Politicians have failed to take on board the severe consequences of failing to cut world carbon emissions, according to Nicholas Stern, the economist commissioned by Gordon Brown to analyse the impact of climate change.

His stark warning about the potentially "devastating" consequences of global warming came as scientists issued a desperate plea last night for world leaders to curb greenhouse gas emissions or face an ecological and social disaster.

More than 2,500 climate experts from 80 countries at an emergency summit in Copenhagen said there is now "no excuse" for failing to act on global warming. A failure to agree strong carbon reduction targets at political negotiations this year could bring "abrupt or irreversible" shifts in climate that "will be very difficult for contemporary societies to cope with".

In a significant break from the scientific tradition not to comment directly on policy, the experts insisted politicians must stand up to "vested interests that increase emissions" and "build on a growing public desire for governments to act". They called for a "shift from ineffective governance and weak institutions to innovative leadership in government, the private sector and civil society".

Katherine Richardson, a climate scientist at the University of Copenhagen, who organised the three-day summit, said: "We have to act and we have to act now. We need politicians to realise what a risk it is they are taking on behalf of their own constituents, the world's societies and, even more importantly, future generations. All of the signals from the Earth system and the climate system show us we are on a path that will have enormous and unacceptable consequences."

Speaking after giving a keynote speech, Stern said he feared that politicians had not grasped the seriousness of the crisis. "Do the politicians understand just how difficult it could be? Just how devastating four, five, six degrees centigrade would be? I think not yet. Looking back, the Stern review underestimated the risks and underestimated the damage from inaction."

This week's conference was arranged to offer an update on scientific thinking on global warming, and underpin political attempts to agree a new global climate treaty to replace the Kyoto protocol. Campaigners have called for such a deal to be signed at UN talks in December, also in Copenhagen, though officials have warned the discussions could drag on into next year.

Scientists at the Copenhagen meeting will publish its full findings in June, but last night they issued their conclusions as a strongly worded statement"

"The climate system is already moving beyond the patterns of natural variability within which our society and economy have developed and thrived. These parameters include global mean surface temperature, sea-level rise, ocean and ice sheet dynamics, ocean acidification, and extreme climatic events. There is a significant risk that many of the trends will accelerate, leading to an increasing risk of abrupt or irreversible climatic shifts."

The summary adds: "There is no excuse for inaction. We already have many tools and approaches - economic, technological, behavioural, management - to deal effectively with the climate change challenge. But they must be vigorously and widely implemented."

In the conference centre that will also host the December UN negotiations, experts at this week's meeting presented a string of new studies that suggested global warming could strike harder and sooner than expected.

They said carbon emissions have risen more in recent years than anyone thought possible, and the world's natural carbon stores could be losing the ability to soak up human pollution.

The conference also heard that:

· A 4C rise could turn swaths of southern Europe to desert.

· Sea levels will rise twice as fast as official estimates predict.

· Modest warming could unleash a carbon "time bomb" from Arctic soils.

· A failure to cut emissions could render half of the world uninhabitable.

· Rising temperatures could kill off 85% of the Amazon rainforest.

Several experts at the conference warned that temperatures are likely to soar beyond the 2C target set by European politicians, though they are reluctant to say so publicly. "The 2C target is gone and 3C is difficult. I think we're heading for 4C at least," one said. Oxford University yesterday announced that it would hold a conference in September to discuss the implications of a rise of 4C or more.

Kevin Anderson, research director at the UK Tyndall Centre for Climate Change Research, said: "The scientists have lost patience with our carefully constructed messages being lost in the political noise. And we are now prepared to stand up and say enough is enough."

Peter Cox, a climate expert at Exeter University, said: "People have been saying this individually for a long time. This is just a much louder and concerted shout from our community."

Rob Bailey, senior climate adviser for Oxfam said: "The verdict of the world's top scientists is clear. The big question now is whether the worlds richest countries, who created the climate crisis, will act before it's too late. Our climate is changing fast and if left unchecked its impacts, particularly on the world's poorest people, will be devastating."