As many as one billion people could lose their homes by 2050 because of the devastating impact of global warming, scientists and political leaders will be warned today.
They will hear that the steady rise in temperatures across the planet could trigger mass migration on unprecedented levels.
Hundreds of millions could be forced to go on the move because of water shortages and crop failures in most of Africa, as well as in central and southern Asia and South America, the conference in London will be told. There could also be an effect on levels of starvation and on food prices as agriculture struggles to cope with growing demand in increasingly arid conditions.
Rising sea levels could also cause havoc, with coastal communities in southern Asia, the Far East, the south Pacific islands and the Caribbean seeing their homes submerged.
North and west Africans could head towards Europe, while the southern border of the United States could come under renewed pressure from Central America.
The conference will hear a warning from the UN High Commissioner for Refugees (UNHCR) that the developed world should start preparing for a huge movement of people caused by climate change.
The event, which is being organised by the Institute for Public Policy Research (IPPR), will also be addressed by a Kenyan farmer and a United Nations worker based in Sudan. They will give first-hand accounts of previously fertile land that has already become parched in recent years as the desert spreads.
Craig Johnstone, the UNHCR deputy high commissioner, said yesterday that humanity faced a "global-scale emergency" whose effects would accumulate over the next four decades. He said it was impossible to forecast with confidence the numbers of people who would lose their homes through climate change. But he pointed to assessments of between 250 million and one billion people losing their homes by 2050. He said: "This will be a global-scale emergency, but ... it will take place gradually and over a long period of time."
Mr Johnstone rejected the suggestion that the industrialised West should shoulder the burden because it was to blame for much of climate change. But he said: "It's the obligation of the people who have the means to be helpful to help. They have an obligation to humanity to help."
He said the UNHCR already assisted in natural disasters such as earthquakes and the Asian tsunami of 2004 and added: "Perhaps even more challenging and more inevitable are the consequences of global changes."
Currently the status of refugees – defined as people escaping personal persecution by the state – is controlled by the Geneva Convention of 1951. The agreement, however, would not cover people who become homeless, or even stateless, because of changes to global weather patterns.
Pressure is therefore growing for the international community to reach a formal consensus on ways of dealing with the issue. Mr Johnstone said: "We're strongly in favour of there being adequate international mechanisms to cope."
Danny Sriskandarajah, head of migration at the IPPR, said: "The displacement of millions of people will be one of the most dramatic ways in which climate change will affect humankind."
Hilary Benn, the Environment Secretary, said a global agreement must be reached. "Climate change is the most serious long-term threat to development in poor countries, and if unchecked millions of people may be forced to migrate to escape the effects of drought, flooding, food shortages and rising sea levels," he said.