Particulate Pollution Delayed Climate Change Over Eastern United States

Climate US© Harvard

Eco Catch 22?

It's important to clean up the air because of things like acid rain and respiratory diseases, but under certain conditions, particulate matters in the air can reflect some heat back into space and create a regional cooling effect. It's kind of an environmental catch 22: Either you get bad stuff caused by air pollution, or you get bad stuff caused by global warming.

That's what Climate scientists at the Harvard School of Engineering and Applied Sciences (SEAS) have found studying the Eastern US. They have discovered that "particulate pollution in the late 20th century created a 'warming hole' over the eastern United States—that is, a cold patch where the effects of global warming were temporarily obscured." But thanks to relatively recent cuts that we've made in these PM emissions, the warming in this region should ramp up to match the global trend.

Of course, this is not a real catch 22, since this pollution is only masking some effects of global warming regionally and temporarily. Any benefits from this air pollution were minimal compared to their downsides, and I'm absolutely not implying that we should pollute the air to fight global warming.


Here's the highlights from this research:

Until the United States passed the Clean Air Act in 1970 and strengthened it in 1990, particulate pollution hung thick over the central and eastern states. Most of these particles in the atmosphere were made of sulfate, originating as sulfur emissions from coal-fired power plants. Compared to greenhouse gases, particulate pollution has a very short lifetime (about 1 week), so its distribution over the Earth is uneven.

"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."

This effect has been known for some time, but the new analysis demonstrates the strong impact that decreases in particulate pollution can have on regional climate.

The researchers found that interactions between clouds and particles amplified the cooling. Particles of pollution can act as nucleation sites for cloud droplets, which can in turn reflect even more sunlight than the particles would individually, leading to greater cooling at the surface. [...]

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. 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.

Via Harvard

See also: Beautiful Interactive Animated Wind Map of the U.S.

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