Global warming will force up to 150 million "climate refugees" to move to other countries in the next 40 years, a new report from the Environmental Justice Foundation (EJF) warns.

In 2008 alone, more than 20 million people were displaced by climate-related natural disasters, including 800,000 people by cyclone Nargis in Asia, and almost 80,000 by heavy floods and rains in Brazil, the NGO said.

President Mohamed Nasheed of the Maldives, who presented testimony to the EJF, said people in his country did not want to "trade a paradise for a climate refugee camp". He warned rich countries taking part in UN climate talks this week in Barcelona "not to be stupid" in negotiating a climate treaty in Copenhagen this December.

Nasheed urged governments to find ways to keep temperature rises caused by warming under 2C. "We won't be around for anything after 2C," he said. "We are just 1.5m over sea level and anything over that, any rise in sea level – anything even near that – would wipe off the Maldives. People are having to move their homes because of erosion. We've already this year had problems with two islands and we are having to move them to other islands. We have a right to live."

Last month, the president held a cabinet meeting underwater to draw attention to the plight of his country.

The EJF claimed 500 million to 600 million people – nearly 10% of the world's population – are at risk from displacement by climate change. Around 26 million have already had to move, a figure that the EJF predicts could grow to 150 million by 2050. "The majority of these people are likely to be internally displaced, migrating only within a short radius from their homes. Relatively few will migrate internationally to permanently resettle in other countries," said the report's authors.

In the longer term, the report said, changes to weather patterns will lead to various problems, including desertification and sea-level rises that threaten to inundate low-lying areas and small island developing states. An expert at the Institute for Sustainable Development and International Relations in Paris recently said global warming could create "ghost states" with citizens living in "virtual states" due to land lost to rising seas.

The UN's Intergovernmental Panel on Climate Change (IPCC) predicts sea-level rise in the range of 18-59cm during the 21st century. Nearly one-third of coastal countries have more than 10% of their national land within 5 metres of sea level. Countries liable to lose all or a significant part of their land in the next 50 years, said the EJF report, include Tuvalu, Fiji, the Solomon islands, the Marshall islands, the Maldives and some of the Lesser Antilles.

Many other countries, including Bangladesh, Kenya, Papua New Guinea, Somalia, Yemen, Ethiopia, Chad and Rwanda, could see large movements of people. Bangladesh has had 70 climate-related natural disasters in the past 10 years.

"Climate change impacts on homes and infrastructure, food and water and human health. It will bring about a forced migration on an unprecedented scale," said the EJF director, Steve Trent. "We must take immediate steps to reduce our impact on global climate, and we must also recognise the need to protect those already suffering along with those most at risk."

He called for a new international agreement to address the scale and human cost of climate change. "The formal legal definition of refugees needs to be extended to include those affected by climate change and also internally displaced persons," he said.