Did pesticides help cool the South after World War II?
(Photo courtesy USDA Natural Resources Conservation Service.)
One of the few places on Earth where temperatures cooled during the 20th century was across the southeastern United States, where average readings dropped by as much as 1–2°C. A group based at Environment Canada suggests that the widespread use of dichloro-diphenyl-trichloroethane (DDT) and other pesticides during the mid-20th century may have played a small but noteworthy role in the South's cooling trend. The study, led by Jianmin Ma, appeared on 9 October in the online version of Environmental Science Technology.
The group examined usage statistics for two organochlorine pesticides: DDT, which was banned in 1972, and toxaphene, a common replacement for DDT that was restricted in 1982. About 40% of the global mass of both these pesticides was applied in the United States, mostly across cotton-growing areas of the Deep South. While much of the pesticide dissipated within a few days, large amounts entered the soil, where the pesticide can remain for many years. As the molecules gradually volatilize and evaporate, they draw energy from the surrounding air, providing a cooling effect.
For the period 1947–1976, Ma and colleagues found strong inverse correlations between average air temperature and the amount of DDT and toxaphene present in soil residue across five southeastern states. The pesticides only account for about 3–5% of the total temperature drop observed, the authors note, so "it is likely that the strong cooling trend in the southeastern U.S. was induced largely by natural climate variability." However, given that many other pesticides could have a similar added effect, "we postulate that the use of [these pesticides] and the cooling trend is not a coincidence." The team is now studying temperature trends in other areas of heavy pesticide use, such as eastern China, where weak summertime cooling was observed from 1950 to 2000.