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

the deep ocean current and triggering climatic destabilisation across the world.

      Obviously today there are no gigantic ice lakes waiting to flood into the North Atlantic, but global warming could still interrupt the formation of deep water by melting sea ice and causing greater freshwater run-off from Siberian rivers. Despite the rapidly melting ice cap, however, for many years there was no evidence that changes in the Atlantic MOC were actually happening, and many oceanographers had begun to pooh-pooh the theory. That was until the RSS Discovery, a scientific research vessel owned by the British government, began a routine cruise across the Atlantic in 2004. The ship's scientific team set themselves the task of sampling seawater at various depths on a line drawn between the Canary Islands in the east and Florida in the west, aiming to repeat similar measurements taken in 1957, 1981, 1992 and 1998. They had not expected to discover anything terribly exciting; in fact the team leader Professor Harry Bryden confided to one journalist: ‘In 1998 we saw only very small changes. I was about to give up on the problem.’

      But 2004 was different. Bryden and his colleagues found that less warm water was flowing north at the surface and less cold water was flowing south at depth. Overall, the Atlantic circulation had dropped by 30 per cent, equivalent to the loss of 6 million tonnes of water flow per second. No wonder Professor Bryden admitted that he was ‘surprised’. Suddenly the slowing-down of the great Atlantic current system was no longer just a hypothesis postulated for the distant future. It was already happening.

      The media reaction was instantaneous. ‘Current that warms Europe weakening’, warned CNN. NPR's All Things Considered show led with Atlantic Ocean's heat engine chills down'. In Europe, the response was one of understandable concern. Alarm over dramatic weakening of Gulf Stream', reported the UK's Guardian newspaper on 1 December 2005. ‘Global warming will bring cooler climate for the UK’ was the Telegraph's take on the same story. A couple of paragraphs down, the paper reported one expert as confirming that ‘an average temperature drop of a degree or two within decades would herald more extreme winters’.

      Older readers would have shuddered at the thought of a return to winters as bitter as that of 1962-3, when the UK was blanketed in snow for more than three months, and temperatures hit a low of–16°C in southern England. In places the sea froze, and ice floes appeared in the river Thames at London's Tower Bridge. That season was about 2.7°C colder than average-almost exactly the temperature drop predicted for London in one modelling study investigating the possible result of a 50 per cent drop in the warm Atlantic current. Was Europe's new ice age just around the corner?

      Apparently not. Almost exactly a year later, and with much less fanfare, Science magazine reported that ‘a closer look at the Atlantic Ocean's currents has confirmed what many oceanographers suspected all along: there's no sign that the ocean's heat-laden “conveyor” is slowing’. Instead of just the snapshot data generated by a few irregular ship cruises, nineteen permanent instrument-laden sensors had now been stretched across the Atlantic between West Africa and the Bahamas-and they were able to deliver a much more consistent picture. A year of continuous monitoring, Harry Bryden now reported to a conference in Birmingham, showed that his original 30 per cent decline was just a part of random natural variability after all, the sort of thing that happens constantly from one year to the next.

      This result was a triumph for the modellers, most of whom had for years been pouring cold water on the European ice age theory. They agreed that huge volumes of freshwater would need to surge into the North Atlantic in order to shut off the Gulf Stream-far more than currently being generated by melt from Greenland or higher precipitation in Siberia. Rather than plunging overnight, the ocean circulation might decline by a stately 25 to 30 per cent or so, but only after at least 100 years of sustained greenhouse gas emissions. Even then, it wouldn't cool Europe-it would simply moderate the otherwise rapid rise in temperatures.

      As the IPCC concluded in 2007: ‘it is … very unlikely that the MOC [Atlantic Meridional Overturning Circulation] will undergo a large abrupt transition during the course of the 21st century’. Although all of them showed some weakening by 2100, none of the models assessed by the IPCC supported the collapse scenario. And even with this MOC slowdown, the IPCC reported that ‘there is still warming of surface temperatures around the North Atlantic Ocean and Europe due to the much larger effects of the increase in greenhouse gases’. The IPCC's judgement was final: there would be no new ice age for Europe.

