Six Degrees: Our Future on a Hotter Planet

O and

O (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 from CNN. ‘When we photographed the mountain in February of 2000, we were down to 2.2 square kilometres. If you look at the area of decrease, it's linear. And you just project that into the future, sometime around 2015 the ice will disappear off Kilimanjaro.’

If there was an urgency in Thompson's voice, this was because he knew that recent melting had already begun to destroy the unique record of past climate preserved in Kilimanjaro's glaciers.

In their analysis of dust layers in the ice, the scientific team found evidence of a marked 300-year drought four thousand years ago; a drying so severe that it has been linked to the collapse of several Old World civilisations across North Africa and the Middle East. The ice also indicated much wetter conditions even longer ago, when huge lakes washed over what is now Africa's dry Sahel. Close to the surface Thompson's team discovered ice containing a layer of the radionuclide chlorine-36, fallout from the American ‘Ivy’ thermonuclear bomb test on Eniwetok Atoll in 1952. With this precise time control, the scientists could tell that ice which would have preserved a record of climate fluctuations since the 1960s had already melted away.

Moreover, the oldest ice at the base of the cores proved to be over 11,000 years old, showing that at no time since the last glacial epoch has the peak of Kilimanjaro been free of ice. This discovery made Thompson's ice cores even more valuable, for the simple reason that within as little as ten years the sawn-up circular cores in Ohio State University's walk-in freezer will be the only Kilimanjaro ice left anywhere in the world. With this in mind, Thompson and his team have already decided that some of the ice will be kept intact for future generations of scientists to dissect with new technologies, possibly unlocking climatic secrets still undreamt of today.

The efforts of climate change deniers to suggest that there is something special about the disappearance of Kilimanjaro's glaciers are undermined by similar changes taking place in mountain ranges right across the world, not least in the Rwenzori Mountains of Uganda, nearly a thousand kilometres to the north-west. In this remote region, where Uganda borders the Democratic Republic of the Congo, the fabled ‘Mountains of the Moon’ generate such heavy rainfall (about 5 metres per year) that the cloud-shrouded peaks are only visible on a few days out of every year, and form the main headwaters of the river Nile. At the top of the highest peak, the 5,109-metre Mount Stanley (named after the explorer, who passed by in 1887), ice and snow deny the summit to all but the most determined mountaineers. Yet as at Kilimanjaro, glacial retreat in the Rwenzoris has been profound: the three highest peaks have lost half their glacial area since 1987, and all the glaciers are expected to be gone within the next two decades.

Elsewhere in the world, disappearing mountain glaciers pose a major threat to downstream water supplies. But Kilimanjaro's ice cap is so small that its final disappearance will make little difference to the two major rivers-the Pangani and the Galana-which rise on its flanks. Instead, the crucial water link for Kilimanjaro is not the glaciers, but the forests. The montane forest belt at between 1,600 and 3,100 metres provides 96 per cent of the water coming from the mountain-this lush tangle of trees, ferns and shrubs not only captures Kilimanjaro's torrential rainfall like a giant sponge, but also traps moisture from the clouds which drape themselves almost permanently around the mountain's middle slopes. Much of this water drains underground through porous volcanic ash and lavas, and emerges in waterholes-vital for local people as well as for wild animals-far away on the savannah plains.

So is Kilimanjaro's water-generating capacity safe from global warming? Not quite: rising temperatures and diminishing rainfall increase the risk of fires, which have already begun to consume the upper reaches of montane forest. By the time the glaciers have disappeared, so will the higher forests, depriving downstream rivers of 15 million cubic metres of run-off every year, according to one estimate. In contrast, the loss of glacial water input will likely add up to less than 1 million cubic metres annually: significant, but not catastrophic. The diminishing water supply will affect everything from fish stocks to hydroelectric production downriver in poverty-stricken Tanzania. Much of the mountain's world-famous biodiversity (Kilimanjaro hosts twenty-four different species of antelope alone) will also be threatened by the weather changes.

As the snows disappear, so will much of the wildlife and the verdant forests that tourists currently trek through on their arduous journey to the roof of the African continent.

Ghost rivers of the Sahara

Far to the north of Kilimanjaro, in the Sahel, another drought-stricken area could by this time be experiencing some blessed relief. The Sahelian region of North Africa has long been synonymous with climatic disaster: during the 1970s and 80s famines struck the area with such severity that they sparked massive humanitarian relief efforts like Band Aid and Live Aid. Reporting from Ethiopia's refugee camps in 1984, the BBC's Michael Buerk spoke of a ‘biblical famine’ as the camera swept slowly over the dead and dying. Over 300,000 people perished during earlier famines in the 1970s.

