Desertification, rising sea levels, flooding and storms linked to climate change might displace hundreds of millions of people, according to the report published on Tuesday by the UN University's Institute for Environment and Human Security.
Janos Bogardi, head of the Bonn-based Institute, said: "We're ringing a kind of scientific and political alarm bell. We need to act."
He said the estimated figure of 50 million environmental refugees - roughly the population of Ukraine or Italy - was in some ways a worst-case scenario that would demand billions of dollars in extra aid.
Still, he estimated that about 20 million people were already displaced by problems linked to a damaged environment, ranging from eroded farmland to polluted water supplies.
Such upheavals already affect millions of people in sub-Saharan Africa, India and Asia, he said.
The institute urged acceptance of the idea that "environmental refugees" - people displaced by environmental degradation - should be eligible for food, tools, shelter, medical care and grants similar to political refugees fleeing war or oppression at home.
"We should prepare now, however, to define, accept and accommodate this new breed of refugee"
Hans van Ginkel,
UN Undersecretary-General and Rector of the UN University
Bogardi said that victims of slow-moving environmental catastrophes were too often dismissed as people moving for purely economic reasons, who are usually denied refugee status.
Among threats, the Gobi desert in China is expanding by more than 10,000sq km a year.
The low-lying Pacific island state of Tuvalu has struck a deal for New Zealand to accept its 11,600 population if sea levels rise.
"This is a highly complex issue, with global organisations already overwhelmed by the demands of conventionally recognised refugees," Hans van Ginkel, UN undersecretary-general and rector of the UN University, said in the report.
"We should prepare now, however, to define, accept and accommodate this new breed of refugee," he said.
Americans who fled the Dust Bowl of the 1930s, for instance, were driven by a mix of environmental degradation and poverty caused by their failed crops.
Costs of coping with "environmental refugees" could be huge. The UN refugee agency UNHCR estimates that it has helped 50 million conventional refugees to restart their lives since it was set up in 1950.
For 2005, it has received about $980 million in funds, mainly from governments, led by the US.