This was not a wake-up call. This was "We told you so."

Seventeen years and a zillion computer hours after the world's climate scientists first ran the big calculation, they came up with the same estimate of the warming the planet will see by the end of the century if we go on pouring out greenhouse gases: 3C.

If a single numeral can be accusatory, this one was.

If there are any of our political leaders still snoozing at the back of this science lesson, they stand duly rebuked for not paying attention.

In the past 17 years that mankind has squabbled and allowed its greenhouse gas emissions to go on rising, the blanket of gases that keeps the Earth warmer than space has increased by seven per cent. But when you calculate the radiative forcing potential of that seven per cent — its capacity to force the climate to warm — the increase since the IPCC's first report in 1990 is 30 per cent.

Rajendra Pachauri, the IPCC chairman, said that gives an idea of the urgency of doing something now, before climate change gets beyond our control — and a sense that many opportunities have already been squandered.

The predicament we face if emissions go on rising along the "high" scenario envisaged by IPCC was put colourfully by one of its authors, Kevin Trenberth, of the National Centre for Atmospheric Research in Boulder, Colorado.

"If you come back in 100 years, it will be like Star Trek. You could be on a different planet," he said. "You can move from Miami to New York, but polar bears and trees don't have anywhere to go to."