Despite last year's disappointment at Copenhagen, the U.N. leadership believes "significant progress is possible" at next week's climate conference in Cancun, Mexico, a top U.N. official said Monday.

But Robert Orr, echoing the unanimous assessment of others, told reporters no one expects a final global deal on climate at the 194-nation Cancun conference.

"Climate change wasn't created overnight and won't be solved overnight," said Orr, assistant secretary-general for policy planning.

Negotiators had targeted last year's climate summit in the Danish capital, attended by some 100 heads of state or government, for agreement on mandatory reductions in global warming gases by dozens of countries. But the talks were unsuccessful, producing only a "Copenhagen Accord," a nonbinding political agreement with pledges of voluntary reductions.

A core dispute between the United States and China has stymied progress on this central element of any global climate deal. For 13 years, the U.S. has refused to join the rest of the industrialized world in the Kyoto Protocol, a binding pact to curb fossil-fuel emissions by modest amounts, due to expire in 2012. More recently, as China, India and other emerging economies exempted from the 1997 Kyoto pact have sharply increased emissions, they have rejected calls by the U.S. and others to commit by treaty to restraints.

As a result, at Cancun "pragmatism is the order of the day," Orr said.

He said significant progress is possible in three key areas: establishing a multibillion-dollar fund to aid poorer countries to adapt to climate change and to install clean energy sources; agreeing on more elements of a complex plan to pay developing countries for protecting their forests; and in making it easier for poorer nations to obtain patented technologies from the industrialized world for clean energy and climate adaptation.

On Dec. 7, Secretary-General Ban Ki-moon will open the high-level, ministerial segment of the Nov. 29-Dec. 10 conference, and "will urge countries to work toward a balanced set of outcomes," Orr said.

"Human-caused climate change is happening," said the former U.S. diplomat. "It is happening faster than anyone predicted even a few years ago, and therefore we need to remind ourselves and negotiators need to remind themselves that the longer we delay, the more we will pay, both in terms of lives and in terms of money."

The Intergovernmental Panel on Climate Change, a U.N. scientific network, projects that temperatures will rise this century by up to 6.4 degrees Celsius (11.5 degrees Fahrenheit), depending on whether and how much emissions are rolled back of carbon dioxide and other industrial, transportation and agricultural gases that are warming the atmosphere.

In one example of accelerating impacts, researchers report that the world's warming oceans are rising at twice the 20th century's average rate, expanding from the heat and the runoff of melting land ice and threatening low-lying islands states and eventually coastlines everywhere.