Global surface and ocean temperatures, sea levels, extreme climate events, and the retreat of Arctic sea ice have all significantly picked up more pace than experts predicted only a couple of years ago, they said.

The stark warning comes less than six months before an international conference aiming to seal a treaty to save the planet from the worst ravages of global warming.

A 36-page document summarized more than 1,400 studies presented at a climate conference in March in Copenhagen, where a United Nations meeting will be held in December to hammer out a successor to the Kyoto Protocol. Kyoto expires in 2012.

The report said greenhouse gas emissions and other climate indicators are at or near the upper boundaries forecast by the UN Intergovernmental Panel on Climate Change (IPCC), whose 2007 report has been the scientific benchmark for the troubled UN talks.

There is also new evidence that the planet itself has begun to contribute to global warming through fall out from human activity.

Huge stores of gases such as methane -- an even more powerful greenhouse gas than carbon dioxide -- trapped for millennia in the Arctic permafrost may be starting to leak into the atmosphere, speeding up the warming process.

The natural capacity of the oceans and forests to absorb CO2 created by the burning of fossil fuels has also been compromised, research has shown.

The new report, written and reviewed by many of the scientists who compiled the IPCC document, calls on policy makers to take urgent steps to keep average global temperatures from increasing more than two degrees Centigrade (3.6 degree Fahrenheit), compared to pre-industrial levels.

"Rapid, sustained, and effective mitigation ... is required to avoid 'dangerous climate change' regardless of how it is defined," it said.

"Temperature rises above 2 C will be difficult for contemporary societies to cope with, and are likely to cause major societal and environmental disruptions through the rest of the century and beyond."

The IPCC has said that achieving this goal would require industrialised nations to slash greenhouse gas emissions by 25-40 percent compared to 1990 levels.

The new report suggested that deep and early emissions cuts -- one of the most contentious issues on the table in the UN talks -- are essential.

"Weaker targets for 2020 increase the risk of serious impacts, including the crossing of tipping points" beyond which natural forces begin to push up temperatures even faster.

Many scientists agree that if those boundaries are crossed, it would be difficult, perhaps impossible, to reverse the process.

The new synthesis does not carry the same weight as the IPCC report, which is based on an even wider range of studies and -- most importantly -- is a consensus document, which means even conservative scientific viewpoints are taken into account.

But the IPCC data is at least four or five years old, and a welter of new research suggests the global warming impacts could be even worse, and will arrive sooner rather than later.

Climate modelers at MIT, for example, recently calculated that unless huge efforts are made to slash carbon pollution, Earth's surface temperatures will jump 5.2 C (9.4 F) by 2100, more than twice as high as their own predictions in 2003.

The Copenhagen report will be presented to Danish Prime Minister Lars Loekke Rasmussen on Thursday during a European Union summit in Brussels.

Rasmussen, who will host the UN conference in December, has called on scientists to provide "concrete advice" to policy makers.

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