COPENHAGEN, Denmark - Delegates converged on the Danish capital yesterday for the grand finale of two years of sometimes bitter talks on a climate treaty, and U.N. officials calculated that pledges in the last few weeks to reduce greenhouse gases put the world within reach of keeping climate change under control.

Yvo de Boer, the United Nations' top climate official, said on the eve of the 192-nation conference that despite unusually broad concessions, both industrial countries and emerging nations now need to dig deeper.

"Time is up," de Boer said. "Over the next two weeks governments have to deliver."

The key issue now seems to be how to fund an international plan to combat climate change. Billions of dollars are needed almost immediately and hundreds of billions of dollars annually within a decade.

Nations also must need to commit to even larger emissions reductions, de Boer said.

South Africa yesterday became the latest country to announce a national emissions target.

It said that during the next 10 years it would reduce emissions by 34 percent from "business as usual," the level it would reach under ordinary circumstances.

"This makes South Africa one of the stars of the negotiations," said the environmental group Greenpeace.

President Obama's decision to attend the conclusion of the two-week conference - rather than join it in its initial stages this week, as he had originally planned - is seen as a signal that an agreement is near.

More than 100 heads of state and government have said they will attend the last day or two, making Copenhagen the largest and most important summit ever held on climate.

"Never in the 17 years of climate negotiations have so many different nations made so many firm pledges together," de Boer said. "It's simply unprecedented."

Some were arriving to the summit on trains splashed with a green stripe to symbolize efforts to reduce the convention's carbon footprint. One train carried 450 U.N. officials, delegates, climate activists and journalists from Brussels and more trains were leaving from other European capitals.

Along with an estimated 15,000 delegates and at least 100 world leaders, officials expect many protesters to descend on Copenhagen for the climate conference. Authorities were beefing up security in preparation.

A study released by the U.N. Environment Program yesterday indicated that pledges by industrial countries and major emerging nations fall just short of the reductions of greenhouse gas emissions that scientists have said are needed.

"For those who claim a deal in Copenhagen is impossible, they are simply wrong," said U.N. Environment Program Director Achim Steiner, releasing the report compiled by British economist Lord Nicholas Stern and the Grantham Research Institute.

Environmentalists had warned that emissions commitments were dangerously short of what U.N. scientists have said are needed to keep average temperatures from rising more than 3.6 degrees Fahrenheit.

But most of those warnings were based on pledges only from industrial countries. The U.N. report includes pledges from China and other rapidly developing countries.

All countries together should emit no more than 44 billion tons of carbon dioxide by 2020 to avoid the worst consequences of a warming world, the report said.

Computing the high end of all commitments publicly announced so far, the report said emissions will total some 46 billion tons annually in 2020. Emissions today are about 47 billion tons.

"The gap has narrowed significantly," Steiner said.

Negotiations on a new climate treaty began in earnest two years ago with the aim of crafting a successor to the 1997 Kyoto Protocol, which bound industrial countries to cutting emissions of carbon dioxide and other Earth-warming gases from 1990 levels, but which laid no obligations on countries such as India and China.

That omission caused much resentment and the United States rejected Kyoto.

Months of deadlock were broken in recent weeks when China and India announced voluntary targets for lowering the greenhouse gas component of economic growth. Emissions would continue to climb, but at a lower rate.

At the same time, Obama said he would commit to an emissions cut of 17 percent from 2005. The cut has to be approved by Congress.