We hear the term “greenhouse gas” a lot—but what is it? Humans add various gases to the earth’s atmosphere every day; these gases (known as “greenhouse gases”) consist primarily of carbon dioxide, ozone, chlorofluorocarbons, nitrous oxide, and methane, and tend to warm the earth. Trees help counter greenhouse gas production during photosynthesis, by taking in carbon dioxide as waste material and producing oxygen, which of course we all need to survive. Scientists predict that the daily addition of greenhouse gases to the atmosphere, combined with daily removal of large portions of the world’s forests, will raise the earth’s average temperature by several degrees in the next century. This in turn will raise the level of the sea and potentially create significant changes in weather patterns on a global scale. As we move into the future, many climatologists expect that most of the United States will warm. What we do not know yet is how to scientifically predict which parts of the nation will become wetter or drier. We do know there is likely to be an overall trend toward increased precipitation and evaporation, and more intense weather systems, in the form of violent rainstorms, blizzards and sun-baked, drier soils.
The Facts—What Do We Already Know About Changing Global Conditions (Global Warming)?

Global temperatures are rising. Observations collected over the last century suggest that the average land surface temperature has risen 0.45-0.6°C (0.8-1.0°F) in the last century.
Precipitation has increased by about 1 percent over the world's continents in the last century, with more rain in high latitudes and less rain in many tropical areas.
Sea levels have risen approximately 15-20 cm (6-8 inches) worldwide in the last century, due to melting mountain glaciers and ocean expansion from warmer ocean temperatures.
The Future—What Predictions Can We Make