Of warming and warnings
SCIENCE has spoken, said Ban Ki-Moon, the UNs secretary general. Time is not on our side. Leaders must act. He was reacting to the latest assessment of the state of the global climate by the Intergovernmental Panel on Climate Change (IPCC), a group of scientists who advise governments. Rajendra Pachauri, the IPCC's chairman, agreed. We have little time before the window of opportunity to stay within 2C of warming closes, he said. (Governments have promised not to let global temperatures rise by more than that amount compared with pre-industrial levels.) Bill McKibben, an American climate campaigner, went for broke, calling the report just short of announcing that climate change will produce a zombie apocalypse, plus random beheadings, plus Ebola. The assessment, it should be said, is sobering. But it does not justify alarmism. The IPCCs overview is its fifth since 1990. Things have moved on since the previous one, in 2007. Scientists have become ever more certain that human activity is to blame for climate change: about 95% certain, in fact (the first report said climate change was as likely as not a product of natural variation). The report spells out the evidence that the climate is indeed changing. Average land and sea-surface temperatures rose by 0.85C in 1880-2012; sea levels rose by 3.2mm a year in 1993-2010, twice as fast as in 1901-2010; the acidity of the oceans surface has risen by 26% since the start of the industrial revolution. At the moment, the impact of all this change can be seen mostly on natural systems. Arctic sea ice, for example, is shrinking by around 4% a decade, and the Greenland and Antarctic ice sheets are losing mass. Marine species are shifting their ranges, heading toward the poles to find cooler waters. In contrast, the impact on human welfare has so far been modest. The report calls the effect on health relatively small [and] not well-quantified. It expresses low confidence in the idea that the frequency and sizes of floods have been affected by climate changethough that is partly because the records are poor. True, it says, climate change in the form of heat and drought may have reduced yields of maize and wheat. But the effect on rice and soyabeans, the worlds other staple crops, has not been so bad. Although humans are damaging the climate, it is less clear that climate change is so far damaging humans that much. If climate change is an emergency, then it is not of the kind that can be quickly reversed. Rather, the report says, actions taken now will have little impact for decades, mainly because the climate has exceptionally long response times. The Earth now has what is known in the parlance as a stock problem, not a flow problem. The flow of greenhouse gases into the atmosphere can be adjusted, but the stock of them already accumulated means that the expected rise in surface temperatures between 2016 and 2035 is roughly the same in a range of projections for how things might now go. Claims that the report is all doom and gloom therefore refer to the middle of the century and later. Then, the IPCC suggests, there could be severe, pervasive and irreversible impacts. Inevitably, though, forecasts that far ahead come with significant qualifications. The size of the population, for example, makes a big difference to carbon emissions and climate change. But the difference between the UNs highest and lowest projections for 2050 is 2.5 billion people. The climate models themselves are a work in constant progress. Taken in sum, this latest assessment is a stern warning, but it is not yet a promise of disaster. Correction: An earlier version of this article suggested that the severe, pervasive and irreversible impacts mentioned by the IPCC would only occur in its worst-case projections. In fact, the report makes no such qualification. Sorry.