"Experts estimate that by 2050 some 200 million people will be displaced by environmental problems, a number of people roughly equal to two-thirds of the United States today," the University said in a statement.

Bogardi said present the number of environmental migrants could be between 25 million and 27 million. Unlike political refugees fleeing their country, many seek a new home in their own country.

He said it was important to work out ways of tracking the numbers of people forced to leave their homes for reasons such as repeated crop failures caused by global warming, so that governments and aid groups could work out how to help.

"The main step towards helping is recognition," Bogardi told Reuters.

In the past, many such people would be listed as economic migrants. However economic migrants, for example, were often young men looking for work.

"Environmentally-motivated migration is expected to feature poorer people, more women, children and elderly, from more desperate environmental situations," it said.

Experts from almost 80 countries will meet in Bonn from Oct. 9 to 11 to discuss how to help environmental migrants.

A study of 22 developing countries by Bogardi's institute and several other European research institutes into reasons for migration showed worries that human trafficking networks could gain from damage to the environment.

In Bangladesh, "women with children, whose husbands either died at sea during cyclone Sidr or are away as temporary labour migrants, are easy prey for traffickers and end up in prostitution networks or in forced labour in India", it said.

Similar patterns were found in at least one more national study. "Exploitation of people on the move by smugglers is reported more and more as the flow of informal or illegal migrants swells," it added.

-- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/ (Editing by Andrew Dobbie)