The pace of global temperature rise over the past century was roughly 0.6 degrees Celsius, but is now (from calculations over the last two decades) equivalent to 1.0 degrees Celsius per century.
             Annual precipitation over the continental U.S. has increased 10% during this century. The number of days with rainfall exceeding two inches has also increased 10% over the past century. Similar trends have been observed in Canada, Japan, Russia, China, and Australia.
             Many factors aid the process of global warming. There are a number of natural variables that can contribute, such as changes in the sun's energy input to Earth, volcanic activity, and regional climate phenomena like El Nino-Southern Oscillation. However, as teh American Geophysical Union concluded: "It is scientifically inconceivable that -- after changing forest into cities, puting dust and soot into the atmosphere, putting millions of acres of desert into irrigated agriculture, and putting greenhouse gases into the atmosphere -- humans have not altered the natural course of the climate system."
             Greenhouse gases such as carbon dioxide, methane, ozone, and others are being added to the atmosphere largely as a result of burning fossil fuels, tropical deforestation, and other human activities. These gases he
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