(November 2000) Most of the sun's radiation passes through Earth's atmosphere to warm the planet. Earth's surface in turn reflects this energy back toward space in the form of infrared (heat) radiation. Certain gases in the atmosphere such as carbon dioxide and water vapor absorb and re-emit this energy, which slows down the process of releasing the solar energy back into space. This natural trapping of heat keeps the planet surface warmer than it would otherwise be, making it more hospitable to life.

Over the past two centuries, all kinds of human activities, from powering steam engines to surfing the World Wide Web, have added to this "greenhouse" effect. The Industrial Revolution, particularly the widespread use of fossil fuels such as oil and coal, has led to the release of enormous amounts of heat-absorbing gases into the atmosphere. The four greenhouse gases whose atmospheric concentrations are most influenced by human activities are:

  • Carbon dioxide or CO2 (produced by burning solid waste, wood and wood products, and fossil fuels such as oil, natural gas, and coal);
  • Methane or CH4 (emitted by livestock or by the decomposition of organic wastes in municipal solid waste landfills);
  • Nitrous oxide or N2O (generated by the combustion of fossil fuels and solid waste);
  • Chlorofluorocarbons (also called CFCs, manufactured by industry for use in coolants and insulation).

Global emissions of carbon dioxide from human activities have increased more than 1000-fold, from 660 million to 931 billion metric tons between 1795 and 1995, according to the U.S. Department of Energy's Carbon Dioxide Information Analysis Center (CDIAC). As a result, atmospheric concentrations of carbon dioxide have risen substantially over the past two centuries.

The implications of rising levels of greenhouse gases in the atmosphere have received a great deal of scientific attention over the past decade. To assess the mounting body of scientific, technical, and socioeconomic research on the climatic effects of rising greenhouse gas concentrations, the World Meteorological Association and the United Nations Environment Program jointly created the Intergovernmental Panel on Climate Change (IPCC) in 1988. In 1995, the IPCC concluded in its Second Assessment Report that "the balance of evidence suggest[ed] a discernible human influence on the global climate," a determination that helped lead to the setting of emissions targets in the Kyoto Protocol.

The IPCC's Third Assessment Report — completed in October 2000 — cited "stronger evidence" than ever before linking human activity to climate change. It also argued that man-made greenhouse gases have probably already "contributed substantially to the observed warming over the last 50 years." The IPCC also revised its estimate of the amount of global warming that is likely in the future: If greenhouse gas emissions are not curtailed, the Earth's average surface temperatures can be expected to increase by 1.5 to 6.1 degrees Celsius (2.7 to 11 degrees Fahrenheit) by 2100, substantially more than the panel estimated in its 1995 report.

While this amount of temperature change might not sound catastrophic, the impact on human populations and natural ecosystems would be significant. Such an increase in global temperature would probably lead to a rise in sea level of 15 to 95 centimeters (about six to 37 inches) or even higher, causing damage to many coastal communities. Global warming would also be accompanied by more extreme weather patterns, heat waves, and more severe droughts. As the most recent assessment noted, the changes may already be underway: The U.S. National Oceanographic and Atmospheric Administration reported in 1998 that each of the first eight months of that year were the hottest since 1880, when global average temperatures were first recorded.

 Bingham Kennedy, Jr. is an associate editor at the Population Reference Bureau.  

For More Information

To read the IPCC's Third Assessment Report, visit their website at www.ipcc.ch/.