But a rise of three degrees, according to the Stern Economic Review of Climate Change, would leave up to 170 million more people facing increased threats from severe coastal floods and as many as 550 million more at risk of hunger. It would also leave up to half of all species facing extinction.

However, 102 of the world’s poorest countries, including the grouping of small island states – acknowledged as being among the most threatened by the impact of climate change – and African nations, have argued that this increase as targeted by negotiators, would turn millions of people on the continent into environmental refugees, given the consequences of such a rise on their livelihoods, such as a sharp decline in tropical crop yields, more flooding and droughts. They have been pressing for emission cuts resulting in a temperature increase of no more than 1.5 degrees Celsius.

The UN analysis, drafted by the UN secretariat running the Copenhagen summit, therefore suggests much deeper cuts will have to be agreed on in order to achieve the objective of limiting temperature rises to two degrees celsius.

The document also indicates an existing gap of up to 4.2 gigatonnes of carbon emissions between the present pledges and the required 2020 level of 44Gt, which is required to stay below a two degrees Celsius rise.

“Unless the remaining gap of around 1.9-4.2Gt is closed and Annexe 1 parties – essentially comprising industrialised countries – commit themselves to strong action before and after 2020, global emissions will remain on an unsustainable pathway that could lead to concentrations equal or above 550 parts per million, with the related temperature rise around 3C,” it says. It does not specify a time when 3C would be reached, but observers believe it is likely to be 2050.

Greenpeace campaigner Joss Garman, was quoted as saying: “This is an explosive document that shows the numbers on the table at the moment would lead to nothing less than climate breakdown and an extraordinarily dangerous situation for humanity.

The internal paper says: “Further steps are possible and necessary to fill the gap. This could be done by increasing the aggregated emission reductions in industrialised countries to at least 30 per cent below the baseline levels, further stronger voluntary actions by developing countries, such as China and India, to reduce their emissions by at least 20 per cent below ‘business as usual,’ and reducing further emissions from deforestation and international aviation and marine shipping.”

Earlier in the week, Rajendra Pachauri, a Nobel Peace prize winner jointly with former U.S. Vice President Al Gore – who heads the UN’s Intergovernmental Panel on Climate Change, said that even with 1.5 C rises, many communities would suffer.

“Some of the most vulnerable regions in the world would be worst affected. These will be the largest countries in the developing world. They have little infrastructure that might protect them from climate change. The tragedy of the situation is that those countries that have not at all contributed to the problem of climate change will be the ones most affected,” he said.