       Africa's shining mountain

      The amateur adventurer Dr Vince Keipper had waited years for this day. Nearing the summit of Kilimanjaro, the highest point on the African continent, Keipper and his group were looking forward to panoramic views of the surrounding Kenyan and Tanzanian plains. They had climbed through the steep and treacherous Western Breach and past the towering ice cliffs of the Furtwängler Glacier. The weather was perfect, with only a few clouds far beneath. Then, not far from the top of the 5,895-metre peak, a loud rumbling sound from behind them brought the group to a sudden halt. ‘We turned around to see the ice mass collapse with a roar,’ remembered Keipper. ‘A section of the glacier crumbled in the middle, and chunks of ice as big as rooms spilled out on the crater floor.’ Keipper and his group knew they had had a lucky escape: they might have been buried had the collapse happened only a few hours earlier. They also knew that the event they had just witnessed had a powerful symbolic resonance: right in front of their eyes, the highest peak in Africa was melting.

      Kilimanjaro has become something of a poster child for the international climate change campaign. The Swahili words kilima and njaro translate as ‘shining mountain’, testament to the power of this massive volcano to inspire awe in onlookers through the ages. A recent aerial photo of the crater, with little more than a few ice fragments encrusting its dark sides, was the centrepiece for a touring global warming photography exhibition sponsored by the British Council in 2005. During the 2001 UN climate change conference in Marrakech, Morocco, Greenpeace sent a team to Kilimanjaro to hold a press conference by video link from beside one of the mountain's disappearing glaciers. Kilimanjaro's international celebrity status has also attracted the attention of climate change deniers, who suggest that deforestation on the mountain's lower slopes is more to blame for glacial retreat than global warming.

      None of the contrarian rhetoric cuts any ice, so to speak, with Lonnie Thompson, a glaciologist at Ohio State University and a man who is deservedly one of America's most celebrated natural scientists. Thompson pioneered the drilling of ice cores in inaccessible mountain regions, bringing back ice tens of thousands of years old from glaciated peaks as remote and far apart as Peru's Nevado Huascarán and Tibet's Dasuopu, often pushing himself to the edge of human endurance in the process. In 1993 Thompson and his drilling team camped for 53 days at 6,000 metres between the two peaks of Huascarán, perhaps setting a world record for high-altitude living. (I stayed there for one night in 2002-one of the most freezing, wind-blasted and wretched nights of my life.) At one point a gale blew Thompson's tent, with him inside, towards a precipice-until he jammed his ice axe through the floor. ‘I don't understand,’ he once remarked, ‘why anyone would want to climb a mountain for fun.’

      As Thompson was one of the first to recognise, this mountain ice contains a unique record of climate variations down the ages-preserved in layers of dust, isotopes of oxygen and tiny bubbles of gas trapped within the frozen layers of water. Once carried down in freezer boxes and analysed in the laboratory, these icy signatures trace everything from droughts to volcanic eruptions from decades and centuries past. They also tell a story about past temperature changes: the two isotopes of oxygen, ¹⁶O and ¹⁸O (which have different atomic weights due to the presence of two more neutrons in the latter's nucleus), vary in abundance with water temperature, so their proportions in ice cores are a good ‘proxy’ record of ancient climates.

      Thompson and his team also drilled on three of Kilimanjaro's remaining glaciated areas, and in October 2002 concluded that 80 per cent of the mountain's ice had already melted during the past century. The news made international headlines, along with Thompson's prediction that the rest of the ice would be gone by between 2015 and 2020. As he readily admitted, this prediction was not based on complex computer modelling or any other advanced techniques. ‘In 1912 there were 12.1 square kilometres of ice on the mountain,’ he told journalists