Supermodels
IN AN APPROPRIATELY sweltering lecture theatre at the University of Pierre and Marie Curie in Paris, scientists gathered earlier this year to discuss a phenomenon called the global-warming hiatus. Between 1998 and 2012 humans pumped unprecedented quantities of greenhouse gases into the atmosphere, but the average global temperature barely rose. Why? Because much energy went into melting ice, explained one. Because it was absorbed by the oceans, said another. Because many small volcanic eruptions threw particles into the atmosphere, deflecting solar radiation, explained a third. Nonsense, said a fourth. There was no hiatus at all1998 was a freakishly hot year, so it was hardly surprising that temperatures bumped around the same level for a few years. At the end, the moderator summed up: Well, thats science! This sort of thing drives green-minded politicians mad. It is hard enough to persuade voters that global warming is a serious danger that they must pay to avoid, in the form of higher energy bills and unsightly wind farms. If the scientists seem unsure, the task becomes impossible. Despite appearances, though, key parts of climate science are settled. Although the remaining uncertainties are a little larger than green groups generally admit, they are not nearly as big as global-warming sceptics suppose. The greenhouse effect itself is straightforward; it just does not work much like a greenhouse. About one-third of the energy that pours into the Earth from the sun reflects off clouds and the planets surface and heads back into space. Much of the rest is absorbed by the land and the oceans, which then emit it largely in the form of infra-red radiation. This is absorbed by trace gases in the atmosphere, which in turn release infra-red upwards, sideways and downwards to the Earths surface. It is this bouncing around of energy that is known as the greenhouse effect. It is essential to life on Earth; without it, the average temperature at the Equator would be -10C. The most important greenhouse gas is water vapour. Were there no water vapour or clouds, the greenhouse effect would be only about one-third as powerful as it is. Carbon dioxide is the second most important, followed by methane, then chlorofluorocarbons (CFCs), industrial chemicals that were cracked down on in the 1980s and 1990s because of their ozone-depleting properties but are still hanging around. These gases are more or less potent and durable. Tonne for tonne, methane is a much more powerful greenhouse gas than carbon dioxide, but it breaks down more quickly. Carbon dioxide, which reaches its maximum warming effect about ten years after being released, is so stable that even 1,000 years after a bump in emissions, atmospheric levels will still be substantially higher than normal. The basic science is hardly novel. In the 1890s a Swedish scientist, Svante Arrhenius, ran some tedious calculations on the greenhouse effect and went on to explain how burning fossil fuels might intensify it (living in a cold country, he thought this a thoroughly good thing). Things get complicated when scientists try to work out what happens to the extra energy that remains in the Earth system and how other human activities, beyond emitting greenhouse gases, might also affect the climate. Beware the feedback loops Greenhouse warming sets off a cascade of effects known as feedbacks, which are harder to measure. On balance, warming begets more warming. Higher temperatures enable the atmosphere to hold more water vapour. Oceans absorb huge amounts of carbon dioxide, keeping a lid on climate changebut as they warm up, their absorption capacity declines. Melting ice produces dark pools of water that absorb more energy. Partly for this reason, the Arctic is warming faster than other places. Inadvertently, though, humans also cool the Earth. Although the overall effect of deforestation is to warm the planet, replacing trees with crops or grassland makes the land paler and more reflective. Particles created from sulphur dioxidethe cause of acid rainreflect lots of light back into space. China has probably been shielded from higher temperatures by air pollution, and might heat up quickly if it gets serious about scrubbing its skies. The greatest mystery is the effect of human activity on clouds. Because clouds grow on aerosol particles, more of them are likely to form in a more polluted atmosphere. Clouds are also affected by temperature changes. But precisely how is unclearand this matters, because whereas high clouds tend to keep the Earth warm, low clouds tend to cool it. Part of the problem in measuring their effect is that many clouds are small. Climate models tend to simplify the world by dividing the atmosphere and the oceans into boxes, perhaps 50km by 50km in the horizontal plane, and treating these as pixels in a giant three-dimensional computer simulation. To capture cloud processes properly might require climate models with cells just tens of metres square. No computer in the world could handle that. Add up all these difficulties, throw in some problems with measuring temperatures, and you get a lot of uncertainty. The chart on the previous page, which shows the estimates by the International Panel on Climate Change (IPCC) of radiative forcingperturbations to the Earths energy system from human and other activitiescontains black bars showing 95% confidence ranges. Some of those bars are long. It is especially hard to be sure of the effect of aerosols. If the past is a little hazy, the future is more so. Not only does it depend on the outcome of physical processes that are inadequately understood. It also depends on human actions. How many people will be living in 2100? How rich will they be? Will they make strenuous efforts to cut greenhouse-gas emissions, do nothing, or something in between? If mankind makes heroic efforts, the Earth system will remain within familiar bounds, making predictions easier. If concentrations of greenhouse gases increase steeply, though, things become highly unpredictable. Passing irreversible tipping points, such as the collapse of the Greenland ice sheet, becomes more likely. If nothing were done to avert climate change by 2200, the IPCC estimates, the world would probably warm by between 3C and 10C. That enormous range is manageable at one end, unimaginable at the other. Much remains unknown, then. But, equally, much has been settledit is just that the settled stuff generates fewer papers and conference panels, because researchers have moved on. Some possibilities that seemed troubling a few years ago have been probed and revealed to be less so. It now appears unlikely, for example, that climate change will lead to the irreversible collapse of the Gulf Stream. Melting permafrost will emit methane, but not as much as some once feared. Even those mysterious clouds are giving up some of their secrets. Satellite-based radar and laser measurements have enabled scientists to peer into clouds; small-scale models designed to capture their behaviour have been refined and plugged into global models. It seems increasingly likely that low cloud cover will diminish as the Earth warms, speeding the process. Most important, the basic proposition of climate changethe causal relationship between greenhouse-gas emissions and higher temperatureshas become almost unassailable. As it happens, the interesting debate about the global-warming hiatus has a boring coda: 2013 turned out slightly hotter than 1998, and 2014 was roasting, setting a new record. That will not stand for long.