Scientists say the three degree rise would likely have severe consequences on human development for centuries, and may well trigger "tipping points" that cause uncontrollable climate change.
The document, marked "confidential very initial draft - do not distribute", shows the pledges made to date would fall well short of the stated aim of world leaders, including that of the Prime Minister Kevin Rudd, to hold world temperature rises to the safer level of two degrees.
"Unless the remaining gap of around 1.9 to 4.2 Gt (billion tonnes of greenhouse gases) and Parties commit themselves to strong action ... global emissions will peak later than 2020 and remain on an unsustainable pathway that could lead to concentrations equal 550 ppm (parts per million of carbon dioxide in the atmosphere) with the related temperature raise 3 (degrees celcius) or above 550 ppm," the document reads.
Couched in the bureaucratic language of the UN, this is a stark warning that carbon emissions cuts are on the wrong track.
The analysis is, however, in keeping with the trajectories developed by the UN's peak global warming body, the Intergovernmental Panel on Climate Change.
The estimated impacts of a three degree temperature rise include half of the world's animal species facing extinction and half a billion people threatened with starvation.
The average global temperature has not consistently been three degrees above pre-industrial levels since about three million years ago, well before modern humans existed.
The document, dated December 15, has the name "Bill McKibben" written in handwriting on it - a reference to the founder of the campaign which aims to limit the global temperature rise to a safer 1.5 degrees.
Mr McKibben told the Herald he had no part in the leak, and guessed a UN staffer may have released the material.
"What this shows, to me, is that the world leaders think political reality is more important than scientific reality," Mr McKibben said from Copenhagen. "Somehow they think they are going to be able to outmanoeuvre physics."
Australia currently proposes minimum emissions cuts of five per cent on its 2020 level, rising to 25 per cent if there is a binding global deal.
To keep the world within the two degree temperature rise, cuts of between 25 and 40 per cent on a baseline year of 1990 would be required, according to UN estimates.
Ben Cubby is the Herald's Environment Reporter