"Let no one say we cannot afford to act," Annan declared, in a reference to those, such as the Bush administration, who contend that reducing global-warming gases would set back economies too much.
The U.N. chief also lamented "a frightening lack of leadership" in fashioning next steps in reducing global emissions. "Let us start being more politically courageous," he urged the hundreds of delegates from some 180 member nations of the 1992 U.N. climate treaty.
Their two-week annual meeting, entering its final three days, has been working on technical issues involving the Kyoto Protocol, which obliges 35 industrial nations to reduce emissions of greenhouse gases to 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 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.
Annan told delegates the Kyoto accord, though "crucial," is "far too small" to deal with what he called "an all-encompassing threat."
Closed-door talks here focused on how to set emissions quotas for the post-2012 period — a regime others hope will include the United States, the biggest emitter.
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.
Cabinet ministers from around the world were arriving here for high-level bargaining on key issues. They must "show to the world that there is a continuation of the Kyoto Protocol," said Catherine Pearce, of the environmental group Friends of the Earth.
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 ratifications 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 utilize 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.