At an emergency climate summit in Copenhagen, scientists agreed that "worst case" scenarios were already becoming reality and that, unless drastic action was taken soon, "dangerous climate change" was imminent.
In a strongly worded message that, unusually for academics, appealed directly to politicians, they said there was "no excuse for inaction" and that "weak and "ineffective" governments must stand up to big business and "vested interests".
Steps should be "vigorously and widely implemented", they said, to reduce greenhouse gases. Failure to do so would result in "significant risk" of "irreversible climatic shifts", the statement added.
The plea came as Lord Stern, the former chief economist of the World Bank whose report two years ago drew attention to the possible results of global warming, told the conference that unless politicians grasped the gravity of the situation it would be "devastating".
Increases in average temperatures of six degrees by the end of the century were an increasing possibility and would produce conditions not seen on Earth for more than 30 million years, he said.
That could mean massive rises in sea level, whole areas devastated by hurricanes and others turned into uninhabitable desert, he claimed, forcing billions of people to leave their homelands.
He told the summit that politicians continued to underestimate the impact of climate change and that scientists needed to redouble their efforts to get them to understand. "Much of southern Europe would look like the Sahara. Many of the major rivers of the world, serving billions of people, would dry up in the dry seasons or re-route.
"What would be the implication? Hundreds of millions of people would have to move, probably billions. What would be the implication of that? Extended conflict, social disruption, war essentially, over much of the world for many decades."
Prof Kevin Anderson, the research director at the Tyndall Centre for Climate Change Research in Manchester, said: "Scientists have lost patience with carefully constructed messages being lost in the political noise. We are now prepared to stand up and say enough is enough."