Brown said climate catastrophe would occur if world leaders fail to agree on a climate protection deal at a major climate conference this December in Copenhagen, Denmark.
"If we do not reach a deal at this time, let us be in no doubt: Since once the damage from unchecked emissions growth is done, no retrospective global agreement, in some future period, can undo that choice. By then it will be irretrievably too late," Brown told the Major Economies Forum, a conference bringing together representatives from the world's 17 biggest polluters in London.
World leaders are gathering this December in Copenhagen for a U.N.-mandated conference that is aimed at producing a comprehensive climate protection deal.
The accord to be born at Copenhagen -- to feature binding emissions-reductions targets, adaptation measures and their funding -- is due to replace the Kyoto Protocol, which runs out in 2012. Several meetings have tried to prepare a negotiation text for Copenhagen, but Brown admitted that a lot of work still needs to be done.
"We must frankly face the plain fact that our negotiators are not getting to agreement quickly enough," Brown said. "So I believe that leaders must engage directly to break the impasse."
He urged leaders of the world's major economies to lead in combating climate change and reminded developing countries not to make the West's mistakes again.
Carbon-intense economic growth, he said, "would be unsustainable, and soon overwhelmed by its inevitable consequences: greater energy insecurity, greater pollution and ill-health and -- as a result of climate-induced migration and poverty in the poorest countries -- almost inevitably, greater conflict."
Brown said that for the major economies represented in London, it should be a priority to help poorer countries adapt to the effects of climate change -- a phenomenon they have done the least to create.
On Saturday the president of the Maldives held a Cabinet meeting underwater to highlight the threats to his islands, which are facing catastrophe if ocean levels continue to rise because of melting arctic ice.
But the effects of climate change are not confined to developing countries.
The heat wave of 2003 caused 35,000 extra deaths in Europe, Brown said, and the future looks even bleaker.
"In Britain, we face the prospect of more frequent droughts and a rising wave of floods."
The London conference brings together the world's two biggest greenhouse gas emitters, China and the United States, several leading European economies and major emerging players including India and Brazil.
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