Global greenhouse gas emissions may have to be cut to zero within the next few decades to avert  a dangerous rise in global temperatures . This is the view of two teams of scientists  who have published their findings in separate journals over the past few weeks. They assert  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 according to their models change precipitation patterns and dry up sources of water worldwide.

Although many nations have agreed to take  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.

But given US dithering over ratifying the Kyoto Protocol is the goal of zero emissions politically achievable?

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."

In the journal Global Biogeochemical Cycles, Oregon State University professor Andreas Schmittner 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.

"This is tremendous," Schmittner said. "I was struck by the fact that the warming continues much longer even after emissions have declined. . . . Our actions right now will have consequences for many, many generations. Not just for a hundred years, but thousands of years."

While natural cycles remove roughly half of human-emitted carbon dioxide from the atmosphere within a hundred years, a significant portion persists for thousands of years. Some of this carbon triggers deep-sea warming, which keeps raising the global average temperature even after emissions halt

The  new studies outline the challenge in greater detail, and on a longer time scale, than many earlier studies. Schmittner's study, for example, projects how the Earth will warm for the next 2,000 years.

But some climate researchers who back major greenhouse gas reductions said it is unrealistic to expect policymakers to think in terms of such vast time scales.

Steve Gardiner, a philosophy professor at the University of Washington who studies climate change, said the studies highlight that the argument over global warming "is a classic inter-generational debate, where the short-term benefits of emitting carbon accrue mainly to us and where the dangers of them are largely put off until future generations."

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