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  • Dick writes about Earth and planetary science for Science magazine.
 

Humans Emulate Volcanoes in the Stratosphere

24 July 2009 (All day)
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D. Hofmann et al., Geophys. Res. Lett. (2009)

Up and up. The thickening stratospheric haze (blue line) may reflect the world's increased burning of coal.

By the 1970s, people were spewing so much soot, ash, and other tiny particles into the lower atmosphere that climate researchers called the effect the "human volcano." Now it looks like humans are imitating volcanoes in yet another part of the atmosphere. New research blames this decade's thickening of the haze in the stratosphere on the burst of coal burning around the world. The chief offender appears to be China.

Until now, scientists could see no sign that anything but volcanic eruptions packed enough punch to pollute the stratosphere. Smokestacks also spew sulfur, creating acid rain and attacking people's lungs, but no smokestack can send it up through the lower atmosphere and into the stratosphere where it turns into haze.

Or can it? In a paper in press in Geophysical Research Letters, atmospheric scientist David Hofmann of the University of Colorado, Boulder, and colleagues report a human role in stratospheric haze pollution. Since 1994, National Oceanic and Atmospheric Administration scientists have used lidar to probe stratospheric haze in the skies above Mauna Loa in Hawaii and, since 2000, above Boulder. Like weather radar, which uses microwaves to measure rain, upward-pointing lidar bounces electromagnetic waves off distant objects--microscopic haze particles in the case of haze.

Both the Mauna Loa and Boulder lidars show a long-term upward trend in stratospheric haze since about 2001 of 4% to 7% per year (blue line in figure), the group reports. "This trend is quite large," says Hofmann. Volcanoes aren't to blame, he says, as the most recent major volcanic activity--Mt. Pinatubo's "eruption of the century" in 1991--occurred long enough ago that any sulfurous gases it blasted into the stratosphere to form haze are long since gone.

So Hofmann turned to humans. Sulfur emitted by coal-burning industries and power plants could spread through the lower atmosphere, he explains, and then less than 1% of it might have been caught in the strong updrafts of tropical storms--the equivalent of a volcano's plume--that could loft it into the stratosphere. The world's 5.2% per year increase in sulfur emissions from 2002 to 2007--mainly from China--could readily account for the increase in stratospheric haze, the group calculates.

However, experts in stratospheric circulation caution that there's an alternative explanation for increasing haze. Rather than the human-induced increase in pollutant sulfur in the lower atmosphere, the cause could be an acceleration of the updrafts that loft air and any sulfur in it into the stratosphere. The speedup could be natural or another effect of global warming.

Still, Hofmann notes that humans don't hold a candle to volcanoes when it comes to stratospheric pollution. Even if, as expected, China dramatically increases its coal burning by 2022, the resulting doubling of stratospheric haze would be just 5% of another Pinatubo--resulting in a slight cooling of the stratosphere and a tiny amount of ozone damage. The corrosive effects of China's coal burning in the lower atmosphere would be another matter.

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