Hazy changes on high

A big boost in coal burning, especially in China, is adding more aerosols to the stratosphere

Ground-based observations reveal that the amount of light-scattering aerosols in the stratosphere has been increasing substantially in the past decade, probably due to a dramatic rise in coal-fired power plants worldwide.

Aerosols in the stratosphere, the layer of the atmosphere between altitudes of about 10 kilometers and 50 kilometers, come from three main sources: major volcanic eruptions, upwelling of the lower atmosphere at tropical latitudes and the slow upward drift of aerosols created by industrial emissions worldwide. Now, a nearly two-decade-long lull in large volcanic eruptions has enabled scientists to discern that anthropogenic aerosol levels in the stratosphere are on the rise, says Michael E. Trudeau, an atmospheric scientist with NOAA’s Earth Systems Research Laboratory in Boulder, Colo. He and his colleagues report their finding in the Aug. 16 Geophysical Research Letters.

The 1991 eruption of Mount Pinatubo in the Philippines, which sent prodigious amounts of tiny sulfuric acid droplets high into the stratosphere where they reflected sunlight back into space, temporarily caused the global average temperature at Earth’s surface to drop by around 0.5 degrees Celsius, says Trudeau. But ground-based observations show that natural processes had largely cleansed that eruption’s aerosols from the stratosphere by 1996, he notes. Because there hasn’t been a major volcanic eruption since Pinatubo, scientists have been able to get a clear look at trends in other sources of stratospheric aerosols.

To make those assessments, the researchers fired lasers into the sky from two observatories, one atop Hawaii’s Mauna Loa volcano and the other near Boulder. Since 2000, the amount of the laser light scattered by aerosols between altitudes of 20 and 30 kilometers has been rising between 4 and 7 percent each year, says Trudeau. Other observations suggest that atmospheric upwelling in tropical latitudes hasn’t increased substantially since 2000, so Trudeau and his colleagues pin the blame for rising aerosol concentrations on human-made sources.

The largest sources of human-made aerosols are coal-burning power plants, which spew sulfur dioxide, a byproduct of the combustion of sulfur-tainted coal. Between 2002 and 2007, coal consumption worldwide jumped more than 36 percent — mostly in China, Trudeau and his colleague note, where sulfur dioxide emissions rose an average of 5.4 percent per year from 2000 to 2005. In 2006, some scientists estimate, China passed the United States to become the world’s largest emitter of carbon dioxide.

Most industrialized nations scrub sulfur dioxide emissions from power plants, and concentrations of ground-level aerosols in those nations, especially in Europe, have been dropping for decades (SN: 2/14/09, p. 9). Such emission controls are rare in China. But even if China’s coal use doubles by 2022 and power plants there continue to spew SO2, the resulting doubling in stratospheric aerosols would have little climatic effect, cooling the Earth by only about 0.03 degrees Celsius, the researchers estimate.

Even though the climatic effect of the recent increase in stratospheric aerosols is small, it’s discernible and will help scientists validate their climate models, says Alan Robock, an atmospheric scientist at Rutgers University in New Brunswick, N.J. Such aerosols “are just another tracer that scientists can use to show that their models are working correctly,” he notes.

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