Global warming is related to the more general phenomenon of climate change, which refers to changes in the totality of attributes that define climate. In addition to changes in air temperature, climate change involves changes to precipitation patterns, winds, ocean currents, and other measures of Earth’s climate. Normally, climate change can be viewed as the combination of various natural forces occurring over diverse timescales. Since the advent of human civilization, climate change has involved an “anthropogenic,” or exclusively human-caused, element, and this anthropogenic element has become more important in the industrial period of the past two centuries. The term global warming is used specifically to refer to any warming of near-surface air during the past two centuries that can be traced to anthropogenic causes. Giving voice to a growing conviction of most of the scientific community, the Intergovernmental Panel on Climate Change (IPCC) reported that the 20th century saw an increase in global average surface temperature of approximately 0.6 °C (1.1 °F). The IPCC went on to state that most of the warming observed over the second half of the 20th century could be attributed to human activities, and it predicted that by the end of the 21st century the average surface temperature would increase by another 1.8 to 4.0 °C (3.2 to 7.2 °F), depending on a range of possible scenarios. Many climate scientists agree that significant economic and ecological damage would result if global average temperatures rose by more than 2 °C [3.6 °F] in such a short time. Such damage might include increased extinction of many plant and animal species, shifts in patterns of agriculture, and rising sea levels. The IPCC reported that the global average sea level rose by some 17 cm (6.7 inches) during the 20th century, that sea levels rose faster in the second half of that century than in the first half, and that—again depending on a wide range of scenarios—the global average sea level could rise by another 18 to 59 cm (7 to 23 inches) by the end of the 21st century. Furthermore, the IPCC reported that average snow cover in the Northern Hemisphere declined by 4 percent, or 1.5 million square km (580,000 square miles), between 1920 and 2005.
7/12/08
