08/05/2013 10:18 BST | Updated 07/07/2013 06:12 BST

Have Our Hopes of Avoiding Dangerous Climate Change Just Melted Away?

The planet was saved on the 18 December 2009, when the largest meeting in UN history produced an accord which, reflecting the 'scientific view', agreed to limit warming to two degrees centigrade in order to avoid dangerous climate change. The world has so far warmed by 0.8 degrees above the average global temperature during the pre-industrial period. Thankfully that means we are not even half way to the point at which climate change will become dangerous, giving us plenty of time to develop and implement the necessary low carbon technologies.

Of course there is that troubling issue of the record extent of Arctic ice melt in 2012, which the UK Met Office has linked to the recent increase in the extent and severity of UK winters. And yes, scientists are suggesting that the weakening of the jet stream may be linked to the recent record breaking droughts and floods in places as far flung as the US, UK and Australia, events which are already starting to impact on the availability of food . But, if world leaders and scientists are saying that climate change won't become dangerous until the planet has warmed by more than double the current levels of warming, one must suppose these weather extremes are nothing to do with global warming.

Except that climate scientists have not identified two degrees of warming as the point at which dangerous climate change begins. Science has shown that humans have never lived in a world with such high concentrations of greenhouse gases. But predicting what will happen, where and when as a result of this transformation is simply not possible. However, we do know that the changes won't be uniform, so fixing one limit for the whole globe is unlikely to protect everyone from danger. We also know that some people are more vulnerable to changes than others. And what counts as dangerous is not the same for everyone - different people have different attitudes to risk. For these and other reasons, all the scientists I have spoken to argue that it is not for science to define how much warming is too much. Instead those sorts of choices need to be made by politicians, ideally as a result of an inclusive social process.

The world leaders who decided at Copenhagen how much warming is too much chose not to explain the political reasons for choosing two degrees. They instead deceitfully claimed that scientists told them this was the dangerous limit. This poses two important questions - why did the politicians describe the two degree limit as scientific fact and, if it is not scientific fact, how do we know climate change will not become dangerous until the planet has warmed by two degrees? The answer to the second question is that we don't know. We must begin to take seriously the possibility that dangerous climate change has already begun. In so doing we raise some very difficult questions about the two degree limit. If it is not a line separating dangerous climate change from safe climate change then what role does it have in our understanding of climate change and how we decide to respond? This is tied up with questions about why politicians have described the limit as scientific fact. The answer to that question is that politicians have described the limit as scientific fact as a substitute for democratic political processes. Climate change policy is, like foreign and economic policy, one of those big questions best kept out of political debate in case the people give the wrong answer. What if the public agreed that the best way to reduce emissions was to create a world of less work, less consumption and a more equal distribution of resources? That is simply too big a risk for the political class to take. Much better to claim climate change is a long term problem not requiring radical knee-jerk responses, but to be solved by carrying on with business as usual while looking for a magic bullet techno fix. Unfortunately it is time to recognise that opportunity has gone, and that working to a two degree limit is a planetary suicide note. We need to start the properly democratic deliberations that scientists have identified as the means by which we can, together, work out how move forward from this point. Physically, our world has entered a new epoch. We need our political processes to do the same.