COPENHAGEN (Reuters) - Pledges so far by all nations for curbing greenhouse gases would mean a world temperature rise of 3 degrees Celsius, above many estimates of “dangerous” climate change, according to a U.N. document seen on Thursday.

The refinery in the central Croatian town of Sisak is seen on a foggy late autumn morning December 2, 2009. REUTERS/Nikola Solic

The internal U.N. paper, dated December 15, was leaked to Reuters as climate talks reached a climax in Copenhagen, where about 120 world leaders were gathering to try to agree a global climate pact, including ambitious carbon cuts.

Industrialized nations have set a 2 Celsius warming as the maximum limit to avoid dangerous climate changes including more floods, droughts and rising seas.

The note by the U.N. Climate Change Secretariat said that present pledges were not enough, exceeding safer emissions limits by about 1.9-4.2 billion tonnes of greenhouse gases a year by 2020.

That would result in an “unsustainable pathway that could lead to ... a temperature rise of about 3 degrees,” it said.

Further steps were “possible and necessary,” it said.

The paper suggested that rich countries may have to cut their emissions by a more ambitious goal than previously thought. It also assessed emissions curbs announced by countries including China, India and South Africa.

A U.N. panel of climate scientists, the IPCC, recommended in 2007 cuts of 25-40 percent below 1990 levels by 2020 to give the world a 50 percent chance of staying below a 2 degrees Celsius temperature rise.

The U.N. paper said rich nations should cut greenhouse gas emissions by 30 percent by 2020 versus 1990 levels to stay within 2 degrees.

A safe limit required “the aggregated emission reductions by Annex 1 Parties (rich countries) of at least 30 percent below the base-line (1990) levels,” said the paper.

To avoid more than 2 degrees warming developing countries should also “reduce their emissions by at least 20 percent below business as usual,” the paper added.

The IPCC suggested developing nations should limit emissions by 15-30 percent below current trends by 2020.