Climate experts have warned for years that a doubling of heat-trapping carbon dioxide in the atmosphere could have serious consequences worldwide. Now a growing number of scientists and policymakers say it will be difficult if not impossible to avoid such a rise.

The reason, they say, is that the world's economic and political systems cannot depart from business as usual rapidly enough. In one metaphor that acquired some currency at talks on global warming, which ended inconclusively here on Friday, it is like trying to turn a supertanker in a sea of syrup.Carbon dioxide is emitted by the burning of coal, oil and natural gas. These fuels are an integral part of the modern economy, and weaning the world from them has always been acknowledged as a difficult task at best.

In the long debate over global warming, a doubling of carbon dioxide from pre-industrial levels in the 19th century has been such a frequent object of analysis and anxiety as to become almost a talismanic benchmark.

Mainstream climatologists say that such a doubling would raise the average surface temperature of the globe anywhere from a moderate 3 degrees Fahrenheit to a potentially catastrophic 8 degrees, disrupting the Earth's climate and causing the seas to rise. By comparison, the world has warmed by 5 to 9 degrees since the depths of the last ice age, about 18,000 to 22,000 years ago.

While there are dissenters, a growing number of scientists and policymakers now say a doubling may be unavoidable late in the next century. This is so, they say, despite whatever steps to limit emissions of the gases emerge from international talks that were held here and will conclude in Kyoto, Japan, in early December.

For the most part, attention until now has focused on the next two decades or so, in the belief that the most important thing is to make a start in reducing emissions. But it is widely agreed, based on proposals on the table, that any action emerging from Kyoto would be insufficient to prevent an eventual doubling of greenhouse gases.

The proposals outline a range of relatively modest reductions by industrialized countries, none of which are enough to prevent overall atmospheric concentrations of carbon dioxide from continuing to rise.

"What Kyoto will do, almost predictably, is produce a small decrease in the rate of increase," said Dr. Jerry Mahlman, director of the National Oceanic and Atmospheric Administration's Geophysical Dynamics Laboratory at Princeton University.

The Kyoto conference has been universally viewed as a first step, and doubts about the long term are forcing people to pay more attention to a basic but long-deferred policy question: What should be the ultimate goal in stabilizing carbon dioxide concentrations? One objective of Clinton's climate plan, announced Oct. 22, is to establish a scientifically sound goal with the assistance of the National Academy of Sciences.

But if the experts' new fears are borne out, a rough doubling of carbon dioxide concentrations, or perhaps a bit less, may be the best to which the world can aspire. Under the most likely outcome, this could mean a rise in sea levels that would inundate low-lying coastal areas and small island nations, more frequent and severe floods and droughts, a shift in climatic zones, and disruption of natural ecosystems.

Not everyone agrees that a doubling is inevitable. Environmentalists say it is possible and necessary to stabilize concentrations at 450 parts per million by volume - about one and a half times the pre-industrial concentration of 280 parts per million.

This, according to a study by the Environmental Defense Fund, would limit the global temperature increase to 1.5 to 2 degrees Fahrenheit over the next 100 years. The estimate assumes that the climate's sensitivity to carbon dioxide is in the middle of the range implied by mainstream scientists' conclusion that a doubling would produce a warming of 3 to 8 degrees.

Atmospheric carbon dioxide is now about 360 parts per million, and the average surface temperature of the globe has risen by about 1 degree over the last century. At 450 parts per million, if the environmentalists are right, the total temperature increase by 2100 would be about 2.5 to 3 degrees. This would still make the world warmer than at any time in the last 10,000 years, and some further warming would occur after 2100.

Virtually nobody believes it is possible to stabilize atmospheric concentrations below 450 parts per million, and a number of experts say it will be very difficult if not impossible to stabilize them even at 550 parts per million, the next plateau on which some policymakers are beginning to focus. That number is close to a doubling of the pre-industrial concentration.

While it is physically possible to stabilize concentrations at 550 parts per million, "I don't think the emissions levels it would take are possible from a political point of view," said Dr. Luiz Gylvan Meira Filho of Brazil, who is in the thick of the political battle and the scientific debate. In the talks leading up to the Kyoto conference, he has headed the crucial negotiating subgroup dealing with targets and timetables for emissions reductions. A physicist, he is also a vice chairman of the Intergovernmental Panel on Climate Change, which produces the world's most authoritative assessments of the global warming problem. He said he was speaking for himself, not any of the official groups.

Many others share Meira's assessment in general. It will be "clearly challenging to stay below 550," said David Sandalow, a member of the Clinton administration's National Security Council and Council on Environmental Quality.

Dr. Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research at Boulder, Colo., says there has been a general sense that: "If we want to stop this experiment, we can do so with really strong, willful action. I think that's not the case." He said he believed a doubling of carbon dioxide is likely, and that "we need to pay more attention to adapting to climate change."

What appears to be going on is "a subtle shift in the declaration of what constitutes success," said Mahlman.

It looks now, he said, as if the long-term task is to keep atmospheric carbon dioxide from soaring beyond a doubling to catastrophic levels of as much as three or four times pre-industrial concentrations.

Concentrations could as much as triple by 2100 and continue growing after that if Third-World countries follow business as usual, even if the most ambitious emissions reduction proposals for rich countries were adopted, said a report made public here by the intergovernmental panel.