How hot and how soon

In his keynote address, Professor Hans Schellnhuber, the director of the Potsdam Institute for Climate Impact Research and a man regarded with some awe by those present, reminded us that at its meeting in L'Aquila in July 2009, the Group of Eight wealthy nations finally agreed to adopt the two-degree target. Two or three years ago this would have been regarded as a breakthrough, but by mid-2009 it was clear to the climate science community that aiming for two degrees would not meet the aim of the Framework Convention of avoiding 'dangerous climate change'. When the world warms by two degrees, Schellnhuber observed, we will lose all coral reefs. 'But who needs coral reefs?' he added, the first instance of the grim humour that seems to go naturally with reflection on climate science. He showed us a diagram mapping the historical relationship between global temperature and sea levels. A planet 2.5 degrees warmer means most of the ice eventually melts, leaving the oceans 50 metres higher than they are today. Already the Arctic is melting, as are the Himalayan glaciers (sometimes referred to as the 'third pole') that feed the rivers of South Asia. Without the summer run-off a billion people will be without water. The 'really big giant', he noted, is the methane trapped in the permafrost in Siberia and northern Canada, estimated to be equivalent to twice the total amount of carbon dioxide in the atmosphere. 'If this is ever released we will be toast,' Schellnhuber said in his soft German accent.

Kevin Anderson later developed the theme. Until recently it was heresy to question the two-degree target; it is, after all, official European Union policy. But two degrees of warming 'will kill a lot of poor people', he said, although we in the northern hemisphere think we can get away with it. The international community is fixated on setting emissions-reduction targets like 80 per cent by 2050, but the scientific understanding has now shifted. (I confess to a pang of sympathy for the policy-makers, for the political process inevitably moves more slowly than the science in a rapidly evolving area like climate change.) Just getting emissions down at some point is not enough; it is cumulative emissions that matter. Total carbon dioxide added by humans to the atmosphere over the next decades will determine our fate. This is the so-called budget approach that had emerged over the previous year or two. It was adopted by Oxford physicist Myles Allen and colleagues in their analysis showing that total anthropogenic emissions must not exceed a trillion tonnes of carbon if warming is to be limited to two degrees. The first half of that trillion has already been emitted and at current rates of emissions the entire budget will be spent sometime between 2030 and 2050.

The budget approach rewrites the chronology of mitigation efforts, said Anderson, because it rules out delay; a tonne of carbon dioxide emitted now counts as much as one emitted in 2050, so setting a target for emissions levels in 2050 is folly if it allows governments to postpone emission cuts. A number of speakers referred to the striking paper by Susan Solomon and others (which, along with the paper by Allen et al., I referenced in Chapter 1) showing that, unlike other greenhouse gases, most carbon dioxide stays in the atmosphere for a thousand years and more so that any warming will be with us for many centuries. Overshooting strategies such as that advocated in the Stern review are based on bad science because they are physically impossible, unless methods are found to extract large amounts of carbon from the atmosphere cheaply, permanently and quickly. Yet, such is its allure that overshooting seems to have embedded itself into political negotiations, with consequences that will prove disastrous.

The presentations from Schellnhuber, Anderson and Allen impressed on us that there are two numbers on which the future of humanity rests—the year in which global emissions peak and the rate of reduction of emissions thereafter. The curves that can be drawn to reflect different combinations of these two numbers describe what Schellnhuber called 'vicious integrals', because the areas underneath them are the carbon budgets we have to gamble with. The later the peak occurs, the faster emissions must fall for the world to remain within its emissions budget. So the peak year matters enormously; to have any chance of limiting warming to two degrees global emissions must peak in 2015, with rich countries starting to cut their emissions right now and pushing them to 25-40 per cent below 1990 levels by 2020. It is the imminence of 2015—and the need by then to transform the way we generate and use energy—that inspired a May 2009 meeting of Nobel laureates to urge the world to recognise 'the fierce urgency of now'.3

Anderson outlined the task with devastating simplicity. If developing-country emissions peak in 2030 and decline at 3 per cent per year thereafter (where 3 per cent is probably the maximum rate of fall consistent with continued economic growth), and developed-country emissions peak in 2015 and decline by 3 per cent a year thereafter, then the world has a 50:50 chance of limiting warming to four degrees. Read that again: four degrees. In his view, we will limit warming to four degrees 'if we're lucky'. As may have been apparent in Chapter 1, in my view Kevin Anderson is the scariest man on the planet. Yet we must be grateful for his unblinking honesty and compassion.

The pace of climate change thus depends on the trajectory of global emissions over the next couple of decades. The biggest influences will be the rate of growth of the world economy, driven disproportionately by growth rates in China, India and Brazil, and the extent of efforts by governments in the major economies to restrain emissions.

Climate scientists are not political analysts but the more savvy ones know enough about the world to understand that peaking in 2015, with rich countries cutting emissions by 25-40 per cent in 2020, is impossible. A 2020 peak in global emissions seems out of the question too. Without some unforeseeable stroke of luck, a warming of four degrees and more appears very likely. The best estimate is that we will reach that level in the 2070s or 2080s, although if things go badly it could be as soon as the 2060s. In other words, children alive today can expect to be living in a world on average four degrees warmer. Because the oceans warm more slowly, this means five to six degrees hotter on land.

It all comes down to the politics. Schellnhuber told us that he has several times had the opportunity to 'speak the unspeakable' to German Chancellor Angela Merkel—she was a physicist and seems to grasp what is at stake—but he believes it will be another decade before our political leaders collectively understand and are willing to act. So we will definitely go beyond two degrees and perhaps as high as five degrees, he said, confessing his deepest worry that the politicians may then say 'Well, let's just let it [the climate] go and adapt to it'. The only response, he argued, is to 'bombard politicians with scientific information every day'.

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