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Beware the atmospheric brown cloud
Maybe cars aren’t so bad after all.
OK, OK — they are still a huge source of pollution, but a recent study adds to the debate on the importance of cars relative to forest fire pollution in Southeast Asia.
Every winter over Southeast Asia and the Indian Ocean, an enormous atmospheric brown cloud (ABC) tiptoes in on little cat feet. The ABC is caused by aerosols in the lower atmosphere, and has generally been attributed to urbanization and the associated increase in fossil fuel combustion.
Fossil fuel combustion, though, has been modeled to account for anywhere from 10–90% of the aerosol loading in the ABC. Given this enormous range, cars and other fossil fuel combustion devices could be the problem, or they could be almost entirely beside the problem. Which is it?
This latest study uses radiocarbon dating of specks of so-called “black carbon” in the aersols to determine the pollution’s source. Because fossil fuels are much, much older than recently felled trees, they contain different amounts of carbon-14 (which has a half life of 5,730 years). These scientists grabbed a bunch of aerosol out of the atmosphere (I am imagining a giant bucket tied behind an airplane, although the actual technology is likely a little bit more advanced), and determined that, surprisingly, fossil fuel combustion and burning forests each account for roughly half of the aerosols.
As if you needed more reasons to be concerned with forest fires in Indonesia, you can now add the ABC to a list of major environmental impacts. In this case, the implications are complex: aerosols actually have a net cooling effect over the region they cover. So reducing aerosol production via decreased fossil fuel combustion and/or reduced deforestation acts as a positive feedback to warming. Adding further complexity, the sheer size of the ABC is enough to affect weather patterns over the entire Asian subcontinent. What this means for monsoon rains and heat waves across India remains unknown.
Like most climate science, the results are complicated (cars *and* trees are responsible for the ABC) and sometimes counterintuitive (pollution *reduces* warming). But no one is suggesting we should be pumping aerosols into the lower atmosphere: that stuff is known as smog, and most everybody agrees that smog is bad.