The Sahel is an immense area, stretching in a wide belt east to west across northern Africa from Senegal on the Atlantic coast to Somalia on the Indian Ocean. For the most part savannah and thorn scrub, it is a climatic transition zone between the hyper-arid Sahara to the north and the lush tropical forests which grow nearer to the equator in the south. Intermittent rains mean that nomadic cattle herding has long been a dominant way of life, with people wandering far and wide through the seasons in search of grazing for their livestock. It is often assumed that global warming will further desiccate the Sahel, allowing the Saharan dunes to march south into Nigeria and Ghana, and displacing millions in the process. Although the forecasts are tentative and uncertain, both palaeoclimatic studies and computer models suggest that the reverse might be true. As other parts of Africa shrivel in the heat, could the Sahel end up as a refuge?

For clues to how the area's climate might alter, we need to venture north into the great Sahara. Here, the world's largest desert has also seen the highest temperature ever recorded on Earth: a truly blistering 58°C. The Sahara covers an area so huge that the entire contiguous United States would comfortably fit inside. This desert doesn't just have sand dunes, it has sand mountains, some reaching to nearly 400 metres in height. It is so completely uninhabitable that only a sprinkling of people get by in a few dwindling oases and at the desert's edge.

But scattered over this enormous area are clear signs that a very different Sahara existed many thousands of years ago. Neolithic paintings and rock carvings have been discovered in places where settled human existence is utterly impossible today. This ancient art depicts elephants, rhinoceroses, giraffes, gazelles and even buffalo-all animals which currently roam only hundreds of kilometres to the south. In Egypt's hyper-arid Western Desert, where less than 5 mm of rain falls on average each year, arrowheads and flint knives for hunting and butchering big game have been unearthed by archaeologists. At one site in south-western Libya, archaeologists even discovered tiny flint fish-hooks-again in an area where no trace of surface water persists now.

Other indications of a wetter past have also been discovered. Although anyone crossing Egypt's dry Safsaf Oasis by camel would today see little more than rock and dunes, radar pictures taken from the space shuttle Endeavour in 1994 clearly show whole river valleys buried beneath the sands. These ghostly watercourses even include major tributaries to the Nile flowing out through modern-day Sudan, all long-dry and forgotten beneath the dust. In southern Algeria, huge shallow lakes once gathered, supporting plentiful populations of fish, birds and even Nile crocodiles. The carbon dating of freshwater snails and desiccated vegetation preserved in these old lake beds shows that between five and ten thousand years ago the desert edge retreated 500 kilometres further north, and at different times almost disappeared altogether.

On the borders of what is today Chad, Nigeria and Cameroon, an immense lake, over 350,000 square kilometres in area, extended across the southern Sahara. Nicknamed Lake Mega-Chad, after its modern-day remnant Lake Chad, this gigantic inland sea was the largest freshwater body that Africa had seen for the last two and a half million years. It would have been only slightly smaller than today's largest lake, the Caspian Sea. Strange ridges of sand, which today lie marooned far away in the desert, reveal the shores of the old lake, as do the shells of long-dead molluscs which once thrived in its warm, shallow waters. The flat landscape between the marching dunes testifies to the erosive power of its long-vanished waves.

Common sense suggests that a major 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 state's normally cold North Slope, with 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 mountain. Fast-acting police managed to clear the area of holidaymakers just before the mass of rocks and mud washed by.

When he heard about the two near-disasters, the glaciologist Wilfried Haeberli had no doubts about the cause. ‘The Matterhorn relies on permafrost to stay together,’ the Zurich University scientist told reporters. But Switzerland had just been suffering its strongest-ever heatwave. With the fierce summer heat having melted all the winter snow much earlier than usual, the permafrost and glaciers themselves were beginning to melt down. Once that process begins, Haeberli warned, ‘water starts to flow, and large chunks of rock begin to break away from the mountain’.

Most ground in the Alps above about 3,000 metres remains permanently frozen throughout the year, and is anchored, as Haeberli says, by permafrost. But in the summer of 2003 the melt zone reached as high as 4,600 metres-higher than the summit of the Matterhorn, and nearly as high as the top of Mont Blanc, western Europe's highest mountain. And whilst the Matterhorn climbers were lucky to get down safely on 15 July, at least fifty other climbers were less fortunate during that boiling summer-most were killed by falling rocks.

Haeberli, a world expert on permafrost, has since co-written a scientific paper on the impacts of the 2003 hot summer in the Alps. He and colleagues calculated that the thaw experienced during that heatwave outranked anything the mountains had suffered in recent history, and that most rock fall as a result took place during the hottest months of June, July and August.