At the C40 Summit in Sao Paolo last week, former President Clinton urged participating cities and the World Bank to make a dramatic reduction in methane and black carbon. He said it would help the earth buy some time on climate change. He has reasons to be worried: In Cancun last year, parties agreed  to stabilize average global temperatures at a level not exceeding 2 degree C above pre-industrial levels. This looks difficult as 0.8 degree C warming has already taken place and GHG emissions continue to grow.
Developed countries  collectively reduced greenhouse gas (GHG) emissions by a mere 6.1% from 1990-2008. Compared to the fast track for warming, humanity is on the slow train to reducing carbon dioxide (CO2) emission reductions.
President Clinton’s statement follows two recent reports that point to emerging scientific awareness that a climate change strategy focusing exclusively on carbon dioxide (CO2) is neither the quickest nor the most effective way to achieve long-term climate stabilization. These reports focus on non-CO2 emissions that stay in the atmosphere for a shorter period of time than CO2. As a result, reducing emissions of these non-CO2 gases will result in a slowing of temperature rise over the first half of the 21st century, buying time both to adapt and to transition away from carbon.
The first report , produced by UNEP and WMO, assesses black carbon and tropospheric ozone. Black carbon—basically soot—is produced by incomplete combustion of carbon fuels, particularly diesel, wood, and coal. It is a dark suspended particle or aerosol, technically not a GHG. It is frequently emitted together with light-colored aerosols (sulfates and organic carbon) which cool the climate. The latest research indicates that, on balance, the warming effect of black carbon overpowers the cooling effect of its companions. It stays in the atmosphere for only a few weeks before falling to earth. Its warming contribution comes from its black color, making it absorb heat while in the air. If it falls onto mountain or polar snow, it accelerates glacial melt.
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