Wealthy nations should establish a fund that could spend $100 billion a year helping the world's poorest countries to tackle climate change, British Prime Minister Gordon Brown said Friday.

Calling for the fund to be in place by 2020, Brown said that wealthier nations would be able to raise funds through the expansion of international carbon trading market — in which carbon reductions achieved by one company can be sold to another — and existing development aid.

Environment ministers from industrialized nations first discussed proposals for the creation of such a fund at a conference in Denmark last month. Campaigners praised Brown for drawing further attention to the scale of assistance the developing world is likely to need to make reductions to greenhouse gas emissions.

Brown's comments followed a report issued Thursday by the Netherlands Environmental Assessment Agency which said developing countries are now emitting more than half of the world's carbon dioxide, the greenhouse gas blamed for climate change.

"We must help the developing countries adapt to the changes in climate which are already now occurring and which over the next few decades, however much we cut emissions, we cannot now avoid," Brown said, delivering a speech at the London Zoo.

Brown said the U.N. climate change conference in Copenhagen, Denmark, in December must bring a new global pact on reducing greenhouse gas emissions. He called for all nations to agree to cut theirs to 50 percent below 1990 levels by 2050.

"There are very few moments in history when nations are summoned to common decisions that will reshape the lives of every man, woman and child on the planet for generations to come," Brown said. "Copenhagen must be the moment we do so."

He said developed countries will likely need to reduce their emissions by at least 80 percent by 2050, in order to achieve an overall global target of 50 percent.

"If we act now, if we act together, if we act with vision and resolve, success at Copenhagen is within reach. But if we falter, the earth itself will be at risk," Brown said.

Leaders hope to use the Copenhagen summit to negotiate a new global climate treaty to succeed the 1997Kyoto treaty, which expires in 2012. That treaty called on 37 countries to cut carbon emissions by a total of 5 percent from 1990 levels by 2012. The U.S. rejected Kyoto, arguing it failed to make demands on rapidly expanding developing countries.

Nations are expected to agree in Copenhagen to make carbon emissions a global commodity, with a universally accepted price.

Representatives from the Maldives, Rwanda and Bangladesh — countries likely to rely on any climate change fund — attended Brown's speech, the British leader's office said.

British charity Oxfam praised Brown's decision to call for the new fund, and urged leaders of the Group of Eight industrialized nation to discuss specifics at a meeting being held in Italy later this month.

"By becoming the first major leader to put a figure on how much money is needed, he has shown signs of leadership on climate change that have so far been sorely lacking," Doug Parr, chief scientist at environmental charity Greenpeace, said in a statement.