'Warming Holes' Delayed Global Warming In Some US Regions
April 27, 2012

‘Warming Holes’ Delayed Global Warming In Some US Regions

Certain areas of the United States were spared the effects of climate change thanks to the presence of tiny particles in the atmosphere, suggests new research from climate scientists at the Harvard School of Engineering and Applied Sciences (SEAS).

Lead author Eric Leibensperger, a graduate student in applied physics at SEAS at the time of the study, and principal investigator Daniel Jacob, a professor of atmospheric chemistry and environmental engineering at SEAS and a professor of earth and planetary sciences at Harvard, used a 50 year model to study the effect of particulate pollution on regions in the eastern US, the school said in a Thursday press release.

They discovered that the particles formed what they are calling a "warming hole" or a cold patch where the impacts of global warming were temporarily obscured, due to the way they interacted with clouds. Those particles of pollution can act as nucleation sites for cloud droplets, they said, which ultimately reflects more sunlight than they would have individually. The interaction between the pollutants and the cloud cover resulted in greater cooling of surface temperature.

"Since the early 20th century, global mean temperatures have risen -- by approximately 0.8 degrees Celsius from 1906 to 2005 -- but in the U.S. 'warming hole,' temperatures decreased by as much as 1 degree Celsius during the period 1930-1990," the SEAS press release said. "U.S. 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."

"Besides confirming that particulate pollution plays a large role in affecting U.S. regional climate, the research emphasizes the importance of accounting for the climate impacts of particulates in future air quality policies," it added.

The study was the result of analysis of two different, complex models of our planet's systems. The GEOS-Chem model, which was originally created at Harvard, was used for pollution data, and the general circulation model developed by NASA's Goddard Institute for Space Studies (GISS) was used for climate-related information.

"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," Leibensperger said. "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."

"Something similar could happen in China, which is just beginning to tighten up its pollution standards. China could see significant climate change due to declining levels of particulate pollutants," added study co-author Loretta J. Mickley, a senior research fellow in atmospheric chemistry at SEAS. "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."

Joining Leibensperger, Jacob, and Mickley on the study were co-authors Wei-Ting Chen and John H. Seinfeld of the California Institute of Technology; Athanasios Nenes of the Georgia Institute of Technology; Peter J. Adams of Carnegie Mellon University; David G. Streets of the Argonne National Laborator; Naresh Kumar of the Electric Power Research Institute (EPRI); and David Rind of the GISS.