Global warming + trees = regional cooling?

Global warming + trees = regional cooling?

3 June 2009

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USA — This morning, for the first time this year, the experts who monitor air pollution in these parts issued an “orange” alert. Folks who might have trouble breathing should minimize outdoor activity. As we live in a rural area near the leeward side of the Great Smoky Mountains, this is always a reminder of just how bad the smog from the coal-fired plants upwind of us in Tennessee and Kentucky can be. To make matters worse, a few hours after hearing that news, I came across a recently published paper in PNAS that suggests things are going to get worse if the world continues to warm.

The good news is, that same global warming might actually lead to regional cooling, or at least a reduction in the magnitude of the warming, here in the U.S. Southeast. While I welcome the thought that it might be possible to avoid some of the heat my Canadian metabolism so dreads, a close read of the science behind the PNAS paper only reinforces how difficult it is to anticipate the effects of meddling with the climate.

The paper, Biogenic carbon and anthropogenic pollutants combine to form a cooling haze over the southeastern United States, is the result of research not by some southern scientists, but Allen H. Goldstein and three colleagues at the University of California, Berkeley. They write that a lot of the organic aerosols emitted by smokestacks and tailpipes are reacting with “natural biogenic volatile organic compound (BVOC) emissions” to produce a haze that has a cooling effect over Georgia, Alabama and the rest of the U.S. Southeast.

BVOCs are released by trees, and there are, thankfully, still lots of trees around here. (So although Reagan wasn’t right when he said trees cause acid rain, there is a grain of truth in the idea.) The problem is warmer temperatures increase the rate of BVOC production, so global warming will lead to more haze in Appalachia. And because aerosols have a cooling effect, that might temper the expected rise in temperatures in the Southeast. Goldstein et al.

predict higher regional summertime aerosol levels in the future and a regional negative, albeit small, feedback on warming and a less rapid increase in BVOC emissions…

Assuming the world gets its act together and manages to implement serious climate change mitigation measures, however, anthropogenic fossil-fuel emissions should decline. If we do what’s necessary to avoid dangerous temperature rises, we’ll basically have to shut down all the coal-fired plants or find an cheaper and simpler way to scrub the CO2 from their smokestacks.

If those emissions do decrease, the secondary organic aerosols (SOAs) produced by the reaction with the BVOCs will also decrease, and so the cooling effect comes to an end.

More important are the implications for climate change science in general. According to Goldstein, current climate models don’t take into account the contributions of BVOC, focusing more on anthropogenic aerosols:

Managing regional haze and visibility in the SE U.S. and modeling air quality interactions with future climate will require improved quantitative understanding of the photochemical processes controlling BVOC and anthropogenic emissions reacting to form SOA.

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