COPENHAGEN, Denmark — Delegates converged Sunday for the grand finale of two years of tough, sometimes bitter negotiations on a climate change treaty, as U.N. officials calculated that pledges offered in the last few weeks to reduce greenhouse gases put the world within reach of keeping global warming under control.
Yvo de Boer, the United Nations' top climate official, said on the eve of the 192-nation conference that despite unprecedented unity and concessions, industrial countries and emerging nations need to dig deeper. "Governments have to deliver," he said.
President Barack Obama's decision to attend the conclusion of the two-week conference, after phone consultations with other heads of state, was taken as a signal that an agreement was getting closer. He originally planned to make an hourslong stop in the Danish capital this week.
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.
A study released by the U.N. Environment Program on Sunday 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 have warned that emissions commitments were dangerously short of what U.N. scientists have said were 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 included pledges from China and other rapidly developing countries, which in turn were contingent on rich-country funding.
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 about 46 billion tons annually in 2020. Emissions today are about 47 billion tons.
"We are within a few gigatons of having a deal," Steiner said. "The gap has narrowed significantly."
South Africa on Sunday became the latest country to announce an emissions target. It said over the next 10 years it would reduce emissions by 34 percent from "business as usual," the level they would reach under ordinary circumstances. By 2025 that figure would peak at 42 percent, effectively leveling off and thereafter begin to decline.
"This makes South Africa one of the stars of the negotiations," said the environmental group Greenpeace.
Finance — billions of dollars immediately and hundreds of billions of dollars annually within a decade — was emerging as the key to unblocking an agreement that would bind the global community to a sweeping plan to combat climate change.
Some participants 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, Belgium, and more trains were leaving from other European capitals.
Along with roughly 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.
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 cut emissions of carbon dioxide and other Earth-warming gases from 1990 levels, but which made 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 the last few 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, even though those cuts have not been approved by Congress.
The United States can show flexibility, even if it cannot raise its emissions offer, said Jonathan Pershing, the senior U.S. delegate at the talks. He said the Obama administration was showing its seriousness through budget allocations and regulatory actions on big polluters.