The advanced, industrial Anasazi culture of the southwestern United States reached its zenith between 1050 and 1125 A.D., before experiencing a dramatic collapse. No written record exists that explains this phenomenon. Climatic evidence, however, points to significant weather changes, such as the Great Drought.

The Southwest is once again experiencing a Great Drought; evidence from dendrochronology, the science that studies tree rings, shows that 2002 was the driest year since 1685. But the Great Droughts of antiquity were triggered by naturally occurring climate shifts of uncertain cause. Today, human actions — particularly the release of greenhouse gases, which has dramatically increased of late — are triggering modern-day climatic disasters.

Planet Earth has a fever, and her temperature has spiked abruptly within the last decade. The five hottest years since the 1890s have all occurred since 1998. The six greenhouse gases (water vapor, carbon dioxide, nitrous oxide, ozone methane, and chlorofluorocarbons), while essential to the Earth’s energy balance, cause warmer temperatures when in excess because of their ability to emit and absorb radiation, a process known as the greenhouse effect.

Warmer weather will amplify the basic dynamics that determine rainfall. Since warm air holds more vapor, a hotter atmosphere contains more moisture. This does not translate into an increased amount of rain, but changes in rainfall patterns. Climate models predict that the polar and subpolar regions will become wetter while the subtropics will become drier. The scientific consensus — that the southwestern United States, southern Africa and southern Australia will all become more arid — is confirmed by the current record droughts affecting the Four Corners region of the United States, Australia’s Murray-Darling Basin and the sub-Saharan African Sahel.

Other climate-related precipitation problems arise from not only the amount but also the type of precipitation. One-sixth of the world’s population lives in areas where freshwater supply is dependent on glacial runoff or seasonal snowmelt. A warmer world means more rain and less snow, breaking this ancient storage system.

Drought may already be contributing to international conflict and refugee crises. The Office of the United Nations High Commissioner for Refugees considers climate change a main driver of forced displacement and tensions in Africa, linking the fighting in Darfur to skirmishes between farmers and nomadic herders in search of water. Only 6 percent of African cropland is irrigated, making the region alarmingly vulnerable to weather variations.

Ironically, the modern, man-made continental scale plumbing that sustains the American Southwest may prove no match for an epic drought caused by human-induced climate change. Drought has already proven its ability to destroy — or at least disperse — entire civilizations. Pre-industrial peoples lacked both the expertise to study weather patterns and the means to create deleterious climatic impacts through their daily activities. Our culture has the knowledge to not only understand and impact our environment, but also the ability to envision and assuage climate change — skills we should use to avert human catastrophe.