The United States 1930s drought was especially severe, Many farms failed, and there was a huge migration of the country's population from the central plains to other portions of the country.
What caused the drought? Computer simulations suggest that the drought should not have been as severe as it was, and should not have been centered in the middle of the country.
Understanding the factors that caused the severity of this drought is not simply of historical interest. It may also be of guidance for current times, in the face of future global climate change.
Benjamin Cook (Lamont-Doherty Earth Observatory and NASA Goddard Institute for Space Studies, New York) and coworkers have shed light on this question. Their research suggests that while regularly oscillating ocean currents started the drought, the collapse of agriculture helped amplify the drought far beyond its usual severity.
The drought hypothesis.
The scientists' hypothesis of why the 1930s drought was especially severe is that the effects of agricultural collapse amplified the drought. This is not a fundamentally new hypothesis.
In the 1920s, agriculture spread into the Great Plains, replacing drought-resistant native grasses with drought-sensitive crops. Partially due to uninformed agricultural practices, at the onset of the drought, farms were left barren, which exposed the soil to the wind.
This soil exposure led to huge dust storms, and sent large quantities of dust ino the atmosphere. The scientists' new contribution to the drought hypothesis is to quantitatively link the impact of vegetation loss and atmospheric dust particles on the 1930s drought.
Testing the drought hypothesis.
The scientists utilized the Goddard Institute for Space Studies atmospheric general circulation model. When coupled with sea surface temperatures, this model accurately predicts other sea surface temperature-induced droughts, such as that of the 1950s.
The original model incorporates the effect of sea surface temperatures only. It does not include the effects of dust or barren landscapes.
They incorporated atmospheric dust particles into a second model, consistent with realistic 1930s estimates of 369 million metric tons of dust in the atmosphere per year. In a third model, they incorporated the presence of barren soil, again consistent with 1930s conditions.
In a fourth model, the scientists incorporated both of these effects. This should realistically mimic the conditions of the 1930s drought.
Simulation results.
As expected, the model which includes only the effects of sea surface temperatures predicts limited drying over the Great Plains during the 1930s. Incorporating devegetation into the model predicts warmer temperatures, and incorporating atmospheric dust particles predicts less rainfall.
However, the model which includes sea surface temperatures, atmospheric dust particles, and barren landscapes accurately predicts the elevated temperatures, lack of rainfall, and centralized focus of the drought on the Great Plains. In other words, the total effects of the drought are accurately predicted.
What features of this enhanced model are responsible for its improved drought predicative capability? One feature is that it accurately predicts the loss of water flux from the soil to the atmosphere, as a consequence of the lack of plant photosynthesis (energy production).
Another feature is that it accurately predicts the reduction in atmospheric heat circulation and cloud formation, reducing rainfall and shifting the drought epicenter to the central Great Plains. Combining these two features leads these two effects to work in concert, explaining why the 1930s drought was anomalously amplified.
Implications for the future.
This research suggests why the 1930s "Dust Bowl" drought was so severe in the United States. While the original data from which the scientists based their simulations is limited (e.g., the extent of crop failure), the results still show close agreement with what was actually observed.
This should serve as a warning to developing nations that are destroying their forests, exposing the underlying soil to erosion. Land degradation may amplify normal droughts, and droughts to come courtesy of global climate change, causing severe social hardship far beyond what would have otherwise been encountered.
for more information:
Cook, B. I.; Miller, R. L.; Seager, R.
Amplification of the North American "Dust Bowl" drought
through human-induced land degradation.
Proc. Natl. Acad. Sci. USA 2009, 106, 4997-5001.