Particulate pollution over the eastern United States, dating from the late 20th century, has caused a colder patch where global warming has been held back.
The conclusion is based on pollution data from Harvard's GEOS-Chem model, and the climate data from the general circulation model developed by NASA's Goddard Institute for Space Studies.
"What we've shown is that particulate pollution over the eastern United States has delayed the warming that we would expect to see from increasing greenhouse gases," says Eric Leibensperger of the Harvard School of Engineering and Applied Sciences (SEAS).
"For the sake of protecting human health and reducing acid rain, we've now cut the emissions that lead to particulate pollution - but these cuts have caused the greenhouse warming in this region to ramp up to match the global trend."
Until the United States passed the Clean Air Act in 1970 and strengthened it in 1990, particulate pollution was churned out on a large scale in the central and eastern states. Most of these particles in the atmosphere consisted of sulfate, originating as sulfur emissions from coal-fired power plants.
"The primary driver of the warming hole is the aerosol pollution -- these small particles," says Leibensperger. "What they do is reflect incoming sunlight, so we see a cooling effect at the surface."
Global mean temperatures rose by around 0.8 degrees Celsius from 1906 to 2005 - but in the US 'warming hole', temperatures fell by as much as one degree Celsius between 1930 and 1990. US particulate pollution peaked in 1980 and has since been reduced by about half.
By 2010 the average cooling effect over the East had fallen to just 0.3 degrees Celsius.
"Such a large fraction of the sulfate has already been removed that we don't have much more warming coming along due to further controls on sulfur emissions in the future," says principal investigator Daniel Jacob.
But, says the team, their work has significant implications for the future climate of industrial nations that haven't yet implemented extensive air quality regulations.
"Something similar could happen in China, which is just beginning to tighten up its pollution standards," says co-author Loretta J Mickley. "China could see significant climate change due to declining levels of particulate pollutants."