Today, experts say such refugees may already number in the millions and could reach 200 million by century's end, stoking tensions and potential for conflict.

They point to Inuit communities literally undercut by melting ice in North America and Greenland, the thirsty people around central Africa's fast-shrinking Lake Chad, and the tens of thousands displaced from New Orleans by Hurricane Katrina.

In the future, these ranks could be swollen by refugees fleeing flooded homes, parched farmland or wrecked economies, from small island states in the Pacific to tropical Africa and the Mediterranean rim.

"The issue of environmental refugees promises to rank as one of the foremost human crises or our time," said Norman Myers, an Oxford University professor.

What constitutes a refugee can stoke emotional debate.

Critics of the term say it is a politically-charged misnomer, liable to hype or inaccuracy.

Climate refugees, they argue, should not be confused with people who flee their homes because of ecological stress caused by over-population, pollution, abuse of freshwater or other acts of greed, ill judgement or bad planning.

In the case of Katrina, scientists acknowledge a single extreme weather event cannot by itself be pinned to a long-term phenomenon, although they also point out that warmer seas provide raw fuel to make hurricanes more vicious.