Steve Connor: Climate change is like a disaster in slow motion

There now seems to be a growing disconnection between the message that scientists are sending out about climate change and the corresponding reaction of politicians and the public. As the experts issue increasingly dire warnings about what could happen to the world's climate system if we don't do something about carbon dioxide emissions, politicians prevaricate, the public becomes more sceptical and we all continue to burn more fossil fuels.

The latest assessment by a team of 31 leading scientists from seven countries presents a bleak vision of the path upon which we are now firmly set. It is the worst-case scenario laid out by the Intergovernmental Panel on Climate Change, suggesting average global temperature will rise by 5C or 6C by the end of the century.

Six degrees may not seem like much – it is the difference between one summer's day and another – but in terms of a global average it is catastrophic. The difference between now and the last ice age, for instance, was just a few degrees, and a 6C increase by 2100 would produce a dramatically different world to the one that humans have lived in since the end of the last ice age.

Global climate change is like a disaster in slow motion. It is as if our brains are not programmed to respond to a threat that could take decades to become real: a smoking gun without a bullet. Politicians have to respond to events on a much shorter timescale which is one of the reasons, perhaps, why the climate conference next month in Copenhagen has been such a difficult deadline for them to work to. Far easier to put it off until the next climate meeting in Mexico, or the one after that.

But as Professor Corinne Le Quéré and her 30 colleagues point out in their scientific paper published in Nature Geoscience, we do not have unlimited time. The longer we put aside action on carbon emissions, the more difficult it will be to keep within the "safe" increase of C above pre-industrial times. If by some miracle we manage to curb carbon dioxide emissions now and reach a peak in production in 2012, we will still need to reduce carbon emissions by 4 per cent per year to achieve the C target. If the peak happens in 2015, we will need to reduce carbon emissions by 5 per cent per year thereafter, and if the peak happens in 2020 the reduction will have to be an almost inconceivable 9 per cent annually.

So the longer we leave it, the harder it will become. That is the simple message to emerge from the scientists who know most about the hugely complicated system that is the Earth's climate. There are huge uncertainties, which are ruthlessly exploited by those who question the relationship between man-made fossil fuel emissions and global warming, or even dispute whether we are in fact experiencing any significant warming. The scientists involved in the present study know full well that these uncertainties may affect their predictions, although not to the extent that the climate sceptics would like us to think.

One of the greatest uncertainties concerns what would happen in a warmer world to the natural "carbon sinks" that absorb atmospheric carbon dioxide. All the data suggests that these sinks are more likely to become less effective when temperatures rise. This could lead to potentially dangerous "positive feedbacks" whereby warming temperatures lead to increases in the amount of carbon dioxide in the atmosphere, which lead to even warmer temperatures.

According to Professor Le Quéré and her colleagues, the feedbacks in the carbon cycle have already kicked in. We are now at a dangerous threshold in terms of serious and potentially irreversible climate change. The scientists know it, the politicians should know it. We all need to know it.