According to the National Academy of Sciences, the Earth's surface temperature has risen by about 1 degree Fahrenheit during the past century, with accelerated warming during the past twenty years (Climate). There is new and stronger evidence that most of the warming over the last 50 years can be attributed to human activities, which have altered the chemical composition of the atmosphere through the buildup of greenhouse gases, primarily carbon dioxide, methane, and nitrous oxide (Climate). Although the heat-trapping property of these gases is undisputed, uncertainties exist about exactly how earth's climate responds to them (Climate). .

             It is clear that the concentration of several greenhouse gases have rise dramatically (Green 2001). In fact, from the late 1700's to the present, carbon dioxide levels have increased by some 30 percent, and concentration of methane, which is an even stronger warming gas, have increased nearly 150 percent since the beginning of the nineteenth century, while nitrous oxide levels have increased by some 22 percent from the pre-industrial era (Green 2001).

             Moreover, ozone, which can warm the climate when present at low altitudes, has been increasing in concentration in the Northern Hemisphere over the last three decades (Green 2001). Ozone concentrations in the Southern Hemisphere are uncertain, while at the poles, low-altitude ozone concentrations seem to have fallen since the mid 1980's (Green 2001). Although these gases have major natural sources, the fingerprint of human action is evident in the changing concentrations (Green 2001).

             The main culprit is carbon dioxide released by burning fossil fuels (Lindsay 2001). In fact, its atmospheric concentration has risen from 270 parts per million in 1750 to 368 today (Lindsay 2001). Moreover, methane gas, has more than doubled in concentration over the last three hundred years as the result of cultivating rice, raising livestock, and exploring for oil, is a secondary factor, "as is nitrous oxide, a byproduct of burning fossil fuels and producing nylon and fertilizer" (Lindsay 2001).