Rising seas and increasingly devastating storms grab headlines, but expanding deserts, falling water tables, and toxic waste and radiation are also forcing people from their homes.
Advancing deserts are now on the move almost everywhere. The Sahara Desert, for example, is expanding in every direction. As it advances northward, it is squeezing the populations of Morocco, Tunisia, and Algeria against the Mediterranean coast.
The Sahelian region of Africa -- the vast swath of savannah that separates the southern Sahara Desert from the tropical rainforests of central Africa -- is shrinking as the desert moves southward. As the desert invades Nigeria, Africa's most populous country, from the north, farmers and herders are forced southward, squeezed into a shrinking area of productive land.
A 2006 U.N. conference on desertification in Tunisia projected that by 2020, up to 60 million people could migrate from sub-Saharan Africa to North Africa and Europe.
In Iran, villages abandoned because of spreading deserts or a lack of water number in the thousands.
In Brazil, some 250,000 square miles of land are affected by desertification, much of it concentrated in the country's northeast.
In Mexico, many of the migrants who leave rural communities in arid and semiarid regions of the country each year are doing so because of desertification. U.S. analysts estimate that Mexico is forced to abandon 400 square miles of farmland to desertification each year.
In China, desert expansion has accelerated in each successive decade since 1950. Desert scholar Wang Tao reports that over the last half-century or so, some 24,000 villages in northern and western China have been abandoned either entirely or partly because of desert expansion.
China is heading for a dust bowl like the one that forced more than 2 million "Okies" to leave their land in the United States in the 1930s. But the dust bowl forming in China is much larger, and so is the population: China's migration may measure in the tens of millions.
And as a U.S. embassy report entitled "Grapes of Wrath in Inner Mongolia" noted, "unfortunately, China's 21st century 'Okies' have no California to escape to -- at least not in China."