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NOT MADE IN THE SHADEUnlike the sunny day depicted above, when skies are polluted or slightly cloudy fewer leaves beneath the outer canopy of a forest are shaded. Because these leaves ending up receiving more radiation, overall carbon uptake is enhanced, a new study suggests.IMAGE CREDIT: Sandra Patiño

The world’s vegetation soaked up carbon dioxide more efficiently under the polluted skies of recent decades than it would have under a pristine atmosphere, a new analysis in the April 23 Nature suggests. The trend hints that relying on forests and other vegetation to sequester carbon may not be effective if skies continue to clear, researchers say.

Major volcanic eruptions throw large quantities of aerosols, such as small bits of fractured rock and droplets of sulfuric acid, high into the atmosphere. Those particles scatter incoming solar radiation, preventing some of it from reaching Earth’s surface and thereby cooling climate temporarily (SN: 11/5/05, p. 294).

That scattering also, however, boosts how much carbon vegetation takes in, says Lina M. Mercado, an ecosystem modeler at the Centre for Ecology & Hydrology in Wallingford, England. Although aerosols, including many types of air pollution, decrease the overall amount of light falling onto a tree, the particles diffuse the radiation that reaches the ground so that it actually illuminates more leaves. In that case, leaves below the tree’s outer canopy are less likely to be shaded.

To estimate the way pollution and other aerosols affect the rate at which the world’s plants take up carbon, Mercado and her colleagues adjusted an ecosystem model to include the effects of diffuse radiation on vegetation. Then the team plugged in meteorological data gathered worldwide since 1901.

From the 1950s through the 1980s, many regions received less solar radiation overall — a phenomenon that atmospheric scientists term global dimming (SN: 9/24/05, p. 168) — and received a larger proportion of diffuse radiation. Since the 1980s, however, in many areas — especially some industrialized parts of the Northern Hemisphere, where pollution control measures have been instituted — skies have brightened.

Those atmospheric changes show up in the Earth’s carbon balance, the team’s model suggests. From 1960 through 1980, the researchers estimate, Earth’s land plants stored about 440 million metric tons of carbon each year on average, but from 1980 to 1999 vegetation stored only 300 million metric tons annually.

“Surprisingly, the effects of atmospheric pollution seem to have enhanced global plant productivity by as much as 25 percent from 1960 to 1999,” Mercado notes.

Short-term variations in atmospheric aerosols, such as those seen in the wake of the eruption of Mount Pinatubo in the Philippines in 1991, triggered the same effect. In 1992 and 1993, land plants worldwide stored at least 1 billion metric tons of carbon more than they would have if the eruption hadn’t occurred, the team’s data suggest.

If pollution control measures continue to increase atmospheric clarity, the boost in natural carbon sequestration provided by diffuse radiation will abate to near zero by the year 2100, the researchers note.

“I’m quite impressed that they’ve improved their [ecosystem] model to include the effect of diffuse radiation,” says Dennis Baldocchi, a biometeorologist at the University of California, Berkeley. The productivity-boosting effect of diffuse radiation has been measured at many individual sites but hasn’t been estimated on a long-term basis at the global level with such models before, he adds.

“It takes a long time for such effects to make their way into climate models,” agrees Michael Roderick, an environmental physicist at the Australian National University in Canberra. “This is a big advance.” Researchers, he notes, could use the revised model to estimate the long-term effects of geoengineering — of which artificially adding large quantities of aerosols to the atmosphere to ameliorate the effects of global warming would be one example.

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