WASHINGTON - An international panel of scientists for the first time yesterday put a price tag on what it would take to avoid the worst effects of global warming, concluding that the effort would be affordable and would be partially offset by economic and other benefits.

The most ambitious initiative, one aimed at stabilizing the level of greenhouse gases from fossil fuels by 2030, would require measures that could add $100 to the costs associated with each ton of carbon dioxide pumped into the atmosphere, said the report by the Intergovernmental Panel on Climate Change.

In a telephone news conference, several participants estimated that would translate into the equivalent of an increase in gasoline prices of about $1 a gallon over a number of years.

Despite the likely cost, the consensus report of most governments said nations had no choice but to act.

"If we continue doing what we are doing now, we are in deep trouble," said Ogunlade Davidson, cochair of the group responsible for yesterday's "mitigation" report, the U.N. panel's third this year.

Immediately, however, the White House issued a statement rejecting the more ambitious options outlined by the report. Referring to the highest-cost scenario, James Connaughton, chairman of the White House Council on Environmental Quality, said it "would, of course, cause global recession, so that is something that we probably want to avoid."

Overall, the panel's report said, blunting the consequences of global warming will require changing lifestyles, including increased prices for basics such as gasoline and electricity, and a much greater investment by governments in research and development.

The impacts of those costs would be significantly lessened by the benefits of a less carbon-dependent economy, including a less polluted environment, more secure sources of energy, and in some cases reduced consumer costs, as more energy-efficient cars, appliances and houses become available, said the report, based on the work of about 2,000 scientists worldwide.

After five sometimes contentious days of negotiation to make final the document at a conference in Bangkok, Thailand, the panel issued its "Summary for Policymakers" without specific recommendations on how to address the threat from climate change. Instead, it offered projections of how much carbon dioxide would have to be eliminated to meet various goals for limiting greenhouse gases, along with assessments of hundreds of approaches.

The U.S. delegation embraced parts of the report, especially those that highlighted possible clean-energy technologies of the future.

In an opening statement to the news media after the report was released, conference chairman Rajendra Pachauri said: "It is probably naive to believe that merely developing technologies in labs and workshops is the answer."

It will be necessary to put a price on carbon emissions, Pachauri said - through either taxes or "cap and trade" systems in which polluters would buy and sell the right to put given amounts of greenhouse gases into the atmosphere.

While the report did not specify what that price should be, it outlined how much benefit would come at different cost levels - $20, $50 or $100 per ton of emitted carbon. The world could meet the goal of stabilizing the level of greenhouse gases in the atmosphere by 2030, the report said, at a sacrifice of less than 3 percent of the projected growth in the world's total economic output, or roughly 0.12 percent a year.

The European Union currently has as its goal reductions in emissions that would be consistent with the $20-per-ton target. Jonathan Pershing, one of the lead authors of the report and director for Climate, Energy and Pollution at the World Resources Institute, said estimates of the impact on gasoline and other energy costs were not included because they were based on assumptions that have not been well-studied.

Atmospheric greenhouse gases - from power plants, automobiles, trucks and airplanes, burning forests, some agricultural activity, and methane from decomposing waste - have grown by 70 percent since 1970, the panel concluded previously.

If nations do not begin to control emissions better, that level of heat-trapping gases is projected to increase by an additional 25 percent to 90 percent by 2030, with potentially calamitous results. They would include a surge in ocean levels, the disappearance of a large number of species, abrupt climate changes in tropical zones, and possibly large migrations of displaced people.

The panel's policy recommendations are negotiated by governments until they reach a consensus, a process that critics say has led to a conservative document.

See the U.N. report via go.philly.com/earth EndText