The Dust Bowl drought of the 1930s was one of the worst environmental disasters of the Twentieth Century anywhere in the world. Three million people left their farms on the Great Plains during the drought and half a million migrated to other states, almost all to the West. But the Dust Bowl drought was not meteorologically extreme by the standards of the Nineteenth and Twentieth Centuries. Indeed the 1856-65 drought may have involved a more severe drop in precipitation. It was the combination of drought and poor land use practice that created the environmental disaster.
|Dust storm approaching Elkhart, Kansas, May 1937|
Much of the Plains had been plowed up in the decades before the 1930s as wheat cropping expanded west. Alas, while natural prairie grasses can survive a drought the wheat that was planted could not and, when the precipitation fell, it shriveled and died exposing bare earth to the winds. This was the ultimate cause of the wind erosion and terrible dust storms that hit the Plains in the 1930s. There had never been dust storms like these in prior droughts. In the worst years of the 1930s on as many as a quarter of the days dust reduced visibility to less than a mile. More soil was lost by wind erosion than the Mississippi carried to the sea. Although the numbers are not known, hundreds if not thousands of Plains residents died from ‘dust pneumonia’, a euphemism for clogging of the lungs with dirt. For wonderful books on the topic see Worster (1979, Dust Bowl: The Southern Plains in the 1930s) and Egan (2006, The Worst Hard Time).
But did the dust storms have a meteorological impact? There are two good reasons to ask this question. First we know from studies elsewhere in the world (e.g. the Sahel) that dust can impact circulation and precipitation. Second the Dust Bowl drought was unique in its spatial pattern – further north than is typical for a La Niña forced, or La Niña plus warm subtropical North Atlantic forced, drought. See our page “Was the Dust Bowl Predictable?” Did the dust storms impact either the intensity of the drought or its area of impact?
|Goodwell, Oklahoma, June 4, 1937|
We have addressed this using the Goddard Institute for Space Studies atmosphere GCM which contains a dust module that can lift up dust from the surface, transport it in the atmosphere and allow it to interact with solar and longwave radiation transfer in the atmosphere. See the GISS Science Brief, “Desert Dust, Dust Storms and Climate” and the references therein.
First we ran a small ensemble of simulations with the atmosphere model forced by 1920s sea surface temperatures (SSTs) to act as our base of comparison for the simulated 1930s. Next we created a small ensemble of simulations with the model forced by 1930s SSTs. This created a drought that, as is typical for models forced by 1930s SSTs, was centered too far into the Southwest relative to the observed drought.
Then we introduced an estimate of the increased dust source from crop failure in the 1930s. This was guided by maps of wind erosion prepared in the 1930s by the newly created Soil Conservation Service. Regions of severe wind erosion were put into the model as potential dust sources although the model’s dust module determines the actual lifting up, transport and deposition of the dust. The modeled dust emissions are around half the size of the scanty estimates from 1930s observations so the modeled climate impact may still be on the conservative side. Ben Cook has created an interesting movie of the dust loading in the GISS simulations of April to August 1934. View Movie.
Figure 1 shows a ‘box and whiskers’ plot of the precipitation anomalies over the Great Plains for the modeled 1932-39 period minus the 1920s for both observations and the two model simulations. Here we have used the anomalies for each year in the observations and each year in the model run and for each of the 5 ensemble members. The boxes contain the one standard deviation spread of the anomalies and the whiskers bound the entire distribution. The distribution for the 1920s is of course spread around zero. The SST forcing alone creates a clear drought. The combination of SST forcing and interactive dust forcing intensifies the drought.
|Figure 1: Box and whisker plots for precipitation anomalies, averaged over the central United States (30N-48N, 105W-85W). Shown are data from the GHCN dataset and output from the three GISS model experiments: SST forcing (1920-1929), SST forcing (1932-1939), and SST+Dust forcing (1932-1939). 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. Plots for the GHCN anomalies are based on 10 years (1920-1929) and 8 years (1932-1939); for the model output, each plot represents output from five member ensembles simulations; 50 years for 1920-1929 and 40 years for 1932-1939.|
Figure 2 shows maps of the observed and modeled drought. The modeled drought with just SST forcing is clearly centered too far to the Southwest. With the addition of interactive dust the drought not only intensifies but also moves further to the north, more in line with the observed pattern. Dust intensifies the drought because of a reduction of surface solar radiation by dust loading in the atmosphere which reduces the energy available for surface evaporation. The drought moves northward because of the pattern of dust transport in the southwesterly flow. A paper describing these results has recently appeared in Geophysical Research Letters. Click HERE to view.
|Figure 2: Spatial extent and magnitude of precipitation anomalies. Anomalies are relative to the same reference period 1920-1929.|
These first cut modeling experiments suggest that the Dust Bowl disaster was the result of complex interactions between humans and the environment. First changes in tropical sea surface temperatures created a drought. Poor land use practices then led to exposure of bare soil followed by wind erosion and dust storms. The dust storms interacted with radiation to make the drought worse and move it northward increasing the potential for further wind erosion. That said, even without the human role, the drought would have occurred and the human impact was limited. However the actual dust loss in the 1930s is poorly constrained and we have also ignored so far the potential impact on climate and hydrology, through the surface radiation and moisture balances, of the vegetation loss itself. Consequently the human role in altering the Dust Bowl drought remains a topic for interesting future research.