Analysis:With hard scientific evidence to hand, February 2nd, 2007, will be remembered as the day doubt died on the reality of global warming, writes Frank McDonald, in Paris
So now we know. The warming of the world's climate system is "unequivocal", and that's not what scientists are saying. It has also been accepted as a fact in Paris this week by the representatives of more than 100 countries on the UN's Intergovernmental Panel on Climate Change (IPCC).
"Unequivocal" is the most powerful word in its latest assessment of global warming, based on hard evidence of increases in air and ocean temperatures, widespread melting of snow and ice, and rising sea levels. And this trend is set to continue, even if we manage to cut greenhouse gas emissions.
IPCC scientists are now 90 per cent certain that these emissions - mainly carbon dioxide (CO2) - are to blame for causing climate change. Using different scenarios, they project that global average surface temperatures will rise by 3° within a century, changing the world we live in.
3° doesn't seem like much, but it equates to the difference between our era and the last Ice Age.
As Dr Kevin Trenberth, head of climate analysis at the US Centre For Atmospheric Research, said: "If you came back in 100 years, it would be like Startrek - you would be on a different planet."
A New Zealander, he was one of the 600 scientists who drafted the IPCC's report on the physical science basis for climate change. A veteran of the IPCC process since its First Assessment was produced in 1990, he agreed that what has really changed since then is the level of public - and media - interest in the issue.
Dr Trenberth, who is based in Colorado, will be in Washington DC next week to give evidence on global warning to committees of the US Senate and House of Representatives. "We have to recognise that it's happening. All scientists can do is to try to present information. It's up to politicians to decide what to do."
It is remarkable that the Fourth Assessment's "mid-range projection" of a 3° increase in temperature by 2100 is exactly the same as the forecast made by the First Assessment 17 years ago. This time, however, it is supported by much more sophisticated computer models, as well as what's been happening since 1990.
Eleven of the last 12 years rank among the dozen warmest years on record since instrumental measurements began in 1850. According to Dr Susan Solomon, co-chair of the IPCC working group that produced yesterday's report, the world will warm up by 0.2° per decade with the rise in greenhouse gas emissions.
The projected increase of three degrees by 2010 is based on a doubling of CO2 concentrations.
But even if emissions were reduced to 1990 levels, the rate of warming would still be 0.1° per decade, because of the time-lag in adjusting the climate system. A warmer world is therefore inevitable, whatever happens.
"Warming would continue for centuries due to the timescales associated with the climate system, even if greenhouse gases were stabilised," Dr Solomon said.
And if the average global surface temperature climbed higher, and this continued for a thousand years, the Greenland ice-sheet would disappear altogether, she warned.
Like all IPCC scientists, she is scrupulous about sticking to the science and declined to be drawn on what should be done to combat global warming, either in the context of the Kyoto Protocol or the wider UN Convention on Climate Change.
But what to do is the obvious, and increasingly urgent, issue. If we go on the way we're going, the changes will be even more disastrous than they might be otherwise.
What the IPCC has done is to inject more certainty into the debate; no longer is it credible to challenge the fundamental fact that climate change is already upon us.
There will always be sceptics, paid or unpaid. The American Enterprise Institute - a neo-conservative think-tank based in Washington, which has received funding from Exxon-Mobil - was reported yesterday to have sent letters to scientists and economists, offering them $10,000, plus expenses, if they were prepared to write articles debunking the IPCC's latest findings.
But the working group's report went through a rigorous process. Not only were there 600 authors from 40 countries involved, but it was also "peer-reviewed" by a further 600 scientists and then the 21-page summary for policymakers was gone through over the past week in Paris by representatives of 113 countries.
Peter Stott, of Britain's Met Office, who is a member of the IPCC working group, said the world now had "an opportunity to prevent many of the climate changes this report discusses and avert a lot of their more serious impacts. We can even bring the temperature back down by pulling CO2 out of the atmosphere," he said.
And even though the scientists couldn't be 100 per cent certain that human activity is to blame for global warming, Achim Steiner, executive director of the UN Environment Programme, said there was every reason to act on it. "Every individual can go out the front door and cut emissions without reducing their quality of life."
The drip-feed release of the IPCC's Fourth Assessment of climate change will also keep the issue high on the international agenda this year. Reports from other working groups on the impacts of global warming and what measures might be taken to deal with them are to be published in Brussels next month and in Bankok in May.