Billions of people will face catastrophic drought and food shortages because of climate change in the second half of this century unless governments act now, a new report claims.
The latest assessment of global warming from a United Nations network of scientists predicts massive migration flows and major flooding affecting coastal cities around the world if the atmosphere carries on heating up at current rates.
Two months ago the United Nations Intergovernmental Panel on Climate Change (IPCC), predicted a three-degree average global temperature rise by the end of the century. Now the same panel publishes its worst-case scenario for the impact on ordinary people around the globe.
The report, finalised at four days of talks in Brussels, warns that many of the poorest, least industrialised nations - least to blame for global warming - are also the most vulnerable to its consequences.
It suggests that, unless greenhouse gas emissions can be stabilised, between 1.1 billion and 3.2 billion people will face water shortages and between 200 million and 600 million will confront hunger by 2080 as global warming triggers heatwaves, fires, droughts and storms.
The IPCC, drawing on expertise from 2,500 scientists from 130 countries, says it is more than 90% certain that human behaviour is to blame for the crisis.
Experts fear that rising waters as ice caps melt will flood coastal cities, claiming between two million and seven million victims in Tokyo and New York alone.
Scientists want governments to commit themselves to reversing the rise in greenhouse gas emissions by 2020.
The Chairman of the IPCC, Rajentra Pachauri, said: "It's a very good document which I hope will be paid attention to around the world. This was a complex exercise. It was not an easy document to produce but it has now been approved".
He was referring to last minute arguments about the exact terms of the report, and the scale of the devastation the scientists now predict. The report will be considered by leaders of the industrialised world at a G8 summit in June.
Greenpeace described the latest report as "shocking" and Stephanie Tunmore, Greenpeace International Climate and Energy Campaigner, warned: "This is a glimpse into an apocalyptic future."
"The earth will be transformed by human induced climate change, unless action is taken soon and fast.
"What this report shows is that we are simply running out of time."
The British Red Cross said the report confirmed worst fears about the vulnerability of poor countries.
"Climate change is a humanitarian issue as much as it is environmental, political or economic," said Head of Policy David Peppiatt.