Increased air pollution may have delayed global warming in the eastern U.S. back in the late 20th century.
A higher incidence of particulate air pollution (aerosols) in the eastern United States between 1930 and 1990 (peaking around 1980) created a "warming hole" over the eastern United States late in the 20th century.
Temperature anomaly trend between 1930-1990. Image from GISS.
This "warming hole" was an area where warming that would be expected from increasing greenhouse gases was delayed, according to climate researchers from Harvard University.
While greenhouse gases like carbon dioxide and methane warm the Earth's surface, tiny particles in the air (such as aerosols) can have the reverse effect on regional scales, according to the Harvard School of Engineering and Applied Sciences press release.
Aerosol pollution reflects incoming sunlight, causing a cooling effect at the surface.
Clouds in clean air are composed of a relatively small number of large droplets (below left). As a consequence, the clouds are somewhat dark and translucent. In air with high concentrations of aerosols, water can easily condense on the particles, creating a large number of small droplets (right). These clouds are dense, very reflective, and bright white. Image courtesy of NASA.
Air pollution concentrations in the eastern U.S. began to diminish in the 1980's and 90's after the passage and changes to the Clean Air Act.
Excerpt from the Harvard press release......
"For the sake of protecting human health and reducing acid rain, we've now cut the emissions that lead to particulate pollution," he adds, "but these cuts have caused the greenhouse warming in this region to ramp up to match the global trend."
At this point, most of the "catch-up" warming has already occurred.
"No one is suggesting that we should stop improving air quality, but it's important to understand the consequences. Clearing the air could lead to regional warming," says co-author Loretta J. Mickley, a Senior Research Fellow in atmospheric chemistry at SEAS.
The analysis was based on a combination of two complex models of Earth systems. The results of this study were also published in the journal Atmospheric Chemistry and Physics.
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