NewsAugust 15, 2012

2012 Drought Inches Up In U.S. Historical Rankings

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Andrew Freedman

By Andrew Freedman

The costly and ongoing drought that stretches across a majority of the lower 48 states worsened during the past month, and by one measure it now ranks fifth on the top 10 list of the largest droughts ever recorded in the U.S., according to a new report released Wednesday from the National Climatic Data Center (NCDC). The drought is affecting the broadest swath of land and has the greatest intensity of any drought since 1956, and is comparable to the Dust Bowl era droughts of the 1930s as well, NCDC scientists reported.

Graph showing changes in the percent area of the U.S. very wet or very dry, with spikes in drought seen since 1999. Click on the image for a larger version.Credit: NCDC.

The drought footprint is currently the largest in the 13-year record of the U.S. Drought Monitor. To compare this drought with droughts that occurred prior to this period of record, climate scientists use drought indices such as the Palmer Drought Severity Index, or PDSI, which measures the balance between moisture demand and moisture supply.

Based on PDSI data, 57.2 percent of the lower 48 states were experiencing “moderately to extremely dry” conditions at the end of July, which was up from 55.2 percent at the end of June. This places the 2012 drought in fifth place based on its aerial expanse, otherwise known as the drought footprint. According to NCDC, the last drought of comparable size occured in 1956. The drought's intensity has also worsened, with a 5 percent increase in the area of the country experiencing “severe” to “extreme” drought, to 38 percent. By this measure, the drought ranks as the sixth worst drought on record.

The NCDC report characterized the current drought as part of a longer-term drought pattern that began in 1999, with a series of intense, regional droughts occurring since. Only the West has consistently experienced drought during this time period, though.

Palmer Z Index for the primary corn and soybean growing area of the U.S., showing where the 2012 drought ranks compared to other droughts since 1900. Click on image for a larger version.
Credit: NCDC.

The drought is having major impacts on agricultural producers in the High Plains and Midwest, particularly corn and soybean farmers. According to the report, the combination of extreme heat and dry weather during July took a heavy toll on crops. July was the warmest month on record in the contiguous U.S., beating out the Dust Bowl-era year of 1936 for that dubious distinction. Excessive heat increases the transport of moisture out of soils and vegetation, and into the atmosphere, thereby drying the environment faster than would occur in cooler conditions.

The main corn and soybean agricultural belt in the U.S. experienced its second-warmest and sixth-driest April-to-July period, coming in fourth according to one of the Palmer Drought indices, after 1934, 1936, and 1988. As of the end of July, about 86 percent of this vital agricultural region was experiencing moderate-to-extreme drought, which trailed only the droughts of 1988 and the 1930s.

Fortunately, the weather pattern that has dominated the summer across North America, with a giant, seemingly immovable “heat dome” of High Pressure parked over the Central U.S., is showing signs of breaking down. The outlook for an El Nino event during the fall and winter may bode well for increased rainfall in parts of the drought-stricken region, particularly across the southern tier of the country.