Few Degrees

Global average temperatures are expected to increase by about two to thirteen degrees Fahrenheit by the end of the century. That small change in global average temperature can lead to really large changes in the environment. Only a few degrees have made the difference between ice ages, temperate periods, and a hothouse Earth with green poles. During the last ice age, global average temperatures were only seven to thirteen degrees Fahrenheit colder. About 125,000 years ago, average temperatures were only a degree or so warmer than today, but sea levels eventually rose as much as 20 feet higher.

Small changes in the global average temperature also mean heat waves get hotter and droughts get drier. This happens when small changes in the way air and water circulate around the globe take place. For instance, during the wetter and cooler climate of the last ice age, the Great Basin of California was filled by a great lake ringed with pine forests. When the climate became warmer and drier at the ice age's end, the lake dropped 500 feet and lost 90% of its surface area. The pine forests became sagebrush desert. Such changes in precipitation could be just as important as temperature increases for many parts of the Earth.

Some places and some years are going to be warmer or cooler than average. However, summers will get hotter, not only because of higher temperatures but also because humidity will increase. In addition, models predict that in many places, an extreme heat event that we only see once every twenty years today will happen once every three years by the middle of this century. Winters will also be warmer in many places, reducing heating bills. The number of days with frosts is likely to decrease. This means more snow may fall as freezing rain, making ice more of a problem for humans.

http://www.meted.ucar.edu/broadcastmet/climate/print.htm#page_2.0.0