Air Pollution Holding Back Global Warming

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Over the eastern half of the U.S., a massive patch of air pollution held back the immediate impacts of global warming for many decades, according to a new report by climate scientists from Harvard University. The thick cloud of particulates, blamed for acid rain and a variety of human health problems, helped to mitigate rising temperatures, compared to elsewhere on the planet, and kept high temperatures especially restrained during summer and autumn months.

Over the 100-year period from 1906 to 2005, temperatures around the world climbed roughly 0.8 degrees Celsius. But temperatures across the eastern U.S. dropped around one degree from 1930 to 1990, a difference the researchers attribute in large part to pollution from coal-fired power plants and industrial production.

The research not only helps explain disparities in global warming’s impact in different regions around the world, but it may provide insights that help shape environmental policies in developing nations, where air pollution controls often remain relatively limited.

Sulfur emissions, formed by small particles known as sulfates, are best known for causing acid rain, which eats away at buildings and creates a range of health problems for people. Until it was targeted by federal regulators under the Clean Air Act of 1970, this particle pollution formed a thick blanket over eastern half of the United States. The problem reached its worst in 1980, when levels of particulate pollution were twice what they are today.

But this cloud of particulates also helped to temporarily keep temperatures down, creating what the Harvard researchers labeled a “warming hole” that covered states from Texas to Maine. “The primary driver of the warming hole is the aerosol pollution—these small particles,” said the study’s lead author Eric Leibensperger, who completed the research as a graduate student in applied physics at Harvard. “What they do is reflect incoming sunlight, so we see a cooling effect at the surface.”

The particles also help to draw together cloud droplets, which ratchets up their cooling effect in atmosphere, because the watery cloud droplets also reflect the sun’s energy.

Unlike greenhouse gases such as carbon dioxide and methane, which persist in the atmosphere for decades or centuries, most of the particulate pollution only stays airborne for roughly a week. Its impacts are therefore distributed unevenly, depending on short-term weather conditions.

The study, published this week in the journal Atmospheric Chemistry and Physics, based its conclusions on decades worth of pollution data and relied on Harvard’s GEOS-Chem model, and climate data from NASA’s Goddard Institute for Space Studies.

As the industrial air pollution was cut, greenhouse warming across the U.S. accelerated, the researchers found. By 2010, two-thirds of the cooling effect had dissipated along with the particulate pollution. The area in the warming hole was only 0.3 degrees Celsius cooler, compared to a full degree during earlier decades.

The Harvard scientists noted that fighting air pollution should not make global warming worse in the U.S. in coming years. “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,” said researcher Daniel Jacob (http://www.seas.harvard.edu/directory/djj), professor of Atmospheric Chemistry and Environmental Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

“No one is suggesting that we should stop improving air quality,” agreed co-author Loretta J. Mickley, a senior research fellow in atmospheric chemistry at SEAS, “but it’s important to understand the consequences.”

Not all particulate pollution had a cooling effect, the researchers found. Smaller particulates reflected the sun’s rays, but black soot aerosols had the opposite effect and actively trapped heat from the sun in the atmosphere.

The cooling effects are short-lived and have dramatic downsides. Past studies have found that sulfate particles harm the Earth’s ozone layer, which helps block harmful ultraviolet rays from the sun. This new information is especially important as developing countries work to shape their environmental laws to combat pollution and climate change. When particulate pollution falls, these countries may find they have to prepare for warming effects that have been stalled by the cloud cover in their regions.

“Something similar could happen in China, which is just beginning to tighten up its pollution standards,” said Mickley. “China could see significant climate change due to declining levels of particulate pollutants.”

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