Science & the Public

Janet Raloff
Science & the Public

Air Pollution Can Be So Cool — ing

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Earth has been warming in recent years. That means it takes in more energy than it emits. The net difference amounts to some 1.6 watts per square meter, averaged over the planet’s surface, according to Michael B. McElroy of HarvardUniversity. But that value would be roughly 30 percent greater if it weren’t for some combustion pollution — chiefly the sulfur dioxide emitted by fossil-fuel burning, he told journalists and other attending a mini climate-change conference last week.

Pollution’s ability to offset climate cooling should not be viewed as an excuse to delay programs aimed at reducing emissions of potentially toxic smog contributors, such as sulfates, he argued: “Without any question, the first problem that we should address is pollution.”

His reasoning: Although global warming is a serious issue, its actions play out over a much longer term than do the direct effects of pollution. Because sulfate’s life span in the atmosphere is measured on the scale of just days, pollution can tackled and make an almost immediate improvement on health and the environment.

McElroy spoke April 30 at an event in Cambridge, Mass., that was sponsored by HarvardMedicalSchool’s Center for Health and the Global Environment together with the Society of Environmental Journalists.

In the air, sulfur dioxide turns into sulfate, microscopic particles that serve as park benches on which water-vapor molecules can rest and then turn to liquid. When enough molecules condense, clouds form.

“The more cloud particles you have, the whiter will be the cloud and the more reflective,” McElroy explains. Any sunlight bounced back into space does not contribute to Earth’s warming.

Although the United States has done a reasonable job of scrubbing sulfur dioxide from fossil-fuel exhaust — especially from the stack gases of industrial plants that burn coal — the same can’t be said for more populous nations, the physicist said. “The major source of sulfur dioxide in the atmosphere, for the moment,” McElroy says, “is China. And India is not very far behind.”

The sulfate-cooling phenomenon has prompted ruminations within the research community about whether clever engineers might not be able to fling sulfur into the upper atmosphere, or stratosphere, to deliberately seed it with cooling aerosols. There’s no question that if this could be accomplished, it would help cool the planet, McElroy says, since major volcanic eruptions naturally propel such aerosols into the stratosphere. Shortly afterward, scientists invariably record “a detectable drop in global average surface temperature.”

Of course, it would take a lot of energy to emulate the volcanic thrust of sulfates into the upper atmosphere, something that in itself would likely release tons more pollution — literally. And that would sort of defeat the purpose, I’m guessing. It would also be a pricey tactic.

But McElroy has another reason for dismissing such a proposed global air-conditioning strategy: “It’s not clear at all to me that you won’t generate a totally different type of climate.” One that can’t be predicted beforehand.

So that would argue that we should be moving toward energy sources that are less carbon-centric. Think wind power instead of coal, geothermal instead of diesel. Because as we clean the air of sulfates and other fossil-fuel pollution — thereby losing their thermal buffering — global warming stands to become much, much worse than it is today.

But “there may be a silver lining” in controlling some especially sooty air pollution, argued another panelist at last week’s meeting, Paul Epstein. Associate director of the Center for Health and the Global Environment, he observed that an April Nature Geoscience paper reported that soot, also known as black carbon, aggravates global warming much more than previously realized, largely by making air plumes less reflective, which means better at absorbing heat.

Indeed, owing to soot’s high energy-absorption capacity and regional distribution, it’s global-warming potency ranks second only to carbon dioxide, according to the new Nature Geoscience paper. And in high altitudes and latitudes, where migrating soot can fall out and darken snow or ice, this pollutant may actually be “as important as carbon dioxide” in contributing to the melting of snowpacks and glaciers, its authors conclude.

As such, Epstein argues, curbing sooty fossil-fuel emissions would appear to promise a double benefit: less carbon dioxide and better protection of important freshwater reservoirs — glaciers, polar ice and mountain snowcaps.


Ramanathan, V. and G. Carmichael. 2008. Global and Regional Climate changes Due to Black Carbon. Nature Geoscience 1(April):221.

2008. Covering Climate: The Health Angle. Cambridge, Mass. (April 30). [Go to]
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