Steve Connor: Global warming is not some conspiratorial hoax

The three reports that make up the fourth assessment of the Intergovernmental Panel on Climate Change will be published in full this year and the process begins this Friday in Paris with the publication of the first, which concentrates on the science of climate change. The importance of this and the remaining two reports should not be underestimated.

For the past six years, some 2,000 of the world's leading climatologists, glaciologists, meteorologists, oceanographers and specialists from dozens of other disciplines have trawled through all that has been published in the scientific literature on climate change and related matters. It has been a mammoth undertaking and the fruits of that work will be there for all to see in the final version of the IPCC's first report of its fourth assessment: Climate Change 2007: The Physical Science Basis.

Reading through the technical summary of this draft report, it is clear that no one could go away with the impression that climate change is some conspiratorial hoax by the science establishment, as some would have us believe. Far from it. It is clear from the draft version of the report that there is now overwhelming evidence to link man-made emissions of greenhouse gases over the past 250 years to dramatic changes in the Earth's climate.

With the climate science community, the IPCC has a reputation of being conservative and erring on the side of caution. It is perhaps the inevitable outcome of decision-making by committee, especially one that has come under huge pressure from various governments trying to downplay the scale of global warming. But what is interesting about the IPCC's fourth assessment is just how far the panel has come in terms of recognising one of the great unspoken fears of climate change - that it may be far worse than anyone can predict.

We are due to see temperature rises of about 3C this century, but the IPCC also says that it cannot rule out a rise of 6C or more. This would be cataclysmic given that the difference between global average temperatures now and during the last ice age 12,000 years ago is about the same.

The IPCC recognises that there are many "positive feedbacks" in the climate system - more apparently than the negative feedbacks that tend to modulate climate change - which could make matters worse as levels of carbon dioxide and global temperatures continue to rise. Some of these feedbacks are pretty well understood, but many are not. And there may even be some that we don't even know about. This is one of the reasons why there are still many levels of uncertainty when it comes to the future. The IPCC recognises this in the terminology of probability - "virtually certain" for instance means 99 per cent probability, while "likely" means 66 per cent probability.

But whatever the uncertainties, one thing is clear. We are changing the face of the planet and we have a limited period of time in which to do something about it.