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

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.

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