Heat-trapping greenhouse gases that contribute to global warming include carbon dioxide, methane and nitrous oxide. According to the U.S. Climate Action Report 2010, carbon dioxide emissions rose 21.8 percent from 1990 to 2007, largely due to increased generation of electricity and an uptick in transportation activities. In that same 17-year period, methane emissions declined by 5 percent and nitrous oxide emissions dropped 1 percent. Better technology and management of natural gas plants as well as increased collection and burning of methane gas at landfills account for most of the reduction in methane emissions. (See References 2, page 5)
Global warming can be blamed for striking weather changes across the U.S., according to a 2009 report issued by the U.S. Global Change Research Program. Since the mid-1970s, heavy downpours occur much more frequently in the Southeast and average fall precipitation measures 30 percent more than in 1901 (see References 3, page 111). Heavy downpours drench the Midwest twice as frequently as 100 years ago, and the number of heat waves rivals the Dust Bowl years (see References 3, page 117). The Northwest --- which relies on snowpack for some of its fresh water --- has seen the Cascade Mountains snowpack decline 25 percent from 1940 to 1970 (see References 3, page 135).
A piece of Antarctica's Larson B ice shelf --- the size of Rhode Island --- broke off in 2002, contributing to the general shrinking of the glacier, now 40 percent smaller than in 1995. The entire polar ice cap is melting at a rate of 9 percent each decade, contributing to a rise in sea levels, according to the Natural Resources Defense Council. Should the entire Greenland ice mass melt, sea levels could rise 21 feet, displacing millions of people living in low-lying areas. (See References 4)