SAN FRANCISCO — It may seem counterintuitive, but cleaner air could actually be exacerbating global warming trends.
The soot and other particles that make up air pollution tend to scatter light back out into space. As countries around the globe have cleaned up their act, there are fewer particles to reflect light, meaning more sunlight is reaching the Earth's surface and warming it, Martin Wild, a researcher at ETH Zurich in Switzerland, said Tuesday (Dec. 15) here at the annual meeting of the American Geophysical Union.
That's not to say people can blame global warming on the clearer skies — the underlying cause of climate change is excess carbon emissions into the atmosphere. But air pollution may have counteracted some of that warming caused by excess carbon in the atmosphere, Wild said. [In Photos: The World's 10 Most Polluted Places]
Perhaps surprisingly, the sunlight reaching Earth's surface hasn't remained constant, at least not on the timescale of human civilization, Wild said.
"The sunlight we receive at the Earth's surface is not stable over the years, but undergoes substantial decadal changes," Wild said at a news briefing.
To understand what's going on, Wild looked at the level of solar radiation at 56 spots across Europe between 1939 and 2012. There were big peaks in that period. It turned out that solar radiation spiked in the 1950s, and then decreased until the 1980s, when it started to uptick again.
But what could have been the source? Sunspots, which look almost like moles on the face of the sun, are areas of intense magnetic activity which are cooler than the surrounding regions of the sun. Because they emit less radiation at these cooler temperatures, the number and extent of sunspots can change how much light reaches the Earth. However, cycles between high and low sunspot levels are much shorter than the timescales of the global dimming and brightening trend, and these cycles weren't correlated with those larger changes, Wild and his colleagues found.
It turned out that there was a huge spike in sulfur emissions up until the 1980s, at which point sulfur pollutants dropped, Wild said. The drop in sulfur emissions corresponds with the introduction of legislation in a number of countries to reduce air pollution, Wild said. (Diesel exhaust often contains high levels of sulfur compounds.)
It's not surprising to scientists that higher levels of pollutants could dim the Earth's surface: After volcanic eruptions, for instance, the huge amounts of sulfur spewed into the atmosphere can cool the planet for a few years. That's because the tiny particles can scatter and absorb light, reducing how much of that light ultimately reaches the Earth's surface, Wild said.
Air pollution can also alter the light reaching Earth in other ways.
Here's how the cloud brightening works: Aerosols that are normally in the air are insoluble and act as seeds for water droplets to condense around, and ultimately to form clouds. Polluted air, on the other hand, contains water-soluble particles, leading to clouds with more, yet smaller, water droplets. These numerous and tiny droplets provide more surfaces for light to reflect off, and voilà — brighter clouds.
These brighter clouds also reduce how much light reaches the ground, he added.
What's more, this unintentional geoengineering may have already impacted global warming, Wild said. Global temperatures held fairly constant from the 1950s to the 1980s, and warming only accelerated starting in 1985, when the global brightening seems to have begun, Wild reported in a study published this month in the journal WIREs Climate Change.
He also sees evidence that this unintentional geoengineering affected the world's hemispheres differently. Temperatures held steady until the mid-1980s in the Northern Hemisphere, where most of the world's population lives, and spiked up sharply afterward. By contrast, in the "relatively more pristine" Southern Hemisphere, which has much fewer people, the region experienced a steadier uptick in warming. That suggests that air pollution had a measurable effect on global warming on the globe's northern half, and less so in the southern half, he said.
Of course, there are other reasons to curb pollution. Smog and other air pollution kills millions every year.
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