What is Global Warming?

Is the Earth Getting Warmer?

During the 20th century, the earth's average temperature has increased by about .6º Celsius. It's almost certain (90-99% certain) that the 1990s were the warmest decade, and 1998 the warmest year since 1861. It's likely (66-90% chance) that these have also been the warmest decade & year of the millenium. A rate of warming this high hasnt been seen in at least the past 400 to 600 years.                                                                     - IPCC Scientific Basis Report 2001
                                                                    - Union of Concerned Scientists

The Greenhouse Effect

Scientists have known for quite awhile that the earth is warmed by the sun through a process called the greenhouse effect. Basically, energy from the sun reaches the surface of the earth, causing whatever object it "lands on" to heat up and emit heat back toward space. Some of this heat finds its way out of the atmosphere, but some encounters greenhouse gases (water vapor, carbon dioxide, methane, CFCs, nitrous oxide and tropospheric ozone) which reflect the heat back toward the earth. This natural cycle of energy keeps the air comfortably warm, and is essential for life on earth. Without the greenhouse effect, the planet's average temperature would drop about 34º Celsius, and life as we know it would cease to exist.

The Enhanced Greenhouse Effect

But it's not the natural greenhouse effect that has many scientists, environmentalists and citizens worried. Since the beginning of the Industrial Revolution, the concentration of carbon dioxide in the atmosphere has increased 30%, nitrous oxide concentrations have increased by 15%, and methane has doubled. The reason for this increase is generally attributed to human activity, more specifically; to the burning of fossil fuels in power plants and vehicles (one by-product of which is carbon dioxide) and deforestation/inreased farming (which leads to an increase in termites who in turn produce methane). The continually increasing greenhouse gas concentration in the atmosphere is causing more and more heat to be reflected back toward earth, and this is why the planet is warming.

Facts

The difference between today's comfortable global temperature and the peak of the last ice age is only about  5º C
                   - EPA

It's almost certain (90-99% certain) that the 1990s were the warmest decade, and 1998 the warmest year since 1861. It's likely (66-90% chance) that these have also been the warmest decade & year of the millenium.
                 - IPCC

The rate of warming experienced in the 20th  century is faster than any that the earth has seen in the last 400-600 years.
                 - UCS