Water in the atmosphere traps heat, like glass in a greenhouse. When there's less water vapor in the atmosphere, less heat is trapped. Space Frontiers/Getty Images hide caption

itoggle caption Space Frontiers/Getty Images

Tools like these ax heads from Vinelz, Switzerland, were used by Neolithic farmers and may have been part of their sex appeal to hunter-gatherer women. Sandstein via Wikimedia hide caption

itoggle caption Sandstein via Wikimedia

Smog shrouds the Los Angeles skyline. New ozone standards from the EPA aim to reduce smog, but may actually increase global warming. David McNew/Getty Images hide caption

itoggle caption David McNew/Getty Images

A large crack in the earth is seen in Port-au-Prince, Haiti, after a 7.0 magnitude earthquake Tuesday. Matt Marek/ American Red Cross via Reuters/Landov hide caption

itoggle caption Matt Marek/ American Red Cross via Reuters/Landov

Nuclear power plants like this one in Middletown, Pa., currently supply 20 percent of the nation's electricity. Some legislators want them to provide more, but it's turning into quite a sticky subject. Carolyn Kaster/AP hide caption

itoggle caption Carolyn Kaster/AP