Human-induced climate and hydrological change is likely to make many parts of the world uninhabitable, or at least uneconomic. Over the course of a few decades, if not sooner, hundreds of millions of people may be compelled to relocate because of environmental pressures.

To a significant extent, water will be the most important determinant of these population movements. Dramatic alterations in the relation between water and society will be widespread, as emphasized in the new report from Working Group II of the Intergovernmental Panel on Climate Change. These shifts may include rising sea levels, stronger tropical cyclones, the loss of soil moisture under higher temperatures, more intense precipitation and flooding, more frequent droughts, the melting of glaciers and the changing seasonality of snowmelt.

Impacts will vary widely across the world. It will be important to keep our eye on at least four zones: low-lying coastal settlements, farm regions dependent on rivers fed by snowmelt and glacier melt, subhumid and arid regions, and humid areas in Southeast Asia vulnerable to changes in monsoon patterns.

A significant rise in sea levels, even by a fraction of a meter, could wreak havoc on tens or even hundreds of millions of people. One study found that although coastal areas less than 10 meters above sea level constitute only 2 percent of the world's land, they contain 10 percent of its population. These coastal zones are vulnerable to storm surges and increased intensity of tropical cyclones--call it the New Orleans Effect.


Hundreds of millions of people may be compelled to relocate.


Regions much farther inland will wither. Hundreds of millions of people, including many of the poorest farm households, live in river valleys where irrigation is fed by melting glaciers and snow. The annual snowmelt is coming earlier every year, synchronizing it less and less well with the summer growing season, and the glaciers are disappearing altogether.

Thus, the vast numbers of farmers in the Indo-Gangetic Plain and in China's Yellow River Basin will most likely face severe disruptions in water availability. Yet those regions are already experiencing profound water stress because of unsustainable rates of groundwater pumping performed to irrigate large expanses of northern China and northern India.

In Africa, all signs suggest currently subhumid and arid areas will dry further, deepening the food crisis for many of the world's poorest and most vulnerable people. The severe decline in precipitation in the African Sahel during the past 30 years seems to be related to both anthropogenic warming and aerosol pollutants. The violence in Darfur and Somalia is fundamentally related to food and water insecurity. Ivory Coast's civil war stems, at least in part, from ethnic clashes after people fled the northern drylands of Burkina Faso for the coast. Worse chaos could easily arise.

Each El Niño cycle brings drying to thousands of islands in the Indonesian archipelago, with attendant crop failures, famine and peat fires. Some climatologists hypothesize that global warming could induce a more persistent El Niño state; if so, the 200 million people in Indonesia and neighboring areas could experience lasting drought conditions.

Until now, the climate debate has focused on the basic science and the costs and benefits of reducing greenhouse gas emissions. Attention will now increasingly turn to the urgent challenge of adapting to the changes and helping those who are most affected.

Some hard-hit places will be salvaged by better infrastructure that protects against storm surges or economizes on water for agriculture. Others will shift successfully from agriculture to industry and services. Yet some places will be unable to adjust altogether, and suffering populations will most likely move. We are just beginning to understand these phenomena in quantitative terms. Economists, hydrologists, agronomists and climatologists will have to join forces to take the next steps in scientific understanding of this human crisis.