Speaking at an annual meeting of the American Association for the Advancement of Science, Barnett said climate models based on air temperatures are weak because most of the evidence for global warming is not even there.

"The real place to look is in the ocean," Barnett told a news conference.

His team used millions of temperature readings made by the US National Oceanic and Atmospheric Administration to calculate steady ocean warming.

"The debate over whether or not there is a global warming signal is now over, at least for rational people," he said.

The report was published one day after the United Nations Kyoto Protocol took effect, a 141-nation environmental pact the United States government has spurned for several reasons, including stated doubts about whether global warming is occurring and is caused by people.

Barnett urged US officials to reconsider.

"Could a climate system simply do this on its own? The answer is clearly no," Barnett said.

His team used US government models of solar warming and volcanic warming, just to see if they could account for the measurements they made. "Not a chance," he said.

And the effects will be felt far and wide. "Anywhere that the major water source is fed by snow ... or glacial melt," he said. "The debate is what are we going to do about it."

COLD WINTERS, HOMELESS POLAR BEARS

Other researchers found clear effects on climate and animals.

Ruth Curry of the Woods Hole Oceanographic Institution found that melting ice was changing the water cycle, which in turn affects ocean currents and, ultimately, climate.

"As the Earth warms, its water cycle is changing, being pushed out of kilter," she said. "Ice is in decline everywhere on the planet."

A circulation system called the Ocean Conveyer Belt is in danger of shutting down, she said. The last time that happened, northern Europe suffered extremely cold winters.

She said the changes were already causing droughts in the US West.

Greenland's ice cap, which contains enough ice to raise sea levels globally by 23 feet (7 metres), is starting to melt and could collapse suddenly, Curry said. Already freshwater is percolating down, lubricating the base and making it more unstable.

Sharon Smith of the University of Miami found melting Arctic ice was taking with it algae that formed an important base of the food supply for a range of animals.

And the disappearing ice shelves meant big animals such as walruses, polar bears and seals were losing their homes.

"In 1997 there was a mass die-off of a bird called the short-tailed shearwater in the Bering Sea," Smith told the news conference.

The birds, which migrate from Australia, starved to death for several years running when warmer waters caused a plankton called a coccolithophore to bloom in huge numbers, turning the water an opaque turquoise color.