SAN FRANCISCO â€” It may seem counterintuitive, yet cleaner air could actually be exacerbating global warming trends.
The soot & 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 & 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 â€” the underlying cause of climate alter 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]
p> Changing light
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, yet 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 & 2012. There were huge peaks in that period. It turned out that solar radiation spiked in the 1950s, & 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 & extent of sunspots can alter how much light reaches the Earth. However, cycles between high & low sunspot levels are much shorter than the timescales of the global dimming & brightening trend, & these cycles weren't correlated with those larger changes, Wild & 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. Â (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 rad the planet for a few years. That's because the tiny particles can scatter & absorb light, reducing how much of that light ultimately reaches the Earth's surface, Wild said.
Air pollution can moreover alter the light reaching Earth in other ways.
"Polluted clouds, counterintuitively, become brighter," Wild said. "The polluted clouds can moreover stay longer in the air because their droplets are small." [In Images: Mysterious Night-Shining Clouds]
Here's how the cloud brightening works: Aerosols that are normally in the air are insoluble & act as seeds for water droplets to condense around, & 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 & tiny droplets provide more surfaces for light to reflect off, & voilÃ â€” brighter clouds.
These brighter clouds moreover 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, & 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 moreover 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, & 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, & less so in the southern half, he said.
Of course, there are other reasons to curb pollution. Smog & other air pollution kills millions every year.
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