Sun over the Chukchi Sea. The planet's far northern and southern latitudes are projected to experience the greatest change under increasing global temperatures—and in many cases they already are. (Photo by Chris Linder, Woods Hole Oceanographic Institution)
Global warming is an observed increase in Earth’s average surface temperature. Although the planet has naturally warmed and cooled over long periods of time, there is concern today that human action is causing far more warming than would occur normally.
This is greatly adding to the heat energy in the atmosphere and ocean and contributing to such projected or observed changes as sea level rise, shifts in precipitation patterns, and changes to the lifecycle and habitat of many plants and animals. There is also concern that existing or continued warming could cause unanticipated or abrupt changes for which many living things (including humans) could find it difficult to adapt.
Since the dawn of the Industrial Revolution in the nineteenth century, humans have increased the concentration of several greenhouse gases in the atmosphere roughly 40 percent (from 280 to 390 parts per million). These gases—which include carbon dioxide, water vapor, methane, nitrous oxide, and CFCs—absorb infrared energy radiating from the surface of the planet and, instead of allowing it to pass through the atmosphere to outer space, re-radiate back into the atmosphere.
The greenhouse effect is natural—and necessary for our survival. Without it, temperatures on most parts of the planet would be too cold to support life. However, adding excess greenhouse gases to the atmosphere is trapping increasing amounts of heat, causing the average global temperature to rise about 0.8°C (1.4°F) since the early twentieth century. Sources of these gases include burning fossil fuels and forests, livestock and agricultural production, and industrial processes such as manufacturing concrete and CFC production.
Projections described in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) indicate that during the twenty-first century the global surface temperature is likely to rise another 1.1 to 2.9 °C (2 to 5.2 °F) under the report’s lowest greenhouse gas emission scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) under the highest.