Global Warming and Greenhouse Gases

According to the National Academy of Sciences, the Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century, with accelerated warming during the past two decades. There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities.

Human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases - primarily carbon dioxide, methane and nitrous oxide. The heat-trapping property of these gases is undisputed although uncertainties exist about exactly how earth's climate responds to them.

Scientists know for certain human activities are changing the composition of Earth's atmosphere. Increasing levels of greenhouse gases like carbon dioxide (CO2) in the atmosphere since pre-industrial times have been well documented. There is no doubt this atmospheric buildup of carbon dioxide and other greenhouse gases is largely the result of human activities.

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. This is because other factors, both natural and human, affect our planet's temperature. Scientific understanding of these other factors - most notably natural climatic variations, changes in the sun's energy and the cooling effects of pollutant aerosols - remains incomplete.

Scientists have identified our health, agriculture, water resources, forests, wildlife and coastal areas are vulnerable to the changes global warming may bring. But projecting what the exact impacts will be during the 21st century remains very difficult. This is especially true when one asks how a local region will be affected.

For more information, visit EPA's Global Warming Web site.

Published: April 28, 2003