An increase in the Earth's average surface temperature as the result of a buildup of heat-trapping greenhouse gases in the atmosphere, largely due to human activities such as burning fossil fuels and deforestation.

Carbon dioxide, methane, and other greenhouse gases trap heat in the atmosphere and warm the Earth. This is called the greenhouse effect, a natural phenomenon that's actually necessary for the sustenance of life on Earth. When these gases increase beyond what's normal and healthy, so does the planet's average temperature. That's called global warming. In the past century, average global temperatures have risen 1 degree Fahrenheit. Levels of carbon dioxide are higher now than they have been at any time in more than 650,000 years, and they continue to rise. Scientists overwhelmingly agree that these increased gases are anthropogenic, or a result of human activities. This means that global warming is the result of-and on the bright side, can be fixed by-our own behavior.

Many people use the terms "global warming" and "climate change" interchangeably, but they actually mean different things. When average global temperatures increase, various scientific phenomena occur, including melting glaciers and sea ice, rising sea levels, and more severe and frequent extreme weather events such as prolonged droughts, floods, and hurricanes. These long-term environmental consequences of global warming are collectively called climate change. By definition, "climate" refers to the normal weather conditions of a specific geographic area over a long period of time (usually 30 years). So the term "climate change" refers to the many long-term, cumulative effects of a global temperature increase (i.e. global warming) on the overall weather pattern and environment. Other predicted impacts of global warming include species extinction and increased incidence of human diseases such as malaria.

Global warming does not mean that every spot on the planet is getting warmer. Indeed, temperature and climate changes may vary by geographic location. Some areas of the world will get dryer while others get wetter. Similarly, some areas-such as northern Europe-may even become cooler due to the various climatic cycles that global warming upsets, including the Gulf Stream. But overall, the planet is getting warmer, and it will continue to as we increase the amount of greenhouse gases in the atmosphere by burning fossil fuels such as oil, gas, and coal, by cutting down rainforests, and by industrial activities such as cement manufacturing, aluminum smelting, and large-scale farming.

Increases in both average global temperatures and greenhouse gas concentrations in the atmosphere continue to be documented. The ten warmest years this century were all observed after 1980, and the three hottest were after 1990; in addition, 1998 and 2005 were the two warmest years on record.

History

Prior to the Industrial Revolution of the 1700s, climate change was purely a natural phenomenon. Technological changes in industry and agriculture during that time period meant that, for the first time, human activities began to alter the planet's climate and increase its overall temperature.

In 1957, scientists working on the International Geophysical Year, a global research program, first documented an increase in both the average global temperature and the concentration of carbon dioxide in the Earth's atmosphere. Their predecessors, including the Swedish scientist Svante Arrhenius, had suspected as early as 1896 that certain human activities would put more carbon dioxide into the atmosphere, which in turn would cause the climate to change.

The first high-level government mention of global warming in the U.S. came in 1965, in a report prepared by the President's Science Advisory Committee Panel on Environmental Pollution. One of the pioneering scientists behind this report was the geophysicist Roger Revelle, who was also involved in the 1957 research.

General awareness of global warming continued to grow in the 1970s, especially after the National Academy of Sciences issued its Energy and Climate study in 1997. (Revelle was the panel's chair.) Former Vice-President Al "An Inconvenient Truth" Gore, who was once a student of Revelle's, later co-sponsored the first Congressional hearings about global warming and wrote the pivotal book, "Earth in the Balance."

The United Nations Framework Convention on Climate Change, an international treaty that came out of the 1992 Earth Summit, took effect in 1994 and is currently ratified by 189 countries. It sets guidelines for intergovernmental cooperation in reducing greenhouse gas emissions, starting with information gathering and reporting. The U.S. publishes an annual national greenhouse gas inventory that tracks its national emissions since 1990, in order to meet its convention obligations. The U.S. currently produces more greenhouse gases than any other country.

The Kyoto Protocol, an international treaty adopted in 1997, was born of the UNFCCC. It sets binding greenhouse gas emission targets for developed nations at 5.2% below 1990 levels, on average, to be reached by 2012. Although the U.S. agreed to reduce its emissions by 7% within this timeframe when it signed the treaty, the Bush administration later declined to ratify it. The ratifiers of Kyoto acknowledge that its emissions targets aren't strong enough to stop global warming, but that the mechanisms put into place would ultimately be helpful, with the idea that future talks would occur and new targets would be set. The Protocol went into effect in 2005, and has been ratified by 165 countries to date.

Context

Although atmospheric composition and the climate can vary on their own, there is near-universal scientific consensus that increased greenhouse gas emissions from human activity is causing the planet to warm, and that significant climatic changes will occur if nothing is done to reverse the trend.

Global warming has become a highly politicized issue over the past two decades, partly because reversing it requires significant policy and behavioral shifts, as well as federal legislation and leadership. A small number of well-funded skeptics, mostly with energy industry ties continue to doubt or challenge the consensus of the world's scientific community that global warming has worsened because of human activity. Although their numbers are diminishing, these few argue, falsely, that global warming is either a myth or that there is dissent within the scientific community. This is not the case.

The 2001 Climate Change Assessment Report from the Intergovernmental Panel on Climate Change, a U.N. group, offers the consensus of more than 2,500 climate scientists from roughly 100 countries. The Natural Resources Defense Council has an annotated bibliography of global warming science that summarizes relevant scientific findings by year. In addition, the Pew Center on Global Climate Change offers the article, "Climate Change: Myths and Realities."

The truth is-even if one wishes to argue about what causes global warming or how well we can stop it-no one can deny that it is actually happening. The 1-degree temperature rise this past century has been documented. Snow cover has decreased, mountain glaciers have retreated, and the average sea levels and temperatures have risen. Global warming is happening now.

To get involved

SaveOurEnvironment.org (A coalition of environmental advocacy organizations)

StopGlobalWarming.org

For further reading

Links
Wikipedia - Global Warming

ClimateCrisis.net - An Inconvenient Truth

PBS: NOW's History of Global Warming

"Climate Change: Myths and Realities," Pew Center on Global Climate Change

Union of Concerned Scientists - Global Warming

Natural Resources Defense Council - Global Warming

RealClimate.org

The Intergovernmental Panel on Climate Change (IPCC)

United Nations Framework Convention on Climate Change (UNFCCC)

Kyoto Protocol

U.S. Environmental Protection Agency - Global Warming (archive)

U.S. Environmental Protection Agency - Climate Change

Books

An Inconvenient Truth, by Al Gore

The Weather Makers, by Tim Flannery

The End of Nature, by Bill McKibben