Scientists have predicted that the earth's climate will change because human activities are altering the chemical composition of the atmosphere through the buildup of greenhouse gases – primarily carbon dioxide, methane, and nitrous oxide. The global warming property of these gases is undisputed although uncertainty exists about exactly how earth's climate responds to them. Like many fields of scientific study, there are uncertainties associated with the science of global warming. This does not imply that all things are equally uncertain. Some aspects of the science of global warming are based on well-known physical laws and documented trends, while other aspects range from near certainty to big unknowns. Global warming is not as serious of a threat as scientists are stating.
             There is no doubt that the atmospheric buildup of greenhouse gases is largely the result of human activities. By increasing the levels of greenhouse gases in the atmosphere, human activities are strengthening Earth's natural greenhouse effect. Figuring out to what extent the human-induced accumulation of greenhouse gases since pre-industrial times is responsible for the global warming trend is not easy though. This is because other factors, both natural and human, affect our planet's temperature. Scientific understanding of these other factors–most notably natural climate variations, cooling effects of atmospheric particles such as sulfates, changes in the sun's energy, and the cooling effects of pollutant aerosols–remains incomplete.
             To project future climate patterns, scientists use computer simulations of the interactions among land, air, water, ice, and sunlight. These general circulation models, or GCMs, consist of equations representing the known laws of atmospheric physics and ocean circulation (Monastersky 1). A perfect model, if given enough information about conditions on earth several hundred ye...