During the 20th century, the average temperature of the continental United States rose by almost 1 degree Fahrenheit (0.5 degree Celsius) -- everywhere, that is, except in the Southeast.
There, until the 1980s, the temperature actually decreased slightly. Climate scientists dubbed this peculiar phenomenon the "warming hole," and it was the cause of much speculation. But beginning in the 1990s, temperatures in the Southeast began to warm again, and in the early years of the 21st century this warming has accelerated.
A new study published in the journal Remote Sensing presents evidence that a significant improvement in air quality in the region may have contributed to the disappearance of the warming hole after about 1990 -- and that other polluted regions outside the United States, such as China and India, may experience the same phenomenon.
One major factor in poor air quality is airborne aerosols -- tiny particles of dust, soot from wood burning, coal and oil combustion, or sulfates created by precursor gases emitted from factories and car exhaust, to name a few sources. Aerosols can decrease temperature by dimming sunlight at Earth's surface and by increasing the amount and lifetimes of clouds, which reflect sunlight back into space.
After the warming hole mysteriously disappeared, various studies proposed possible causes: changes in cloud cover, precipitation or in the amount of aerosols produced by air pollution. In 2006, the U.S. Environmental Protection Agency (EPA) began implementing a more stringent cap on the concentration of aerosol particles smaller than about 1/10,000th of an inch (2.5 micrometers) in diameter. To comply with the regulation, many U.S. power utilities and industrial companies began reducing their use of coal and installing filters to reduce emissions.
A similar change to temperature trends occurred in Europe in the 1980s after new regulations improved air quality there. Because reduced aerosol particle concentrations allow more sunlight to reach Earth's surface, the scientists hypothesized that the improvements in U.S. air quality could also be responsible for the temperature change over the Southeast.
To test this hypothesis, a team led by Mika Tosca, a researcher at NASA's Jet Propulsion Laboratory in Pasadena, California (who is now with the School of the Art Institute of Chicago), used three surface temperature data sets. The data sets were compiled by the University of Delaware, the University of California (UC) at Berkeley, and the Global Historical Climatology Network (which compiles surface temperature and precipitation data). They also used aerosol data from two satellite instruments: the Multi-angle Imaging SpectroRadiometer (MISR) instrument on NASA's Terra satellite, launched in 1999, and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite, a joint mission between NASA and the French space agency, CNES, launched in 2006.
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Image via NASA Jet Propulsion Laboratory
