BOSTON — GLOBAL warming is happening at a slower pace than once thought, but even so, nations must act quickly and decisively to protect the planet's atmosphere.
These were two conclusions from a UN-sponsored report, expected to help world leaders draw up a new blueprint for reducing dangerous gasses.
The study was the first broad review of global-warming research in five years by the Intergovernmental Panel on Climate Change, a United Nations-sponsored group of more than 2,500 scientists charged with keeping governments up to date on global-warming research and its implications.
The research is synthesized in an IPCC report released on Saturday, following a week of talks in Rome. The report is designed to guide international negotiators setting targets for lowering greenhouse gas emissions in the next century - just as the organization's first report laid the groundwork for goals set at the 1992 UN Conference on the Environment in Rio de Janeiro.
IPCC recommendations don't have to break the bank, however. Encouraging cleaner burning cars, more efficient appliances, and recycling and mass transportation policies are steps in the right direction for governments. The IPCC cites a range of studies that show economies can actually benefit from the increased efficiencies needed to curb greenhouse-gas emissions.
According to the report, if no changes are made in emissions of greenhouse gases, the overall amount of carbon dioxide in the atmosphere in 2100 would be nearly double its pre-industrial level. But the IPCC's "best estimate" yields an average temperature increase of 2 degrees C between 1990 and 2100, one-third lower than the 1990 estimate.
The panel attributes the change largely to improved climate models that take into account the cooling effect of small particles such as sulfate aerosols, which come from burning fossil fuels. In addition, the estimate of how high the sea-level is expected to rise drops by 25 percent.
Even so, the adverse effects of these changes on human health and welfare will be significant, the report says. The IPCC acknowledges uncertainties in climate-forecasting models and an inability to tie specific effects to specific atmospheric concentrations of greenhouse gases. Yet the persistence of carbon dioxide in the atmosphere and the oceans' lethargic response to temperature changes suggest the effects will keep growing.
THE report indicates that countries must act "quite soon" if they hope to stabilize greenhouse-gas concentrations and prevent catastrophic climate changes, says Irving Mintzer, a senior scholar at the Center for Global Change at the University of Maryland at College Park and one of the authors of the IPCC synthesis report.
This was a touchy point in the talks, he says. "We wanted to include, in narrative form, how long governments could wait to deviate from business as usual" to achieve various greenhouse-gas concentration levels. If the goal is to stabilize the concentration at 550 parts per million, he explains, governments would have a few decades to cut emissions.
"If you want to get to 450 parts per million, you would have to act in this decade. That was too scary a thought for some of our colleagues," he says, referring to representatives from several oil states in the Persian Gulf.
Instead of trying to adopt reduction and mitigation policies to last 100 years, the IPCC says, strategies to combat global warming should be flexible enough to adjust to new information.
Some economists have suggested that trying to curb greenhouse-gas emissions could send industrial economies into a tailspin. "That would be true if you think dealing with global warming means solving the problem this week," Dr. Mintzer says.