Introduction:

Climate change is happening and Nigel Lawson, Jeremy Clarkson, Exxon and the Koch Foundation cannot stop it, though they can dangerously confuse and hinder our responses to it. In 2013 global atmospheric concentrations of carbon dioxide, as measured atop Hawaii's Mauna Loa volcano, reached 400 parts per million, up from around 280ppm before the Industrial Revolution. Such concentrations of CO2 have not existed in Earth for at least two million years. A new geological era – the Anthropocene – has now been proclaimed, where human activities start to have a significant – and negative – global impact on Earth’s ecosystems.

167 countries have endorsed the Copenhagen Accord agreeing that the safest maximum amount that global temperatures should be allowed to rise above the pre-industrial revolution level is 2oc. Some scientists claim that this is too lenient a target, but let us accept it for the time being. Given this the current consensus is that humans can pour roughly 565 more gigatonnes (billions of tonnes) of CO2 into the atmosphere by mid-century and still have a reasonable hope of staying below two degrees. As Bill McKibben (2012) points out "Reasonable" in this case means 'four chances in five, or somewhat worse odds than playing Russian roulette with a six-shooter'. Such probabilities must be borne in mind whenever interpreting scientists predictions, especially since it is common to talk of policies to achieve an only 50-50 chance of avoiding warming of more than 2oC.