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

new meltwater ponds dotting the landscape. These ice masses had previously remained frozen for at least the past three thousand years, indicating how far outside previous historical variability current warming is moving.

      In other parts of the state entire lakes are draining away into cracks in the ground as the impermeable permafrost layer thaws underneath them. More than 10,000 lakes have shrunk or disappeared altogether in the last half-century, highlighting an alarming drop in the state's water table. In 2007, Canadian researchers reported that in Ellesmere Island, Nunavut, ponds which existed for millennia have now become ephemeral as their water evaporates away in the summer heat. Water-dependent species from insect larvae and freshwater shrimps to nesting birds are being wiped out as a result. Vegetation that once grew on these thin, waterlogged soils is now so desiccated that it easily catches fire.

      Arctic mountain glaciers are also responding. On the Seward Peninsula of Alaska, the Grand Union Glacier is retreating so quickly that it is projected to disappear entirely by the year 2035. Other, much larger glaciers elsewhere in Alaska are also thinning rapidly. In the decade up to 2001 alone, the biggest Alaskan glaciers are estimated to have lost 96 cubic kilometres of ice, raising global sea levels by nearly 3 mm. Across the entire Arctic, glaciers and ice caps have lost 400 cubic kilometres of volume over the past forty years.

      Perhaps the clearest bell-wether of change is found out at sea. The Arctic ice cap has been in constant retreat since about 1980, with each successive summer seeing more and more of its once-permanent ice disappearing. Each year on average 100,000 square kilometres of new open ocean is revealed as the ice which once overlay it melts away. In September 2005 alone, an area of Arctic sea ice the size of Alaska vanished without trace. Even in the pitch blackness of the winter months, the sea ice cover has been ebbing-both 2005 and 2006 saw the ice extent fall far below average.

      Here is where the tipping point comes in. Whilst bright white, snow-covered ice reflects more than 80 per cent of the Sun's heat that falls on it, the darker open ocean can absorb up to 95 per cent of incoming solar radiation. Once sea ice begins to melt, in other words, the process quickly becomes self-reinforcing: more ocean surface is revealed, absorbing solar heat, raising temperatures and making it more difficult for the ice to re-form during the next winter. Climate models differ about exactly where the Arctic sea ice tipping point may lie, but virtually all of them agree that once we are past a certain threshold of warming the disappearance of the entire northern polar ice cap is pretty much unavoidable.

      These models suggest that we have not yet reached this critical tipping point-but it may not lie very far away. One model run projects a sudden collapse in sea ice cover after 2024, with four million square kilometres of ice melting away in the following ten years. In this simulation, reported by a US-based team led by Marika Holland of the National Center for Atmospheric Research in Boulder, Colorado, the whole ocean becomes virtually ice-free in summertime by 2040. Whilst other model runs examined by the same team don't cross the tipping point until 2030 or 2040, one simulates a collapse in sea ice production beginning as early as 2012.

      Even so, Holland's team emphasises that ‘reductions in future greenhouse gas emissions reduce the likelihood and severity of such events’-in other words, all is not yet lost. Another team, led by NASA's Jim Hansen, reaches a similar conclusion. Despite major changes already in the system, Hansen and co-authors write, ‘it may still be possible to save the Arctic from complete loss of ice’-but only if other atmospheric pollutants (such as soot, which darkens the ice surface and speeds melting) are reduced as well as carbon dioxide. Implement a dramatic programme of emissions reductions, and we ‘may just have a chance of avoiding disastrous climate change’, the team concludes. We may not have much time left, however: at the time of writing, 10 August 2007, a new historic sea ice minimum has just been reported for the Arctic. With a whole month of summer melting still left to go, the expectation is that the previous record low, recorded in 2005, will be ‘annihilated’. Particularly worrying is that dramatic ice extent reductions are being recorded for every sector of the Arctic basin, whereas in previous years only certain areas were affected. Perhaps this is what a tipping point looks like.

      But why is Arctic sea ice so important? As the following chapter will show, without it emblematic Arctic species like polar bears and seals are doomed to extinction. But the impacts will also hit closer to home, far away from the once-frozen north. As Ted Scambos, lead scientist at the US National Snow and Ice Data Center in Colorado, explains: ‘Without the ice cover over the Arctic Ocean we have to expect big changes in the Earth's weather.’

      These big changes are inevitable because of how the world's climate works. Most mid-latitudinal weather is generated by the contrast between polar cold and equatorial heat: the reason the UK gets year-round rainfall is because of its location on this unstable boundary between these competing air masses-the so-called ‘polar front’. The nor'easter storms which barrel up the eastern US coastline in winter are also generated by this temperature contrast. But with the Arctic warming up, this contrast will lessen and the zone where it takes place will migrate north as rising temperatures contract the world's weather belts towards the poles. In the UK places like Cornwall and Wales which are accustomed to bearing the brunt of stormy winter weather may find themselves in the doldrums for weeks and months at a time, with a much drier overall climate. Only Scotland is likely to hang on to the wetter weather indefinitely. And as chapter 3 will show, the result in the western US is also likely to be drought-but on a scale never before experienced in human history.

      Nor are these predicted changes just conjecture: they are already under way. Satellite measurements over the past 30 years have shown a marked 1° latitudinal contraction of the jet streams towards the poles in both hemispheres. Given that these high-altitude wind belts-narrow corridors of rapidly moving air at the top of the troposphere-mark the boundaries between the different air masses, their gradual movement shows that the location of the world's typical climate zones is already starting to shift in response to rising global temperatures.

      What we have so far witnessed is still only the beginning. As one group of scientists warned recently: ‘The Arctic system is moving toward a new state that falls outside the envelope of recent Earth history.’ As future chapters show, this new ice-free Arctic will see extreme levels of warmth unlike anything experienced by the northern polar regions for millions of years.

       Danger in the Alps

      When the Englishmen Craig Higgins and Victor Saunders left the Hornli hut at 4 a.m. on 15 July 2003, they had no idea that they would end the day being part of the biggest-ever rescue on Switzerland's iconic Matterhorn. The ascent began straightforwardly, with the two climbers scaling three rock towers, after which steep slabs led up to a small bivouac hut midway up the Hornli ridge. Higgins and Saunders had just reached the second hut, at 6 a.m., when an enormous rock avalanche pounded down the eastern face of the mountain. Cowering behind the building as stones bounced all around them, the two climbers would have been well advised at that point to turn tail and descend as quickly as possible. But mountains have strange effects on people's minds, and the two Brits pressed on.

      Then, three hours later, the mountain shook once again as a further gigantic rockfall crashed down, this time from the north face. Shortly after, a third rockfall struck-and this time the Hornli ridge itself was giving way. A Swiss mountain guide found himself inches from disaster as the ground began to crumble just in front of him. With no hope of crossing the dangerously unstable zone, the guide radioed for help. For the next four hours two Air Zermatt helicopters ferried stranded climbers off the ridge and back to the main hut. ‘As we climbed slowly down,’ recalled Saunders, ‘the smoking plume of rock dust and the returning helicopters told us of a major rescue taking place.’ Both British climbers, realising they too were trapped, joined the queue of people waiting to be plucked to safety.

      Ninety people were rescued that day, and amazingly no lives were lost or injuries reported-a tribute to the professionalism of the Swiss mountain guides and emergency services. The mountain remained closed for days afterwards as experts tried to assess the likelihood of further rockfalls taking place. In fact, falling rocks were not the only hazard in the area: on the same day as the Matterhorn drama was taking place, massive chunks of ice broke off from a glacier above the nearby resort of Grindelwald and plunged into a river, causing a two-metre-high wave to flood down the