While the effects of power plant emissions, vehicle exhaust and other manmade aerosols on air quality and public health are well-known, their impact on the climate is not completely understood. Scientists have shown that aerosols can lower surface temperatures either directly, by reflecting sunlight skyward, or indirectly, by increasing the reflectivity of clouds, but until now have not figured out the role these airborne particles play in shaping the distribution of rain and snowfall around the world.

Big hopes are riding on the 2015 United Nations climate change conference planned for Nov. 30-Dec. 11 in Paris, where more than 190 nations will strive to hammer out an international agreement aimed at lowering global temperatures through significant reductions in human-made greenhouse gas (GHG) emissions. But the meeting, known as COP21, or the 21st Session of the Conference of the Parties to the UN Framework Convention on Climate Change (UNFCCC), is also attracting a fair amount of skepticism.For good reason: More than two decades of UN climate summit meetings have yielded limited results. The Kyoto Protocol of 1997 established GHG emissions reduction commitments for a small number of industrialized countries from 2008 to 2012, but was not ratified by the U.S. because it made no demands on developing countries. Overcoming this hurdle, the Copenhagen meeting in 2009 produced voluntary pledges from both developed and developing countries through the year 2020 that promised little headway in keeping global temperatures below the 2 degree Celsius threshold identified by the UNFCCC as necessary to avoid the most serious impacts of climate change.