Secretary-General Kofi Annan told the U.N. conference on climate change Wednesday that those who would deny global warming or delay taking action against it are "out of step" and "out of time."
"Let no one say we cannot afford to act," Annan declared.
The United States is among those who contend that reducing global-warming gases would set back economies too much.
The U.N. chief lamented "a frightening lack of leadership" in fashioning next steps in reducing global emissions.
"I would want leaders around the world to really show courage and to know that if they do, their people and the voters will be with them," he told reporters after his speech.
The chief U.S. delegate, at a later news conference, countered that Washington has been a leader with "groundbreaking initiatives" on clean-energy technology.
Hundreds of delegates from some 180 member nations of the 1992 U.N. climate treaty were entering the final three days of their two-week annual meeting, where they've been working on technical issues involving the Kyoto Protocol, which obliges 35 industrial nations to reduce emissions of greenhouse gases by 5 percent below 1990 levels by 2012.
The United States and Australia are the only major industrialized countries to reject that 1997 treaty annex. U.S. President George W. Bush says it would harm the U.S. economy, and it should have required cutbacks in poorer nations as well.
Scientists attribute at least some of the past century's 0.6-degree-Celsius (1-degree-Fahrenheit) rise in global temperatures to the atmospheric accumulation of carbon dioxide and other heat-trapping gases, byproducts of power plants, automobiles and other fossil fuel-burning sources. Continued temperature rises could seriously disrupt the climate, they say.
In his address, the U.N. chief referred to a recent British government report projecting that unimpeded global warming — with its predicted rising seas, droughts and other climate disruptions — could cost between 5 percent and 20 percent of global gross domestic product each year.
"It is increasingly clear it will cost far less to cut emissions now than to deal with the consequences later," Annan said.
He said the science on climate change "is not science fiction."
Those who try to sow doubt about it are "out of step, out of arguments and out of time," he said.
At her news conference, Paula J. Dobriansky, a U.S. undersecretary of state, stressed U.S. efforts in developing clean-energy technology in partnership with other countries.
"We must act in ways that encourage economic growth," she said.
Asked about Annan's criticism of poor international leadership on climate, Dobrianksy replied, "We think the United States has been leading with its groundbreaking initiatives."
Closed-door talks here are focusing on how to set emissions quotas for the post-2012 period — a regime others hope will include the United States, the biggest emitter. Cabinet ministers from around the world were arriving here for high-level bargaining on such key issues.
At best, however, the conference may simply set a timetable for continuing talks into next year. Many here think real negotiations must await the end of the Bush administration.
"The United States will return to the negotiating table with a serious proposal when a new president takes office in 2009," said veteran conference observer Philip Clapp.
Clapp, president of the U.S. group National Environmental Trust, noted that Democratic and Republican contenders in the 2008 presidential election favor capping U.S. emissions.
Other climate campaigners oppose this strategy of marking time.
"That won't work. It would allow the U.S. to hold the negotiations hostage," said Hans Verolme, Dutch spokesman for Climate Action Network, an alliance of environmentalist groups.
Verolme expressed concern that too many years of negotiating and seeking government ratification of a new agreement would leave a gap after 2012.
"The carbon market is beginning to lose confidence in this process," he said.
A multibillion-dollar market has emerged in which European Union countries, obligated under Kyoto, buy and sell "carbon credits."
Companies overshooting their emissions quotas can buy carbon credits from more energy-efficient enterprises that don't use all their allowances.
At the same time, another market is growing in projects that reduce greenhouse-gas emissions in developing countries, which are not obligated under Kyoto but which can sell such credits to industrial nations.
If it appears the Kyoto regime will end without an immediate successor, it would shatter business confidence in the future of those markets and cause carbon prices to collapse.