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

four weeks of summer. A doubling of what the study calls ‘extremely hot days’ is also projected, whilst land areas around the Mediterranean can expect three to five additional weeks of ‘heatwaves’ (defined as days with temperatures over 35°C). Islands such as Sardinia and Cyprus only tend to escape the worst because of the cooling influence of the sea.

      The high temperatures will be aggravated by drought, with areas in the southern Mediterranean projected to lose around a fifth of their rainfall. Spain and Turkey will also be badly affected, whilst northern areas on average see a 10 per cent decline in rainfall and a corresponding two-to three-week increase in the number of dry days. Up to a month extra of drought can be expected in southern France, Italy, Portugal and north-west Spain. The seasonality of rainfall will also change, playing havoc with agricultural practices: in southern France and Spain, for example, the dry season is projected to begin three weeks earlier and end two weeks earlier.

      Air-conditioning may not always be an option: with peak power demand occurring during the driest part of the year when reservoir levels are already low, hydroelectric power outages could lead to blackouts during the worst heatwaves. Tourists-especially the elderly-will need to stay away because of the danger of heatstroke, whilst Mediterranean locals might actually prefer to spend summers far away in northern Europe in search of cooler temperatures. Lifestyles will have to change, with people perhaps adopting more Middle Eastern or North African living routines to cope with the heat.

      Water shortages will become a perennial problem around the whole Mediterranean basin, particularly as some of the most arid coastal areas of Spain and Italy are also some of the most densely populated. Rich Germans and Britons thinking of retiring to Spain might be well advised to stay put. With searing heat and little fresh water to cool things off, perhaps the lure of the sun won't be so strong after all. The mass movement in recent decades of people from northern Europe to the Mediterranean is likely in the two-degree world to begin to reverse, switching eventually into a mass scramble to abandon barely habitable temperature zones-as Saharan heatwaves sweep across the Med.

       The coral and the ice cap

      Back in 1998, three Canadian geologists took a trip to the Cayman Islands. They were not there to sunbathe or launder money (two activities for which the islands are justly famous) but to investigate a strange raised limestone platform in the Rogers Wreck area of Grand Cayman island. The platform-known to geologists as the Ironshore Formation-is about 20 metres thick, and includes layers of ancient coral hundreds of thousands of years old. The formation sparked the scientists' interest because if they could date the coral accurately, its height above sea level today would help them solve a mystery about how sea levels had changed in the past. Tropical coral reefs form in shallow seas, so if old coral is now above sea level, only two explanations are possible: the land has risen, or the sea level has fallen. After meticulous investigation, the three scientists-Jennifer Vezina, Brian Jones and Derek Ford of the University of Alberta's Earth and Atmospheric Sciences department-ruled out land uplift and concluded that sea levels during the previous Eemian interglacial period were many metres higher than they are now.

      The Canadian scientists' conclusion chimed with other studies from around the world, which have also suggested that sea levels were 5-6 metres above present during the Eemian, 125,000 years ago. Given that global temperatures were then about 1°C higher than now (though slightly higher in the Arctic, thanks to the polar amplification effect), this in turn raised another question: where had all the extra water come from?

      First to come under suspicion was the West Antarctic Ice Sheet. Glaciologists had long suspected that it might be sensitive to small changes in temperature, and in total it contains enough ice to raise global sea levels by 5 metres. Indeed, as early as 1978 a paper in Nature warned that the ice sheet posed ‘a threat of disaster’-a warning which is even more pressing today, as chapter 4 reveals. But attempts to model ice sheet collapse had proven inconclusive, and in 2000 an entirely different contributor to sea level rise was proposed: Greenland.

      The Greenland ice cap contains enough water in its 3-kilometre-thick bulk to raise global sea levels by a full 7 metres, and when scientists investigated cores drilled from the summit of the ice sheet they reached a surprising conclusion. Greenland had indeed shrunk significantly during the Eemian-so much so in fact that most of the southern and western part of the landmass had been completely free of ice for thousands of years. Indeed, evidence has recently emerged that Greenland was once forested in regions that are now under two kilometres of ice-although this may have been in an earlier (and slightly warmer) interglacial than the Eemian. With a lower summit, steeper sides and a drastically reduced extent, the Eemian ice sheet would have contributed, the scientists concluded, between 4 and 5.5 metres to higher global sea levels at the time. This, together with smaller contributions from Antarctica and other glaciers, plus some thermal expansion of seawater, would seem to explain the high sea levels.

      The study raised a few academic eyebrows at the time, but its implications didn't really begin to sink in until several years later. In retrospect, this is perhaps surprising: it contained clear evidence that a climate only a degree or so warmer than today could melt enough Greenland ice to drown coastal cities around the globe, cities that are home to tens of millions of people. Nor was it just a one-off: more recent work confirms that Greenland's contribution to the higher sea levels of the Eemian was indeed somewhere between 2 and 5 metres.

      The 2001 report by the Intergovernmental Panel on Climate Change (IPCC) did conclude that higher temperatures would eventually melt the Greenland ice sheet-but only over centuries to millennia, and very little contribution from Greenland was factored into the twenty-first-century sea level rise projections of between 9 and 88 cm. As warnings go, it wasn't a terribly urgent one: most people have trouble caring about what happens 100 years hence, let alone bothering about whether their distant descendants in the year 3000 might be getting their feet wet.

      One man begged to differ, and he wasn't some sandal-wearing greenie who could be easily dismissed. The new warning came from James Hansen, the NASA scientist whose testimony to Congress back in the hot summer of 1988 did so much to put global warming on the international agenda for the first time. Hansen penned a characteristically straightforward article entitled ‘Can we defuse the global warming time-bomb?’, later published in Scientific American, which asked the key question: ‘How fast will ice sheets respond to global warming?’ The article was critical of the IPCC's assurances that ice sheet melting would be gradual even in a rapidly warming world, words which Hansen felt had downplayed the urgency of our situation.

      Hansen noted instead that a global temperature rise over 1°C could destabilise the polar ice sheets enough to give rises in sea levels far greater than the modest 50 centimetres or so by 2100 that is seen as most likely by the IPCC. At the end of the last ice age, for example, global sea levels shot up by a metre every twenty years for a period of four centuries, drowning tropical coral reefs in Hawaii and submerging low-lying coasts. This dramatic flood, termed ‘Meltwater Pulse 1a’ by scientists, occurred 14,000 years ago as the giant ice sheets of the last glacial age finally crumbled and gave way to the warmer Holocene.

      What has happened before can happen again, argued Hansen, especially given today's enormous atmospheric loading of greenhouse gases, whose climatic impact far outweighs the tiny orbital changes which govern the ice-age-to-interglacial transitions. Just as they were in the past, ice sheet changes in the future could be-to use Hansen's phrase-‘explosively rapid’.

      Hansen had little support, however, until the following year, when a European modelling team put an actual figure on Greenland's critical melt threshold: 2.7°C. This, moreover, wasn't a figure for global warming but instead for regional warming. Because the Arctic heats up faster than the globe as a whole, this tipping point will be crossed sooner in Greenland than the global average: because of polar amplification, reported a second scientific team, Greenland warms at 2.2 times the global rate. Divide one figure by the other, and the result should ring alarm bells in coastal cities across the world: Greenland will tip into irreversible melt once global temperatures rise past a mere 1.2°C.

      That's the bad news. The good news is that according to this study the Greenland ice sheet will contract only slowly, over millennia, to a smaller inland form. With higher levels of warming (up to 8°C