The last time global negotiations collapsed like this was in Doha in 2001. After the trade talks fell apart, the World Trade Organisation assured delegates that there was nothing to fear: they would move to Mexico, where a deal would be done. The negotiations ran into the sand of the Mexican resort of Cancun, never to re-emerge. After eight years of dithering, nothing has been agreed.

When the climate talks in Copenhagen ended in failure, Yvo de Boer, the man in charge of the process, urged us not to worry: everything will be sorted out ''in Mexico one year from now''. Is Mexico the diplomatic equivalent of the Pacific garbage patch - the place where failed negotiations go to die?

We can live without a new trade agreement; we can't live without a new climate agreement. One of the failings of the people who have tried to mobilise support for a climate treaty is that we have made the issue too complicated. So here is the simplest summary I can produce of why this matters.

Human beings can live in a wider range of conditions than almost any other species. But the climate of the past few thousand years has been amazingly kind to us. It has enabled us to spread into almost all regions of the world and to grow into the favourable ecological circumstances it has created. We enjoy the optimum conditions for supporting 7 billion people.

A shift in global temperature reduces the range of places that can sustain human life. During the last ice age, humans were confined to low latitudes. The difference in the average global temperature between now and then was 4 degrees. Global warming will have the opposite effect, driving people into higher latitudes, principally as water supplies diminish.