Soot hastens snowmelt on Tibetan Plateau

Study suggests black carbon pollution has greater effect than carbon dioxide on region’s ice

In high-elevation snowy regions, the warming effects of greenhouse gases pale in comparison to those triggered by soot, new computer calculations show. The finding could help explain the accelerating pace of melting on the Tibetan Plateau, which holds the world’s largest reservoir of ice outside of the polar regions.

MELTDOWN Soot from black carbon pollution can darken snow (shown here on the Tibetan Plateau in Zangzu, China), causing the snow to melt earlier in the spring. Jan Reurink

Located north of the Himalayan range, the plateau’s spring meltwater feeds rivers that ultimately slake much of Asia’s thirst. In recent years, spring melting has been starting earlier, triggering downstream floods and shortening the time that irrigation water is available to farmers.

Until now, most researchers attributed the earlier runoff to global warming induced by greenhouse gases such as carbon dioxide, says Yun Qian of the Department of Energy’s Pacific Northwest National Laboratory in Richland, Wash. But Qian’s team wasn’t satisfied that carbon dioxide increases were large enough to account for the pace of the spring snowmelt’s advance.

So the researchers used a computer program to investigate the melting of snow covered in tiny particles of black carbon — the soot emitted by everything from cookstoves and diesel engines to coal-fired electric power plants. Like a black tarp, these dark particles absorb solar energy and warm the snow beneath. The new simulations indicate that the estimated amounts of black carbon on the Plateau can reduce snow’s reflectivity in spring by 4 to 6 percent. That’s enough to warm the average surface air temperature across the Tibetan Plateau by around 1 degree Celsius, the scientists report online March 2 in Atmospheric Chemistry and Physics.

Warming from soot is comparable to the regional warming attributable to the rise in atmospheric carbon dioxide that’s occurred since the 1700s, the researchers note. But soot’s snow melting potency is substantially higher than that of carbon dioxide — some two to five times higher, depending on the elevation and month of the year, the new study finds.

Unlike the global spread of long-lived carbon dioxide, ground-level soot’s climate impacts tend to be fairly localized — affecting snow but not soil, rock or water. The new simulations show that this effect exaggerates the impact of Asia’s monsoons, “because monsoons are driven by the temperature difference between the land and ocean,” Qian explains.

Qian cautions that the team’s newfound association between soot and melting would be limited only to snow-covered areas, and the computer program does not yet have fine enough resolution to quantify the extent of that area precisely.

Dust poses another wild card, Qian says. Satellite and other monitoring sources don’t quantify the generation and fallout of dust very well. Dust can also significantly darken snow, contributing to earlier snowmelts.

Despite such limitations, computer simulations are essential to understanding impacts of soot and other climate factors on the Tibetan Plateau, says Kenneth Hewitt of Wilfrid Laurier University in Waterloo, Canada. He has conducted field monitoring of climate impacts on ice and snow in this part of the world. “And work on the ground — especially in high Asia — is so difficult that you can’t do much of it,” he says.

Computer simulations can take soot measurements collected carefully from a small area, he says, “and extend them to a larger space, like the whole Tibetan Plateau.”

Climate scientist Teppei Yasunari at NASA Goddard Space Flight Center in Greenbelt, Md., agrees — but cautions that such computer modeling efforts are still very much a work in progress. For instance, he notes, no climate simulator can yet account well for all pollutants simultaneously — especially soot, dust and the potentially insulating role of rocky debris that has been found blanketing and perhaps reducing melting in large sections of Himalayan ice fields.

Photo courtesy of Jan Reurink

Janet Raloff

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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