Science Briefs
Modeling the Dust Bowl Climate Forcings
Recurrent periods of drought are a common feature of North American climate, often the result of colder than normal sea surface temperatures (SSTs) in the eastern tropical Pacific (so-called La Niña conditions). One such drought, the "Dust Bowl" of the 1930s, resulted in widespread crop failure, dust storms, and the displacement of thousands of people.
Image at right: A dust storm strikes Powers County, Colorado, in April 1935. Image: Library of Congress, FSA-OWI Collection, Repro. Num. LC-USF343-001617-ZE DLC. View larger image.
The Dust Bowl drought was atypical for a North American drought in many ways, most notably the fact that it was centered over the Great Plains rather than in the southwest and was accompanied by large scale dust storms that were unprecedented in the historical record. The dust storms themselves resulted from a combination of dry conditions, poor land use practices, and large scale crop failures that exposed easily erodible bare soil to the strong winds of the Great Plains. Many climate models, however, have difficulty reproducing the precipitation pattern of the Dust Bowl drought using SSTs alone. Could the dust storms themselves explain the anomalous drought?
Figure 1, at left: Ensemble mean differences in total atmospheric dust loading, g/m2, Experiment 3 (SST+Dust) minus Experiment 2 (SST only). Outlined are the eight grid boxes that constitute the new dust source in the SST+Dust experiments. View larger image.
The impact of dust on precipitation is an active area of research. Dust in the atmosphere reflects sunlight back to space, reducing temperatures at the surface as well as evaporation. If evaporation is sufficiently reduced, then the supply of moisture for cloud formation and precipitation can also be severely reduced, resulting in decreased precipitation. Thus, there is a strong potential that the added dust in the atmosphere during this drought could have intensified the drought by reducing precipitation over the plains.
Figure 2, at right: Spatial extent and magnitude of precipitation anomalies for 1932-1939. Shown are data from the Global Historical Climatology Network (GHCN) dataset and output from the two GISS model experiments (SST only in middle panel and SST+Dust in bottom panel). For GHCN data, anomalies are relative to GHCN data for 1920-1929. For model experiments, anomalies are relative to the SST forcing (1920-1929) experiment. View larger image.
To test the effect of atmospheric dust, we ran the GISS climate model with observed SSTs for 1932-1939, with and without the presence of a dust source over the Great Plains. Figure 1 shows the increase in model atmospheric dust when this source is added, relative to a simulation without the added dust source. Figure 2 shows the observed precipitation anomaly (top panel), the model generated precipitation anomaly with SST forcing only (middle panel), and the model generated precipitation anomaly with the effects of observed SSTs and the added dust source (bottom panel).
When the effect of dust is included in our climate model, we get a much more realistic simulation of the drought. The SST only drought is centered too far south and is not dry enough compared to observations. When the dust source is added, the drying intensifies and the center of the drought moves north over the Great Plains. This suggest that human land degradation was an integral part of the Dust Bowl story.
Since then, the U.S. has developed strict soil conservation and erosion control measures under the auspices of the Soil Conservation Service to prevent events like the Dust Bowl from reoccurring. In the developing world, however, as population pressure and climate changes pushes farmers onto more and more marginal land, the potential for reoccurence of Dust Bowl-like conditions in these regions is increasingly likely. Vulnerable areas include both interior China and semi-arid regions of Africa, where the landscape is particularly vulnerable.
Reference:
Cook, B.I., R.L. Miller, and R. Seager, 2008: Dust and sea surface temperature forcing of the 1930s "Dust Bowl" drought. Geophys. Res. Lett., 35, L08710, doi:10.1029/2008GL033486.
Contact
Please address all inquiries about this research to Dr. Benjamin Cook.
