In every era there are only one or two moments when nations come together and reach agreements that make history because they change the course of history. Copenhagen must be such a time.

There are now fewer than 50 days to set the course of the next 50 years and more. So, as we convene here, we carry great responsibilities, and the world is watching. If we do not reach a deal at this time, let us be in no doubt: 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.

We must never lose sight of the catastrophe we face if present warming trends continue. Only last week we saw new evidence of the rapid loss of Arctic sea ice. In just 25 years the glaciers in the Himalayas which provide water for three-quarters of a billion people could disappear entirely. If the international community does nothing to assist the rainforest nations in protecting the world's rainforests, the damage, not just to the climate but to biodiversity, to watersheds and to the livelihoods of millions of people will be incalculable.

But the threat is not confined to the developing world. The extraordinary summer heat wave of 2003 in Europe resulted in over 35 thousand extra deaths. On current trends, such an event could become quite routine in Britain in just a few decades' time. And within the lifetime of our children and grandchildren the intense temperatures of 2003 could become the average temperature experienced throughout much of Europe. In Britain we face the prospect of more frequent droughts and a rising wave of floods.

And the threat is not only humanitarian and ecological, it is also an economic one. Three years ago, the Stern Report, which I commissioned, concluded that failure to avoid the worst effects of climate change could lead to global GDP being up to 20 per cent lower than it otherwise would be – an economic cost greater than the losses caused by two World Wars and the Great Depression.

Let us be in no doubt then that this is a profound moment for our world – a time of momentous choice. And we must be the people who act.

This is an edited extract from a speech given by the Prime Minister to the Major Economies Forum in London yesterday