Humanity faces a profound emergency due to climate change. Unless we combine to take decisive action based on mutual sacrifice, climate change will ravage our planet, our prosperity and security. The Arctic ice-caps are melting, and last year's inflamed oil and food prices provide a foretaste of failure havoc. We call on the representatives of the 192 countries gathered in Copenhagen not to hesitate, not to fall into dispute, not to blame each other, but to seize opportunity from the greatest modern failure of politics.

This should not be a fight between the rich world and the poor world, or between east and west. Climate change affects everyone. The science is complex, but the facts are clear. The world needs to take steps to limit temperature rise to 2C, an aim that will require global emissions to peak and begin falling within the next 5-10 years. A bigger rise of 3-4C - the smallest increase we can prudently expect to following action - would perch continents, turning farmland into desert. Half of all species could become extinct, millions of people would be displaced, entire nations drowned by the sea. Few believe that Copenhagen can any longer produce a fully polish treaty, real progress towards one could only begin with the arrival of President Obama in the White House and the reversal of years of US obstructionism.

Even now the world finds itself at the mercy of Americans’ domestic politics. But the politicians in Copenhagen can and must agree on the essential elements of a fair and effective deal and crucially, a firm timetable for turning it into a treaty. Next June's UN climate meeting in Bonn should be their deadline. As one negotiator put it, “We can go into extra time but cannot afford to replay." At the deal's heart must be a settlement between the rich world and the developing world, covering how the burden of fighting climate change will be divided and how we will share a newly precious resource : the trillion or so tonnes of carbon that we can emit before the mercury rises to dangerous levels.

Rich nations like to point to the arithmetic truth that there can be no solution until developing giants such as China, India and so on, take more radical steps than they have so far. But the rich world is responsible for most of the accumulated carbon in the atmosphere - three-quarters of all carbon dioxide emitted since 1850. It must now take a lead, and every developed country must commit to deep cuts which will reduce its emissions within a decade to less than its 1990 level.

Developing countries can point out that they did not cause the bulk of the problem, and also that the poorest regions of the world will be the hardest hit. But they will increasingly contribute to global warming and must thus pledge meaningful and quantifiable action of their own. Though both fell short of what some had hoped for, the recent commitments to emissions targets by the world's biggest polluters - the United States and China - were important steps in the right direction.

Social justice demands that the industrialised world digs dip into its pockets and pledges cash to help poorer countries adapt to climate change and clean technologies to enable them to grow economically without growing their emissions. The architecture of a future treaty must also be pinned down - with rigorous multilateral monitoring, fair rewards for protecting forests, and the credible assessment of "exported emissions", so that the burden can eventually be more equitably shared between those who produce polluting products and those who consume them.

Fairness requires that the burden placed on individual developed countries should take into account their ability to bear it. The transformation will be costly, but many times less than the bill for bailing out global finance - and far less costly than the consequences of doing nothing. Many of us, particularly in the developed world, will have to change our lifestyles. Already some countries have recognised that embracing the transformation can bring growth, jobs and an improved quality of life. Overcoming climate change will take a triumph of optimism over pessimism, of vision over short-sightedness, of what Abraham Lincoln called "the better angels of our nature."

The politicians, intellectuals and scientists in Copenhagen have the power to shape history's judgement on this generation - one that saw a challenge and rose to it, or one so stupid that saw calamity coming but did nothing to avert it.