"The first massive movement of climate refugees has been that of people away from the Gulf Coast of the United States," said the Earth Policy Institute, which has warned for years that climate change demands action now.

Institute president Lester Brown said that about a quarter of a million people who fled the devastating impact of Hurricane Katrina a year ago must now be classed as "refugees".

"Interestingly, the country to suffer the most damage from a hurricane is also primarily responsible for global warming," he said.

The United States is the world's largest consumer of energy, but has refused to sign up to the Kyoto pact aimed at reducing emissions of gases that scientists say are to blame for heating up the Earth.

Many environmentalists had expected the first big population shift to come somewhere like the Tuamotu islands in French Polynesia, the world's largest chain of atolls which rise barely metres (feet) from the Pacific.

Rising sea levels are part of the problem afflicting low-lying places but, experts argue, so are tropical storms that are mounting in ferocity because of warmer ocean temperatures.

Brown said many thousands of people who evacuated last year as Katrina slammed into New Orleans and other populated areas on the Mississippi and Louisiana coasts had no intention of returning.

"We estimate that at least 250,000 of them have established homes elsewhere and will not return," he said.

"They no longer want to face the personal trauma and financial risks associated with rising seas and more destructive storms. These evacuees are now climate refugees."

Many businesses have also deserted the coastal towns left ravaged by Katrina as insurance and other costs soar, the study said.

"As rising seas and more powerful hurricanes translate into higher insurance costs in these coastal communities, people are retreating inland," Brown said.

"And just as companies migrate to regions with lower wages, they also migrate to regions with lower insurance costs."

The study also warned: "The experience with more destructive storms in recent years is only the beginning."

The institute said that since 1970, the Earth's average temperature has risen by one degree Fahrenheit, but by 2100 it could rise by up to 10 degrees Fahrenheit (six degrees Celsius).

Rising temperatures could melt glaciers and polar ice caps, raising sea levels and displacing coastal residents worldwide.

"The flow of climate refugees to date numbers in the thousands, but if we do not quickly reduce CO2 (carbon dioxide) emissions, it could one day number in the millions," Brown said.

The institute's study classed "climate refugees" as part of a larger group of people who have been forced from their homes by man-made environmental change such as overgrazing.

"Overgrazing destroys the vegetation which leads then to local sandstorms ... we are looking at growing flows of environmental refugees in Africa, for example in Nigeria, Senegal, Mauritania or Kenya," Brown told reporters.

Millions of people in northern and western China have abandoned their villages as the land turns semi-arid because of overgrazing, the study said.

China is also the second biggest greenhouse-gas polluter after the United States thanks to the voracious rise in coal, gas and oil consumption to power its economic growth.

The booming port city of Shanghai could be at risk of flooding from more ferocious typhoons linked to global warming as it is only a metre (three feet) above sea level, Brown said.

All rights reserved. copyright 2018 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.