Copenhagen, Dec 18 - World pledges on cutting greenhouse gas emissions by 2020 are nowhere near enough to keep global warming in check, a leaked UN report said Thursday.

But a top climate economist questioned the report, saying that what was important was the world's longterm emissions path, rather than a make-or-break figure in 2020.

The leaked text, circulated by environmental groups, came on the eve of the last day of UN talks on fighting global warming in Copenhagen, as world leaders were expected to push for a final deal.

The analysis, carried out by the UN's climate secretariat, says that "unless the remaining (emissions-reduction) gap ... is closed and parties commit themselves to strong action prior to and after 2020, global emissions will remain on an unsustainable pathway" that could raise global temperatures by up to 3 degrees by 2100.

UN scientists say that an increase in world temperatures of more than 2 degrees could lead to catastrophic climate change.

But Lord Nicholas Stern, author of a key report on the economics of climate change, stressed that the most important goal in the fight against global warming was to bring world emissions to a "path" that would ensure longterm sustainability, rather than pinning all hopes on a single cut-off year.

"You can't define a temperature increase or a probability of a temperature increase on one point in 2020. That's why I always say the challenge is to get on a path which gives us a good chance," Stern told the German Press Agency dpa. (dpa)