Is Soot Melting the Ice?by Bill Chameides | April 14th, 2009
posted by Erica Rowell (Editor)
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Filter sample of aerosols collected in November in Lin ‘an, southwest of Shanghai. The black color indicates the presence of black carbon. Researchers argue that increases in these emissions are responsible for the Arctic's large warming.
A few months back on a flight to the West Coast I looked out the window to see a strange site: instead of being white, the snow-covered Rockies had a gray tinge. A new paper suggests a similar phenomenon may be responsible for melting the Arctic’s polar ice cap.
First some background. They’re too small to see with the naked eye, but the air is filled with tiny particles called aerosols. To a large extent they come from burning — burning wood and fossil fuels.
Of keen interest are aerosols in the “accumulation mode.” Their size ranges from a few tenths of a micron to a few microns in diameter. (A micron is a millionth of a meter.) We call this the accumulation mode because they, well, accumulate — they’re too big to clump onto other aerosols, and they’re too small to rapidly fall out of the atmosphere. Like the porridge in the Goldilocks story, they have just the right size and so stay suspended in the atmosphere for as long as a week or two, building up in concentration.
Aerosols and Climate 101
OK, so now you know a bit about aerosols. Let’s turn to how they affect climate.
The size of accumulation mode aerosols is also just right when it comes to climate. Because their diameters are close to the wavelengths of light from the Sun, the aerosols are able to interact with sunlight.
Most of the accumulation mode aerosols are composed of sulfates and organic carbon. When sunlight shines on a column of air with lots of these aerosols, the light is dispersed in all directions — to the right, left, forward, and backward. You’ve seen this effect when there’s a lot of pollution in the air — it’s called haze.
The net effect of this scattering on the climate is to cool the atmosphere, because a fraction of the light is scattered backward, out of the atmosphere and into space, effectively reflecting some of the sunlight and preventing it from being absorbed by Earth. Additional cooling can arise because these same aerosols can increase the reflective properties of clouds.
Climate models indicate that the net cooling of aerosols is probably quite substantial — the exact amount is uncertain but it’s probably enough to have offset a significant amount of the warming from greenhouse gases. (See the Intergovernmental Panel on Climate Change’s fourth assessment report [pdf] and chart.)
But there is another kind of aerosol — called soot or black carbon, because it absorbs rather than scatters sunlight. These aerosols cause warming, the amount of which is even more uncertain than the cooling from scattering but could be substantial in is own right. This warming could effectively cancel out some of the aerosol cooling that tends to cancel out some of the greenhouse warming.
This sounds complicated but it’s just a negative of a negative, which makes it a positive. But from a climate point of view it’s a negative because it causes warming which is a negative. Got all that?
However, there are two important differences in the climatic effects of greenhouse gases and aerosols. Greenhouse gases stay in the atmosphere for decades or more and so their concentrations respond to changes in emissions very slowly. Moreover, their concentrations tend to be globally uniform so their warming is distributed evenly around the Earth.
In contrast aerosols are short-lived in the atmosphere and so their concentrations (and therefore climatic effects) decrease quickly after their emissions decrease. Also, their concentrations and thus their climatic impact vary spatially.
The Aerosol-Climate-Policy Catch
Here’s an easy solution to the climate change problem: let’s put a lot more of those cooling sulfate and organic carbon aerosols into the atmosphere and get rid of those black carbon ones.
Not a good idea. Why? Because those same sulfates and organic carbon aerosols are basically air pollution. Among other things, they kill people. In fact, nations around the world including the United States are doing their best to get the stuff out of the atmosphere. And at least in the case of sulfates, emissions appear to be going down globally. While that’s clearly a good thing, it’s going to exacerbate global warming.
On the other hand, getting black carbon out of the atmosphere is a win-win for air quality, public health, and the climate. Unfortunately, while sulfate emissions are on a downward trend, emissions of organic black carbon appear to be rising, primarily as a result of developing countries using more wood, coal, and diesel fuels. For climate change, this is a double whammy: more warming from black carbon, less cooling from sulfates.
Is the Aerosol Whammy Showing Up in the Arctic?
This is where the work of Drew Shindell of NASA’s Goddard Institute for Space Studies and Greg Faluvegi of Columbia University comes in. In a paper published in this month’s Nature Geosciences, they show, using a coupled ocean-atmosphere climate model, that Arctic temperatures are highly sensitive to regional warming and cooling of aerosols.
In the past two to three decades the Arctic has warmed more than any other region in the globe. Shindell and Faluvegi argue that this anomalous warming, and presumably the concomitant melting of the polar ice cap, has been the result of declining sulfate emissions and increasing black carbon emissions.
The authors conclude by noting the silver lining in their back carbon story: clamping down on black carbon pollution is technologically feasible, a public good, and would reap quick climate benefits in the Arctic.filed under: Arctic, climate change, faculty, global warming, Planetary Watch
and: aerosols, black carbon