In the balance
IN SCIENCE, more information is supposed to lead to better conclusions and greater consensus. The Intergovernmental Panel on Climate Change (IPCC), which published its latest report on March 31st, certainly has more information. The new study synthesises 73,000 published works (a quarter of them in Chinese). This represents a 100-fold increase in about 30 years. But consensus remains elusive. Richard Tol of Sussex University, in Britain, disparagingly appraised the reports conclusions as the four horsemen of the apocalypse. The final version appears to have been fought over paragraph and comma between those (such as Dr Tol) who want to describe dispassionately what they think is happening and those who want to scare the world into taking action. Every six or so years, the IPCC produces a three-part encyclopedia of the climate. This report is the second tranche of its latest effort. The first, on the science of climate change, came out last September. It argued that the process is accelerating even though the worlds surface temperatures are currently flatlining (a phenomenon most climate scientists regard as merely a pause in an upward trend). This, second, volume asks how the climate is affecting ecosystems, the economy and peoples livelihoods. Profoundly, is the headline answer. It argues that climate change is having an impact on every ecosystem from the equator to the poles. It suggests that although there are some benefits to a warmer climate, most effects are negative and will get worse. It talks of extreme weather events leading to breakdown of...critical services such as electricity, water supply and health and emergency services and it sounds the alarm about the breakdown of food systems, linked to warming. Behind such scares, though, lies a subtler story, in which the effects of global warming vary a lot, climate change is just one risk among many, and the damage it causesand the possibility of reducing that damagedepend as much on other factors, such as health systems and rural development, as they do on global warming itself. Bad, but how bad? The report describes three different sorts of problem. The first are those in which climate is the dominant influence, so that no human action other than stopping it changing will have an effect. The second are those in which the climates influence is modest and where the news is not entirely bad. The third are the ways a changing climate alters which species (both natural and agricultural) thrive wherewhich from a human perspective can be both good and bad. Rising sea levels are an example of the first sort of problem. Thermal expansion of the water in the oceans means that, at current rates, the average sea level could go up half a metre (20 inches) by the end of the century. That would be pure bad news for people living in coastal cities. They now number 271m, a figure which may increase to 345m by 2050, according to the report. Another example of a problem of the first sort is ocean acidification. This is caused by the absorption of carbon dioxide into seawater. The report calls this a fundamental challenge to marine organisms and ecosystems. The second sort of problem, in which the climates influence is more modest and manageable, includes its effects on health. In a warmer world some diseases, such as malaria, are expected to spread. And heat itself can kill. More summer heatwaves will mean more premature deaths. But cold is also a killer, and the number of cold-related deaths will fall. By and large, the report says, the bad impacts will outweigh the good, but in neither case is climate the dominant influence on mortality or morbidity. Public health and nutrition matter more. Malaria cannot spread if it has been exterminated. The third category, the way a changing climate alters species ranges, is in some ways the most intriguing. To the surprise of a lot of conservationists, for example, global warming does not seem to have caused many extinctions. The only ones laid at its door so far are of frogs in Central America. That does not mean change is not happening. In the oceans both animals and plants are migrating from the tropics to temperate latitudes in pursuit of cooler waters. Benthic algaeseaweeds, to the laymanare shifting their ranges polewards at 10km (6 miles) a decade. Their single-celled planktonic cousins are moving much faster: 400km a decade. Since algae are the beginning of marine food chains, everything else changes with them. The result, says the report, is that by 2055 fish yields in temperate latitudes could be 30-70% higher than they were in 2005 (see map). Tropical yields, by contrast, could fall by 40-60%. The yields in question are potential ones, and assume that overfishing has not denuded the oceans by then, but that matter is beyond the IPCCs remit. From the human point of view, though biological changes in the ocean are important, the most crucial such changes will be on land, and will concern where particular crops can be grown. A warmer climate lengthens growing seasons and more carbon dioxide in the atmosphere should stimulate photosynthesis. The previous IPCC assessment, in 2007, therefore said that yields of the worlds main cropswheat, rice, maize and soyabeanswould improve in temperate and cold climates, offsetting declines elsewhere. Some argued, on this basis, that a modest amount of warming might be good for people. The new report pours cold water on that idea. It confirms that tropical yields will decline if the temperature rises by 2C (which is all but inevitable) but finds that the offsetting benefits in temperate zones will be smaller than once thought. Rain-fed crops (as opposed to those watered by irrigation), which are often grown in the tropics, do respond to higher levels of carbon dioxide, but the effect is counteracted by rising temperatures. Plants like long growing seasons but many (especially maize) hate temperature spikes: even one day above 35C at the wrong time of their life cycles can damage them. And rates of photosynthesis in maize, sorghum and sugarcane (called C4 cereals, because of the details of their photosynthetic pathways) do not respond to changes in CO2 concentrations in the way that C3 cereals, such as wheat and rice, do, so the effect of more carbon dioxide on crops is patchy. At the moment, the report concludes, wheat yields are being pushed down by 2% a decade compared with what would have happened without climate change; maize is down by 1% a decade; rice and soyabeans are unaffected. Over time, this could worsen. Roughly half of studies of likely cereal yields over the next ten years forecast an increase, whereas the other half forecast a decline. Forecasts for the 2030s are even more sobering: twice as many predict a fall as a rise. Dividing up the effects of climate change in this fashion leads to different ideas about how to respond. Defending low-lying cities against a rising sea level is difficult and expensive, and it is impossible to adapt to ocean acidification. These problems would best be dealt with (if at all) by attacking the cause: ie, by cutting carbon-dioxide emissions. Problems in the second category, however, can be approached in other ways. As the report itself says, the most effective vulnerability reduction measures for health...are programmes that implement and improve basic public health [like] the provision of clean water. Such measures would be beneficial even if there were no climate change. The third category lies somewhere in between. It requires measures that should be undertaken anyway, but need to be tweaked because of the climate. Farmers are always trying out new crop varieties, but increasingly those varieties will have to be drought-resistant. That may mean choosing between different aims, for there is often a trade-off between drought resistance and yield. This way of looking at the climate is new for both scientists and policymakers. Until now, many of them have thought of the climate as a problem like no other: its severity determined by meteorological factors, such as the interaction between clouds, winds and oceans; not much influenced by lesser problems, like rural development; and best dealt with by trying to stop it (by reducing greenhouse-gas emissions). The new report breaks with this approach. It sees the climate as one problem among many, the severity of which is often determined by its interaction with those other problems. And the right policies frequently try to lessen the burdento adapt to change, rather than attempting to stop it. In that respect, then, this report marks the end of climate exceptionalism and the beginning of realism.