The Long-Term Perspective
By examining sediment deposits in lake beds and measuring old-growth tree rings throughout the Northern Great Plains region, scientists are gaining a longer-term perspective on patterns of drought in North America. The theory is that when lake levels were high, the water was fresher, and when the lake levels were low, the water would have been saltier, which can be measured in the sediment layers deposited during a given period. This theory was confirmed by scientists who studied Moon Lake (North Dakota) sediment deposits from the 1890s and 1930s, both known to be extreme drought years from historical records (Laird et al. 1996). Likewise, Giant Sequoia trees in San Joaquin Valley, California, exhibit less growth during very dry years (Laird et al. 1996).
Analysis of the data from these sources suggests that extreme drought
events in North America were much more frequent, severe, and longer
lasting during the period from about 2,300 years ago to about 1200 AD
(Laird et al. 1996). The data show that there were quite a few droughts
that persisted for more than a century. The Moon Lake region, for
example, shows signs of drought lasting from about 200 to 370 AD, from
700 to 850 AD, and again from 1000 to 1200 AD (Laird et al. 1996).
From sediment data collected in lake beds, scientists developed a long-term profile of rainfall patterns over the North American Great Plains. They found that before 1200 AD, droughts occurred frequently and often lasted for decades or even more than a century. Then, after 1200 AD, climate conditions grew relatively much wetter. (Graph by Robert Simmon, based on data from Laird, et. al.)
Around 1200 AD, with the onset of the Little Ice Age, the climate in North America became generally cooler and wetter and, with the exception of relatively brief periods of drought, these wet conditions have predominated ever since (Laird et al. 1996). Why were North American dry spells before 1200 AD so severe and long-lived? Scientists suggest the answer to that question may have to do with cycles of solar activity and fluctuations of carbon dioxide in the Earth's atmosphere (Laird et al. 1996). Whatever the cause, historical evidence suggests that for about 1,500 years there was a pattern of persistent upper-level anti-cyclones (high-pressure systems) over the Great Plains region.
Could we see a return to the mode of climate behavior that North America experienced before 1200 AD? Researchers cannot answer that question now, but there is mounting concern among Earth scientists that as the level of atmospheric carbon dioxide rises, and average global temperature with it, we will see profound effects on oceanic and atmospheric circulation patterns that will, in turn, significantly affect precipitation patterns. As they continue to analyze the growing body of data from both current and historical data sources, they will also continue to develop newer and better computer models to help them improve drought forecasts-in both timing and severity-months or even years ahead of time.
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