The Anasazi culture of the southwestern United States reached its zenith between 1050 and 1125 A.D. before experiencing a dramatic collapse. Despite their advanced industrial society known for their cliff dwellings and ornate baskets, no authoritative written record adequately explains this phenomenon. Archeologists aren’t even sure what the Anasazi, Navajo for “Ancient Ones,” called themselves.

Climate evidence points to significant weather changes, namely The Great Drought, as a major factor leading to the abandonment of the larger Anasazi settlements, once the size of today’s Albuquerque. Their descendents now are scattered into much smaller bands of Hopi, Zuni, Acoma, and Pueblo peoples and isolated on remote Native American reservations in Arizona and New Mexico.

View larger image
Cliff Palace at Mesa Verde.

The Southwest is once again experiencing a Great Drought. Dendrochronologists, scientists who study tree rings, have reported that 2002 was the driest year since 1685.

The Great Droughts of antiquity were triggered by naturally occurring climate shifts of uncertain cause, but scientists by and large attribute these modern climate events to human actions triggering industrialized societies’ increased releases of greenhouse gases.

Planet Earth, one might say, has a fever, and Earth’s temperature has spiked abruptly within the last decade. The five hottest years since the 1890s have all occurred since 1998. The six principal greenhouse gases or GHGs (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 they emit and absorb radiation, a process generally known as the greenhouse effect. Those warmer temperatures amplify the basic dynamics that determine rainfall.

With warm air holding more vapor, the atmosphere of a warmer world contains more moisture. That increased atmospheric moisture, however, does not translate into more rain but rather to changes in rainfall patterns. Climate models predict the polar and subpolar regions will become wetter while the subtropics will become drier.

An extensive body of scientific evidence indicates that the southwestern United States, southern Africa, and southern Australia will all become more arid, and that evidence is illustrated by current record droughts affecting the Four Corners region of the United States, Australia’s Murray-Darling Basin, and the sub-Saharan African Sahel.

View larger image
An increasingly common image for many would-be recreational boaters?

Climate scientists at the National Center for Atmospheric Research, NCAR, in Boulder, Co., list human-induced atmospheric warming as the leading cause of water scarcity. NCAR data indicate that the proportion of Earth’s land area stricken by severe drought has more than doubled since the 1970s, while 30 percent of the world’s land mass could suffer from extreme drought by 2100.

Increased flooding over the past century is also consistent with this climate model. Downpours have become more intense, even as total precipitation has remained largely unchanged. The United Nations Millennium Ecosystem Assessment reports the number of major floods affecting every continent has increased over the past 50 years. Land use changes and population shifts certainly contribute to these calamities, but a warmer climate increases both the intensity and the frequency of flash flooding from torrential rains – defined by a timescale of less than six hours – and recently experienced in the American Midwest.

Other climate-related precipitation problems arise not only from 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, threatening this ancient storage system.

Extreme weather may already be contributing to international conflict and refugee crises, as droughts and floods can lead to tremendous population displacement. There are currently an estimated 25 million environmental refugees – more than those displaced by war or religious persecution – and the United Nations anticipates 150 million could exist by 2050.

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 as Lake Chad shrinks. Adding to the challenge is the fact that only 6 percent of African cropland is irrigated, making the region very 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. Floods are the top-ranked natural disaster in human history – both in terms of lives lost and property damage – and their intensity promises to worsen despite our society’s modern marvels, such as weather forecasting and flood walls.

Pre-industrial peoples lacked both the expertise to study weather patterns and the means to create deleterious climatic impacts through their daily activities. In contrast, modern cultures have both the knowledge to understand and impact our environment and also the ability to envision and assuage adverse human-caused climate change. Those are skills that can help modern society reduce or avert human catastrophe.

The question for now remains whether we’ll get on with using those skills, and to what end.

Leslie King, MD, MPH, is Founding Director of Flying Physicians International. She currently is completing a one-year mid-career masters at the Yale School of Forestry and Environmental Studies, focusing on communications on impacts of climate change on human health.