Fiona Harvey Tackling soot could help reduce Arctic ice melt

News about the collapse of an Antarctic ice bridge linking the Wilkins ice shelf to two islands was quickly followed by new research showing that the Arctic ice is thinning dramatically as well as shrinking in extent.

One factor that could help to slow the melting of the Arctic, but which has not yet received serious consideration at an international level, would be to cut the amount of “black carbon” – soot – that we spew into the air. Black carbon darkens ice when it falls, causing it to absorb more heat, and may be responsible for half of the warming effect in the Arctic, according to recent research published in Nature Geoscience. Cutting down on soot would not only remove large amounts of air pollution, but, according to some scientists, could be much quicker and easier than cutting carbon dioxide emissions.

On current form, the chances of this being agreed also look small, however.

The news about both the Wilkins bridge and the Arctic ice thinning, which backs up claims that global warming is proceeding much faster than expected, had absolutely no effect on the urgency of climate change talks in Bonn, which crawled along at the speed glaciers used to. Delegates seemed to devote more time and energy to arguing over the timing of future meetings in the lead-up to Copenhagen, the December conference at which a new Kyoto will be negotiated, than they did on the substantive issues of how far to cut emissions and how much money developing countries should get to do so.

A collapsing ice bridge is bad news because it will help to break up the shelf behind it. The disintegration of the shelf, in turn, will lead to glaciers speeding up their progess to the sea. Sea ice shelves act as plugs holding back glaciers – when they disappear, the glaciers have a free run to the sea, so they start to pick up speed. Accelerating warming, as the sea ice disappears and the dark water left behind absorbs more heat from the sun, compounds this process.

So, bad news all around then.

The Nature Geoscience report itself is quite lengthy, but here is a background notes from the Institute for Governance and Sustainable Development:

• During 1976-2007, large changes in mid-latitude emissions have increased the importance of local Arctic forcing. The Arctic warming for 1976-2007 from NH mid-latitude aerosol forcing (28-60N) was +0.6C, and from the Arctic aerosol forcing (60-90N) +0.8C (mostly from BC deposition on snow and ice), with -0.3C from tropical aerosol forcing, for a total of 1.09±0.81C. Tropical forcing comes largely from tropical emissions, and NH mid-latitude forcing from NH mid-latitude emissions, but Arctic forcing is also largely from NH mid-latitude emissions as there is little BC directly emitted in the Arctic.

• From 1976 to 2007, the BC emission increases in the atmosphere from 28-60N have contributed about 0.3C to Arctic warming and the BC emission increases N of 60 (atmospheric and deposited, mostly the latter) have contributed about 0.4C. Black carbon emissions from biofuel and fossil fuel combustion increased ~50% from 1880 to the 1920s, rose ~25% from the 1920s to the 1970s, then rose ~30% from the 1970s to 2000.

• There are few direct BC emissions N of 60. Roughly 80-90% BC are NH mid-latitude emissions, and 10-20% tropics. Of BCs contribution of ~0.5-1.4C to Arctic warming since 1890, about 0.4-0.5C (around half the mid-range 0.95C) is from BC deposition, mostly emissions from 40N.

• Improving aerosol management to halt BC ahead of cooling sulfates could reduce about 85% of the 0.5-1.4C warming in the Arctic since 1890. This means it would be possible to get cooling of ~0.8±0.4C. This is significant compared to total Arctic warming rates. Because it takes the climate a while to respond, it might be necessary to delay sulfate reductions for 20 years while also halting BC emissions.