Scientists warned the planet could be getting much hotter, much faster than scientists anticipated only two years ago, according to a review drawn up ahead of the December 7-18 UN climate conference.

The 68-page report suggests that many of the estimates in a report by the UN Intergovernmental Panel on Climate Change (IPCC) in 2007 were too low.

Now scientists say the planet could warm by seven degrees Celsius (10.8 degrees Fahrenheit) and sea levels could rise by more than a metre (3.25 feet) by 2100, scenarios that two years ago were viewed as improbable.

"This is a final scientific call for climate negotiators from 192 countries who must embark on the climate protection train in Copenhagen," said Hans Schellnhuber, director of Germany's Potsdam Institute for Climate Impact Research (PIK), which oversaw the paper.

"They need to know the stark truth about global warming and the unprecedented risks involved," he said.

Meanwhile the United States, whose stance is regarded as crucial at the global warming summit, raised hopes that major powers were moving towards a deal.

Obama said the world had moved closer to a "strong operational agreement" on climate change at the summit after his talks with Indian and Chinese leaders.

"It's essential that countries do what is necessary to reach a strong operational agreement," Obama told a joint news conference with Indian Prime Minister Manmohan Singh.

But experts say uncertainty remains over Washington's stance. The US is yet to announce concrete targets for cutting greenhouse gas emissions although the Obama administration plans to do so in the coming days.

China has put the issue on the agenda of a meeting with the European Union next week and leaders of the 53 members of the Commonwealth, representing around two billion people, are set to address it at their weekend gathering in Trinidad.

And Australia, the world's heaviest per capita polluter, is attempting to rush legislation through parliament curbing emissions ahead of the Copenhagen talks.

The US position should become clear within days, Obama's administration said late Monday, attempting to remove a major obstacle to the summit goal of finding a global treaty to replace the Kyoto Protocol which expires in 2012.

"Countries will need to put on the table what they are willing to do on emissions," a senior administration official told journalists. "We expect that a decision will be made in the coming days."

An emissions target from the United States, the world's number two polluter and wealthiest country, was essential for the success of the conference, according to United Nations climate chief Yvo de Boer.

"The key issue here at the moment is the United States. My sense is Obama will be in a position to come to Copenhagen with a target and a financial contribution," he said in Brussels on Monday.

It was unclear what the official US target would be but the administration official said it would not differ much from levels set out in legislation before Congress.

A US House of Representatives bill, passed in June, calls for cutting greenhouse gas emissions by 17 percent from 2005 levels by 2020 and by 83 percent by 2050. A bill before the Senate talks of a 20-percent reduction from 2005 levels by 2020.

The European Union has vowed to reduce its emissions by 20 percent from 1990 levels before 2020, raising the target to 30 percent in the event of an international agreement. Japan has offered 25 percent, but attached conditions.

In order to limit warming to two degrees Celsius (3.6 degrees Fahrenheit), a threshold widely adopted as safe, scientists say emissions by industrialized nations must fall by 25-40 percent by 2020 over 1990 levels.

The United States was the world's biggest carbon emitter until it was overtaken by China in 2006, according to the Global Carbon Project, a consortium of leading climate scientists.

China along with India and other developing nations, is reluctant to commit to binding gas reductions at the Copenhagen talks, arguing that wealthy nations bear historic responsibility for carbon emissions.

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