Six Degrees: Our Future on a Hotter Planet. Mark Lynas

lake in such an arid area could only have been maintained by much higher rainfall, and longer-term records do indeed show that the Saharan region has experienced repeated wet and dry episodes over cycles of many thousands of years. The coldest periods of the ice ages tended to be the driest in the Sahara, whilst warm interglacials brought rain-allowing life to emerge once again. During the early Holocene epoch, 9,000 to 6,000 years ago, the northern hemisphere summer sun was slightly stronger than today, thanks to a small cyclical shift in the Earth's orbit around the sun. The increased heating warmed up the giant North African landmass to such an extent that it powered a monsoon-just like the one that brings annual summer rains to the Indian subcontinent today.

      Monsoons are based on the simple principle that land surfaces heat up quicker in the summer than the surrounding oceans. This creates an area of low pressure as the hot air in the continental interior rises, sucking in cooler, moister air from the neighbouring seas. These rain-bearing winds bring torrential summer downpours to monsoonal climates such as India's, where agricultural life revolves with this annual cycle. The African summer monsoon is weaker and less generally recognised, but is still the only source of reliable rainfall for the Sahel. Climate models project that land surfaces will warm much faster than the oceans during the twenty-first century, potentially adding a boost to summer monsoons. So with one degree of global warming, this monsoon could begin to gain power and penetrate once again far into the African continent, greening the Sahara.

      But will it actually happen? Before anyone makes plans to move large-scale food production to the central Sahara, a note of caution needs to be sounded. During the early Holocene, an additional monsoon driver was the difference in the distribution of solar heat between the two hemispheres. This time the whole globe is heating up, so the past is not a perfect analogue for the future. Moreover, it would be wrong to get the impression that the more humid Sahara was some kind of verdant wonderland-rainfall totals mostly only reached 100 mm or so, enough to support only the barest savannah-type vegetation, and wetter phases would also have been interspersed with long droughts. However, computer models can help negotiate a way through the conflicting possibilities-and the answer they provide holds profound implications for all the inhabitants of North Africa.

      The preliminary stage is set by Martin Hoerling and two other climate scientists based in Boulder, Colorado, who used sixty different model runs to confirm that whilst southern Africa dries out with global warming, northern Africa does indeed begin to get wetter. Indeed, the long-term drying trend which caused such misery and devastation during the second half of the twentieth century goes into full reversal after about 2020 (with one-degree global warming or less), when the Sahel sees a long-term recovery in its rainfall. By 2050 the recovery is in full swing, with 10 per cent more rainfall right across the sub-Saharan zone.

      This conclusion is supported by a second study, which projects heavier rains on both the West African coast and into the Sahel as a warmer tropical Atlantic Ocean supplies huge amounts of water vapour to form rain-bearing clouds. With more plentiful rains, crop production can potentially increase, offsetting declines elsewhere-assuming, that is, that temperatures are not so high that people who once died from famine now die from heatstroke.

      However, computer modellers based in Princeton, New Jersey, have come up with a rather different long-range forecast. Their model accurately simulates the terrible 1970s and 1980s drought-but after a short interlude of higher rainfall, it projects even fiercer drought conditions for the Sahel region in the second half of the twenty-first century.

      So why the divergence between the different models produced by the Princeton and the Boulder teams? The Princeton researchers admit that they are stumped. ‘Until we better understand which aspects of the models account for the different responses in this region,’ they caution, ‘we advise against basing assessments of future climate change in the Sahel on the results of any single model.’ Nevertheless, they insist, ‘a dramatic 21st century drying trend should be considered seriously as a possible future scenario’.

      This latter finding also chimes with global studies, which suggest stronger droughts affecting ever-larger areas as the world warms up. One of the most wide-ranging analyses was undertaken by Eleanor Burke and colleagues from the Hadley Centre at Britain's Meteorological Office, who used a measure known as the ‘Palmer Drought Severity Index’ to forecast the likely incidence of drought over the century to come. The results were deeply troubling. The incidence of moderate drought doubled by 2100-but worst of all, the figure for extreme drought (currently 3 per cent of the planet's land surface) rose to 30 per cent. In essence, a third of the land surface of the globe would be largely devoid of fresh water and therefore no longer habitable to humans.

      Although these figures are based on global warming rates of higher than one degree by 2100, they do indicate the likely direction of change. As the land surface heats up, it dries out because of faster evaporation. Vegetation shrivels, and when heavy rainfall does arrive, it simply washes away what remains of the topsoil. It may seem strange that floods and droughts can be forecast to affect the same areas, but with a higher proportion of rainfall coming in heavier bursts, longer dry spells will affect the land in between. This, then, is the most likely forecast for the Sahel: whilst rainfall totals overall may indeed rise, these increases will come in damaging flash-flood rainfall, interspersed with periods of intensely hot drought conditions.

      According to some historians, the greener Sahara of 6,000 years ago was the geographical basis for the mythical Garden of Eden, its original inhabitants expelled not by God for bad behaviour, but by a devastating drying of the climate. Whilst scientists continue to argue over the specifics of the likely climatic future of the Sahara and Sahel, one thing seems clear: humanity will not be returning to Eden any time soon.

       The Arctic meltdown begins

      Over recent years a new phrase has entered the scientific lexicon: ‘the tipping point’. Originally popularised by Malcolm Gladwell's bestselling book of the same name, the understanding that social or natural systems can be non-linear is a crucially important one. An oft-used analogy is of a canoe on a lake: wobble it a little, and stability can return with the boat still upright. Cross the point of no return, the ‘tipping point’, and the boat will capsize and find a new stability-this time upside down, with the ill-advised canoeist floundering underneath.

      Scientists have increasingly realised that the Earth's climate is a good example of a non-linear system: over the ages it has been stable in many different states, some much hotter or colder than today. During the ice ages, for example, global temperatures averaged five degrees cooler than now for tens of millennia. Moreover, the system can ‘tip’ from one state into another with surprising rapidity. Episodic sudden warmings embedded within the last ice age saw temperatures in Greenland rise by as much as 16°C within just a few decades. The reasons why the climate flipped so rapidly are still not completely understood, but it is clear that even tiny changes in ‘forcings’-from greenhouse gases or the Sun's heat-have in the past led to dramatic responses in the climate system. In contrast, our relatively stable climate is highly unusual-the Holocene period, during which all of human civilisation has come about, has seen very little change in global temperatures. Until now.

      Scientists have established beyond reasonable doubt that the current episode of global warming, of about 0.7°C in the last century, has pushed Earth temperatures up to levels unprecedented in recent history. The IPCC's 2007 report confirmed that no ‘proxy records’ of temperature-whether from tree rings, ice cores, coral bands or other sources-show any time in the last 1,300 years that was as warm as now. Indeed, records from the deep sea suggest that temperatures are now within a degree of their highest levels for no less than a million years.

      The part of the globe most vulnerable to this sudden onset of warming, and the part which will likely see the first important ‘tipping point’ crossed, is the Arctic. Here, temperatures are currently rising at twice the global rate. Alaska and Siberia are heating up particularly rapidly; in these regions the mercury has already risen by 2-3°C within the last fifty years.

      The impacts of this change are already profound. In Barrow, Alaska, snowmelt now occurs ten days earlier on average than in the 1950s, and shrubs have begun to sprout on the barren, mossy tundra. Scientists based in Fairbanks, Alaska, have documented a sudden thawing of underground ice wedges on the