In his review of Heat, Clive Hamilton makes many excellent points, and draws on powerful examples. The image of Texans turning up their air conditioning in order to enjoy a log fire is now printed indelibly on my mind. He correctly identifies the association between consumption and identity, and the threat that the need to tackle climate change presents to the traditional view of progress. I believe he may be right when he says that I have been too harsh in my criticisms of the Kyoto Protocol. But I would, of course, like to contest some of his other claims.

Let me begin—as this underpins all the arguments that follow—by explaining why I have chosen an ‘aggressive target’ for cutting carbon dioxide emissions. Hamilton says that seeking to prevent two degrees of warming above pre-industrial levels is ‘a more ambitious target than most.’ That is not correct. As long ago as 1990, the United Nations Advisory Group on Greenhouse Gases proposed that we should seek to confine the rise to a maximum of 1–2°C.footnote¹ An upper limit of 2°C is the European Union’s stated target, which informs the carbon reductions planned by the uk and most of the governments making serious efforts to tackle climate change.footnote²

There is a good reason for this. Two degrees of warming is the point at which up to 4 billion people could suffer water shortages, crop yields could fall in many regions of the poor world, mountain glaciers disappear worldwide and the irreversible melting of the Greenland ice sheet, which could eventually raise global sea levels by 7 metres, is expected to begin.footnote³ It is also the point at which several important positive feedbacks could be triggered. The permafrost of the West Siberian peat bog, for example, contains 70 billion tonnes of methane, a powerful greenhouse gas.footnote⁴ If all of it were released, its warming effect would equate to 73 years of current manmade carbon dioxide emissions. The methane that escapes due to melting would accelerate global warming, melting more permafrost, which releases more methane. A two-degree rise in temperatures could cause the runaway warming of permafrost throughout the Arctic Circle.

For this and other reasons—including the die-back of tropical forest, the accelerating metabolism of soil bacteria, a reduction of the earth’s reflectivity as ice melts—two degrees of manmade warming could cause a total impact of three degrees; and three degrees could lead inexorably to four. In other words, if two degrees of warming takes place, the problem is snatched from our hands. The biosphere becomes a major source of greenhouse gases, and there will be little we can do to prevent further climate change. Two degrees is the only target worth setting.

But while governments might agree that we should strive to keep temperatures below this threshold, they and their advisers fudge the means by which this should be done. Sir Nicholas Stern, for example, spells out the dire consequences of two degrees of warming. He then recommends a target for atmospheric concentrations of greenhouse gases of 550 parts per million, when measured as ‘carbon dioxide equivalent’ (CO2e). Stern admits that this concentration would produce ‘at least a 77 per cent chance—and perhaps up to a 99 per cent chance, depending on the climate model used—of a global average temperature rise exceeding 2°C.’ It would also give us a ‘30–70 per cent’ chance of exceeding 3° and ‘a 24 per cent chance that temperatures will exceed 4°C’.footnote⁵

In other words, 550ppm CO2e is the wrong target, as Stern must know. He is not alone. At a meeting I attended in 2005, Sir David King, the British government’s chief scientist, proposed that a ‘reasonable’ target for stabilizing carbon dioxide in the atmosphere was 550ppm CO2 (which means approximately 630ppm CO2e). It would be ‘politically unrealistic’, he said, to demand anything lower.footnote⁶ Simon Retallack from the Institute for Public Policy Research reminded Sir David that his duty is not to convey political reality but to represent scientific reality. King replied that if he recommended a lower limit, he would lose credibility with the government. It seemed to me that his credibility as a scientific adviser had just disappeared without trace.

So while two degrees remains the nominal upper limit, repeatedly cited by government ministers, politics, not science, informs the carbon reductions they propose in order not to exceed it. The calculations I explain in Heat, which any numerate person can replicate, estimate the cut demanded by the science. Hamilton says that the result—a worldwide reduction of 60 per cent—is ‘far beyond the cuts proposed by anyone else’. This is also incorrect. A paper published recently in the journal Climatic Change shows that in order to obtain a 50 per cent chance of preventing the global average temperature from rising by 2° above its pre-industrial level, we require a global cut of 80 per cent by 2050.footnote⁷