China Soot Heating Pacific Ocean

March 16, 2007 — There may be an unexpected sooty surcharge on all those cheap Chinese imports, say atmospheric scientists. The carbon soot from China is warming and polluting the atmosphere over the Pacific Ocean and all the way to North America, according to a new study.

The effect on the Pacific Ocean, the largest and most influential ocean on the planet, could have global climate implications, the study's authors conclude.

The airborne soot dims sunlight reaching the surface, causing cooling there, but it also heats the air higher where it resides, resulting in an overall heat gain for the climate.

The effect on the North Pacific is that the dimming could be lowering the amount of evaporation, and therefore rain and snowfall, while adding to greenhouse effect.

"It affects the hydrological cycle," said atmospheric researcher V. Ramanathan of the Scripps Institution of Oceanography at the University of California, San Diego. Ramanathan has examined similar effects in the Indian Ocean, where dimming caused by soot has cooled waters and now lessens or blocks the annual Indian Monsoon — with devastating economic and ecological consequences for India and neighboring nations.

"I'm getting concerned what's happening closer to home," Ramanathan told Discovery News, regarding the new study published in the Journal of Geophysical Research.

In that study he, Odelle Hadley and their colleagues used data on soot gathered at air stations in North America, as well as from aircraft, to get a better idea how much soot, also called black carbon, is making it across the ocean.

The researchers used that information in a simulation to calculate the amounts of energy, and therefore heat, being blocked or added at the surface level and at about 7,000 feet above sea level, in March and April.

Over the Pacific it appears that soot allows higher air to absorb 2 to 2.5 watts more sunshine per square meter, the team reported.

Down at the ocean surface, the dimming effect reduces solar heating by almost 1.5 watts per square meter. That means the soot creates a net heat gain to the atmosphere of about 0.5 to 1 watt per square meter.

Soot is a small but critical part of the pollution that blows off of eastern Asia. The sources of the soot include diesel exhaust, agricultural burning and cooking fires.

"If you look at total aerosols (airborne particles), soot is only a small percentage," said Mark Jacobson, an atmospheric researcher at Stanford University in California. Since most aerosols cause dimming of sunlight at the surface, soot's importance is that it also warms the air around it — unlike other common aerosols, like sulfate.

The other chief concern about black carbon is its health effects as an air pollutant, said Jacobson.

The new study also estimates that during springtime, China's black carbon contributions to California's air are equivalent to 75 percent of the state's own black carbon emissions.

And although that's still not a lot, since California has strict controls on diesel use, the effects could still be strong enough to melt snow cover.

"When deposited on snow it has the effect of melting of the snow," said Jacobson.

The earlier mountain snows melt, the less time there is each year for the snowy ground to reflect sunlight back into space and keep things cool. Bare ground, on the other hand, absorbs sunlight and re-emits the light as heat — adding to global warming.

The good news is that unlike the greenhouse gas carbon dioxide, which can last a century in the atmosphere, black carbon lasts only a couple of weeks. So if China cleans up its emissions, this particular climate troublemaker could disappear just as quickly.