New Study Discovers the Worst Drought of the Millennium Occurred in 1934

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According to the National Aeronautics and Space Administration (NASA), the worst drought of the last thousand years occurred in 1934 at the height of the Dust Bowl and Great Depression. Scientists from NASA and the Lamont-Doherty Earth Observatory found that 1934 had the driest and most widespread drought in a millennium. It was 30 percent harsher than the runner-up which occurred in 1580 and extended across 71.6 of the western North American continent. To get an idea of its severity, the drought of 2012 only had an average extent of 59.7 percent.

Scientists used both modern records and tree-ring records from the years 1000 to 2005 to draw their conclusions. Ben Cook from NASA’s Goddard Institute for Space Studies in New York, the main author of the study, stated that 1934 was the worst drought by a large margin and that two sets of conditions fostered its incredible extent and severity. The first condition was the presence of a high pressure system in winter that sat over the U.S. west coast. That pressure system turned away wet weather.

The second condition was the dust storms. These dust storms are largely believed to be the fault of humans and poor land management practices. Dust storms suppress rainfall, leading to the worsening of drought conditions already in place. These two elements, the high pressure system and the dust storms, combined to bring about the worst drought of the millennium to almost the entire United States.

As a result, the study has scientists speculating about the effect of climate change on droughts. A recent report from the Intergovernmental Panel on Climate Change said that the shifting climate is only likely to make North American droughts worse. They might be especially severe for the southwest as well as the central plains. Scientists are especially interested in the human impact upon droughts and compiling a thousand year drought history is one way that they can separate the role that humans play versus natural weather patterns.

For example, scientists can draw upon their study of the 1934 drought to understand the current droughts occurring in California and the western U.S. In both cases, winter storms that would normally affect areas such as California were turned northward by high pressure systems in the atmosphere. Since wintertime storms provide much of the moisture for these locations, the lack of rainfall has led to severe droughts. On top of that, these high pressure systems are hard to predict in computer models.

Monthly 700 hPa vertical velocity anomalies (Pa/s) from October of 1933 through March of 1934, calculated from the 20th Century Reanalysis. All anomalies are calculated relative to the baseline period of 1931–1990.

Monthly 700 hPa vertical velocity anomalies (Pa/s) from October of 1933 through March of 1934, calculated from the 20th Century Reanalysis. All anomalies are calculated relative to the baseline period of 1931–1990. From Cook et al, 2014.

The study from NASA argues that colder sea temperatures in the Pacific Ocean, the phenomena of La Niña, likely brought about the Dust Bowl during the 1930s. In a typical La Niña year, the northwest receives more rain than usual while the southwestern states are normally drier. In the winter of 1933-1934, however, this pattern was overridden by the high pressure system. Then, as the winter ended, that high pressure system moved east, halting the spring and summer rains that usually fall on the plains. The manmade dust storms made conditions worse and even allowed the drought to spread farther east.

Dust storms, like those of the 1930s, are problematic because they prevent sunlight from reaching the surface of the Earth, preventing the evaporation that helps to form rainclouds. The good news, though, is that dust storms are not a problem in the U.S. today, mostly because of improved agricultural practices. Scientists warn, however, that the risk of droughts is expected to increase over time.

Dust Storms; "One of South Dakota's Black Blizzards, 1934"

Dust Storms; “One of South Dakota’s Black Blizzards, 1934”. Photo from National Archives FDR Library.

References

Cook, B. I.R. Seager, and J. E. Smerdon (2014), The worst North American drought year of the last millennium: 1934Geophys. Res. Lett.41, doi:10.1002/2014GL061661.

“NASA Study Finds 1934 Had Worst Drought of Last Thousand Years.” NASA. NASA, 14 Oct. 2014. Web. 14 Nov. 2014. <http://www.nasa.gov/content/goddard/1934-had-worst-drought-of-last-thousand-years/#.VGafk4eTf6g>.