The task of cutting greenhouse gas emissions enough to avert a dangerous rise in global temperatures may be far more difficult than previous research suggested, say scientists who have just published studies indicating that it would require the world to cease carbon emissions altogether within a matter of decades.

Their findings, published in separate journals over the past few weeks, suggest that both industrialized and developing nations must wean themselves off fossil fuels by as early as mid-century in order to prevent warming that could change precipitation patterns and dry up sources of water worldwide.

Using advanced computer models to factor in deep-sea warming and other aspects of the carbon cycle that naturally creates and removes carbon dioxide (CO2), the scientists, from countries including the United States, Canada and Germany, are delivering a simple message: The world must bring carbon emissions down to near zero to keep temperatures from rising further.

"The question is, what if we don't want the Earth to warm anymore?" asked Carnegie Institution senior scientist Ken Caldeira, co-author of a paper published last week in the journal Geophysical Research Letters. "The answer implies a much more radical change to our energy system than people are thinking about."

Although many nations have been pledging steps to curb emissions for nearly a decade, the world's output of carbon from human activities totals about 10 billion tons a year and has been steadily rising.

For now, at least, a goal of zero emissions appears well beyond the reach of politicians here and abroad. U.S. leaders are just beginning to grapple with setting any mandatory limit on greenhouse gases. The Senate is poised to vote in June on legislation that would reduce U.S. emissions by 70 percent by 2050; the two Democratic senators running for president, Hillary Rodham Clinton (N.Y.) and Barack Obama (Ill.), back an 80 percent cut. The Republican presidential nominee, Sen. John McCain (Ariz.), supports a 60 percent reduction by mid-century.

Sen. Barbara Boxer (D-Calif.), who is shepherding climate legislation through the Senate as chairman of the Environment and Public Works Committee, said the new findings "make it clear we must act now to address global warming."

"It won't be easy, given the makeup of the Senate, but the science is compelling," she said. "It is hard for me to see how my colleagues can duck this issue and live with themselves."

James L. Connaughton, who chairs the White House Council on Environmental Quality, offered a more guarded reaction, saying the idea that "ultimately you need to get to net-zero emissions" is "something we've heard before." When it comes to tackling such a daunting environmental and technological problem, he added: "We've done this kind of thing before. We will do it again. It will just take a sufficient amount of time."

Until now, scientists and policymakers have generally described the problem in terms of halting the buildup of carbon in the atmosphere. The United Nations' Framework Convention on Climate Change framed the question that way two decades ago, and many experts talk of limiting CO2concentrations to 450 parts per million (ppm).

But Caldeira and Oregon State University professor Andreas Schmittner now argue that it makes more sense to focus on a temperature threshold as a better marker of when the planet will experience severe climate disruptions. The Earth has already warmed by 0.76 degrees Celsius (nearly 1.4 degrees Fahrenheit) above pre-industrial levels. Most scientists warn that a temperature rise of 2 degrees Celsius (3.6 degrees Fahrenheit) could have serious consequences.

Schmittner, lead author of a Feb. 14 article in the journal Global Biogeochemical Cycles, said his modeling indicates that if global emissions continue on a "business as usual" path for the rest of the century, the Earth will warm by 7.2 degrees Fahrenheit by 2100. If emissions do not drop to zero until 2300, he calculated, the temperature rise at that point would be more than 15 degrees Fahrenheit.