dioxide, which, at 380 parts per million now, has risen more than a third since the start of the industrial revolution and could easily double from the preindustrial level within decades.
The report, which awaits only formal adoption this afternoon, concluded that significant progress toward that goal could be made in the next 25 years with known technologies and policy shifts, but would still need to be followed by a century-long transition to new energy sources that come with no climate impacts.
Several authors, while declining to discuss specific results before the report was formally adopted, said its message was clear.
We can no longer make the excuse that we need to wait for more science, or the excuse that we need to wait for more technologies and policy knowledge," said Adil Najam, an author of one chapter and an associate professor of international negotiation at the Fletcher School at Tufts University. "To me the big message is that we now have both and we do not need to wait any longer."
The report also made clear the risks of delay, noting that emissions of greenhouse gases have risen 70 percent since 1970 and could rise an additional 90 percent by 2030 if nothing is done.
Carbon dioxide is particularly important not only because so much is produced each year — about 25 billion tons — but because much of it persists in the atmosphere, building like unpaid credit card debt.
To stop the rise, report authors said, countries would need to expand adoption of existing policies that can cut emissions — like a fuel tax or the binding limits set by the Kyoto Protocol — while also increasing research seeking new energy options. This work would include pushing for advances in solar and nuclear power.
The meeting ended just after dawn Friday in Bangkok with several authors of the report saying that there had been relatively little last-minute fighting with government officials over details. China had resisted language that implied big cuts would have to be made in fast-growing developing countries, which will soon surpass rich countries as the dominant source of greenhouse gases.
According to several authors, the final version estimates that bringing global carbon dioxide emissions by 2030 to levels measured in 2000 would require a cost on released carbon dioxide of $50 to $100 a ton, roughly on a par — in terms of fossil fuel prices — of an additional 25 cents to 50 cents for a gallon of gasoline.
The report projects that this shift might cause a small blunting of global economic activity, resulting in an overall reduction of perhaps one-tenth of a percentage point per year through 2100 in the world's total economic activity, the authors said.
Some of the experts and government officials involved in the final discussions said in telephone interviews and in e-mail messages that the costs could be substantially greater than that.
But a variety of participants, including some from the United States, said in interviews that it was hard to argue against such an investment, given the potential costs of inaction.
William Moomaw, a lead author of a chapter on energy options and a professor of international environmental policy at Tufts University, said that he saw evidence that big cuts could happen.
"Here in the early years of the 21st century, we're looking for an energy revolution that's as comprehensive as the one that occurred at the beginning of the 20th century when we went from gaslight and horse-drawn carriages to light bulbs and automobiles," Moomaw said. "In 1905, only 3 percent of homes had electricity. Right now, 3 percent is about the same range as the