late-19th-century droughts, Herweijer et al. say that 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 have been primarily 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 "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, 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 ongoing rise in the air's CO2 content has had much to do with the production of our current warmth. On the other hand, there is substantial reason to believe that increasing solar activity may have played a significant role in 20th-century global warming.

Sherwood, Keith and Craig Idso

References
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.

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.