That makes immediate action to slow global warming even more vital, the teams at the National Center for Atmospheric Research in Colorado report in the journal Science.

"Even if we stabilize greenhouse gas concentrations, the climate will continue to warm, and there will be proportionately even more sea level rise," said the NCAR's Gerald Meehl, who led one of the two studies.

"The longer we wait, the more climate change we are committed to in the future."

Virtually no one disagrees human activity is fueling global warming, and a global treaty signed in Kyoto, Japan, aims to reduce polluting emissions. But the world's biggest polluter, the United States, has withdrawn from the 1997 treaty, saying its provisions would hurt the US economy.

Meehl's team ran two computer simulations of climate change -- complex programs, he said, that took months to run on supercomputers.

Those models included as many variables as the researchers could think of, such as human carbon emissions, other pollution, current temperatures and their rate of change, emissions from volcanoes, changes in solar radiation and shifts in the ozone layer.

"Then we ran for the 21st century three different scenarios," Meehl said in a telephone interview.

One scenario assumed human production of carbon dioxide and other so-called greenhouse gases stabilized in 2000 and ran the model to the year 2100.

"We found that just based on the ingredients that have already been put into the atmosphere in the 20th century, we already are committed to another half a degree (0.5 degree C or 0.9 degree F) of global warming," Meehl said.

"That's about what we saw in the 20th century. We are already committed to as much climate change in the 21st century as we saw in the 20th century."

That would mean more extreme weather and a rise in sea levels, not even accounting for melting ice, Meehl said.

Experts say sea levels have risen 4 inches (9 cm) already over the past century and could rise between 4 and 40 inches (9 to 88 cm) more in the next century.

If completely melted, the Greenland ice sheet would add 25 feet (7 metres) to overall sea level and the West Antarctic Ice Sheet would raise it by 16 feet (5 metres) -- enough to swamp most of Florida, Bangladesh and New York City's Manhattan island.

In a second study in Science, the NCAR's Tom Wigley said he used a much simpler climate model to make a similar prediction.