In a United Nations report this month, scientists said the cost of aggressively tackling climate change was comparatively reasonable. By spending a little...

CHICAGO — In a United Nations report this month, scientists said the cost of aggressively tackling climate change was comparatively reasonable. By spending a little more than 0.1 percent of the world’s income each year for 23 years, they say, greenhouse gases could be held nearly in check, avoiding the worst predicted environmental disasters.

The same day, Bush administration officials argued that the same aggressive effort would throw the world’s economy into recession.

The reality, top climate economists say, is that cutting U.S. emissions sufficiently to hold greenhouse-gas concentrations at near-current levels soon could cost the United States twice as much per year as it is now spending on the war in Iraq. But, as the U.N. report essentially urges, spending $1 trillion a year worldwide over two decades to aggressively curb global warming could be a bargain in the long run.

“It isn’t going to be cheap, but there’s an awful lot we can do, and it doesn’t break the bank, especially if we do it cleverly,” argued Robert Socolow, a physicist, co-director of the Carbon Mitigation Initiative at Princeton University and a leading theorist on ways to reduce greenhouse-gas emissions. “I don’t see how we get a recession out of it.”

For the United States, the most aggressive scenario in the new U.N. Intergovernmental Panel on Climate Change mitigation report — holding greenhouse gases in the atmosphere to less than 500 parts per million, up from the current 380 parts per million — could cost $240 billion a year, or 2 percent of the nation’s income, said Robert Mendelsohn, a climate-change economist at Yale University. The Iraq war, comparatively speaking, has cost a little less than $100 billion a year on average since it began in 2003.

That 2 percent of national income figure is much higher than the cost of 0.12 percent of world income quoted in the U.N. report because the United States is the world’s leading producer of greenhouse gases and therefore has more work to do cutting them, Mendelsohn said. Many economists also say U.N. figures suggesting a moderate cost for limiting climate change assume that nations around the world would act quickly and in concert to target the problem, something political leaders say is highly unlikely.

Reducing greenhouse gases vigorously and quickly probably would push Americans’ heating and electric bills up by 50 percent to 100 percent, said Jae Edmonds, a scientist and economist with the Joint Global Change Research Institute, based in Maryland. Gasoline prices would rise between 50 cents and $1 a gallon, he said.

Whether that is a cheap or expensive price to pay for cutting emissions is a matter of perspective, he said.

“Some might look at those numbers and say that’s a pretty good buy to avoid the potential negative implications of climate change,” he said. “Others might think those costs look high and say they’d rather go slower.”

Choosing a sufficiently aggressive plan to stave off the worst effects of climate change without dire economic consequences is a complicated balancing act, economists say, particularly because so many variables remain unknown.

Too vigorous a worldwide campaign could backfire, hurting economic growth and alienating key greenhouse-gas producers. But doing too little too slowly might waste a crucial opportunity to avoid potentially catastrophic impacts of global warming and to dodge greater costs in the future.

The right answer, many economists suggest, is to act quickly to launch tests of potentially useful technology and programs worldwide, then rapidly scale up those that work.

In Mendelsohn’s view, the most aggressive level of greenhouse-gas cuts promoted in the U.N. report is “too radical a recommendation to be supported by mainstream economics.” Because efforts to control greenhouse gases will be effective only if all of the world’s major producers take part, “by starting with a crash program you ensure a lot of countries are not going to join in,” he said.

However, “you don’t want to get sucked into thinking the only choice is to do the crash program or nothing at all,” he said.

He suggests that a much more modest target — limiting atmospheric concentrations of greenhouse gases to perhaps 640 to 750 parts per million — would cost the United States a tenth as much as the most aggressive scenario outlined in the U.N. report. Worldwide, the cost would fall by about half, according to the report. Other scientists and economists say holding greenhouse-gas concentrations to about 550 parts per million, at somewhat higher cost, is a better option.

Under Mendelsohn’s scenario, average global temperatures would be expected to rise by 7 to 11 degrees Fahrenheit by the end of the century, according to the U.N. panel, compared with about 3 to 6 degrees under the most aggressive program.

Because no one knows what temperature increase might trigger disastrous environmental problems — large sea-level rises, worsening flooding and droughts, a disruption of ocean circulation patterns — the lower range of temperature increases is generally thought to be safer.

Development of new technology and creative use of existing technology potentially could cut the costs of reducing emissions dramatically. Because plants draw carbon dioxide from the atmosphere when they grow, using plant fuels rather than fossil fuels effectively cuts emissions of greenhouse gases, Edmonds said.

If engineers are able to find efficient ways to use plants to create fuel and then capture carbon dioxide released from the smokestacks of plant-fueled power stations and pump it into storage underground, the world could potentially lower levels of greenhouse gases in the atmosphere while generating power.

But racing too quickly toward renewable energy and other efforts to cut greenhouse-gas emissions could have problematic consequences as well, Mendelsohn warned.

Using more nuclear power, he said, will lead to renewed concerns about what to do with nuclear waste. Planting billions of acres of new crops for biofuels could lead to accelerating deforestation in places such as Brazil and Indonesia. And efforts to boost hydroelectric generation could result in many of the world’s last wild rivers being dammed.

David O’Reilly, the chief executive of Chevron, points to a Senate bill calling on the Energy Department to develop a plan to cut gasoline consumption by 20 percent by 2017, 35 percent by 2025 and 45 percent by 2030, largely by substituting ethanol and other renewable fuels.

Under the Senate proposal, the amount of alternative fuels used in U.S. motor vehicles would rise to 8.5 billion gallons by 2008 and 36 billion gallons by 2022.

The problem, O’Reilly said, is that U.S. farmers cannot currently produce enough corn to make more than 15 billion gallons of fuel. Producing 36 billion gallons would require huge corn imports or a massive overhaul of the U.S. agricultural economy. And Chevron is not just protecting its fossil-fuels turf; the company already produces 70 percent of the ethanol made in the United States. “We’re dealing with a massive economy and a massive energy infrastructure that was developed to supply this economy,” O’Reilly said. “You can’t turn that around in just a couple of years.”