Читать книгу Six Degrees: Our Future on a Hotter Planet - Mark Lynas - Страница 10

China's thirsty cities

Take a train from Hohhot to Lanzhou in northern China, and you pass through a strange area of heavily eroded badlands, where steep gullies and cliffs crowd in around the railway track as it weaves its way through a narrow river gorge. At many points caves have been hacked out of the cliffs-their history is murky, but perhaps they were used by vagrants, or people expelled from the cities, or even by Communist dissidents exhorting the peasantry to rise up against the Nationalists during the 1930s. A more prosaic explanation is that they were carved out by railway construction workers as they laboured to lay track through cold, windy and inhospitable territory.

These badlands are the edge of China's loess plateau, a gigantic area of compacted dust many hundreds of metres deep, deposited over thousands of years by dust storms and strong winds roaring down from the Gobi Desert of Mongolia. This dry plateau may not be much good for agriculture, grazing or anything else (bar digging caves) but it is a treasure trove for palaeoclimatologists, who use its finely preserved layers of dust and sand to reconstruct the fluctuations of ancient climates across the whole region of northern China.

It was with this purpose in mind that a team of Chinese scientists based in Lanzhou trekked out to four sites on the loess plateau in 1999, drilling more than 30 metres down into the compacted soil before carefully extracting their sections and carting them back to the lab. Near the base of each section was the target of the research: a layer of prehistoric soil-‘palaeosol’ in the jargon-dating from the Eemian interglacial, the previous warm period before the start of the last ice age. The weather records preserved in this inauspicious red-brown layer would prove to hold clues not just about the past, but also about the future.

Like Africa and the Indian subcontinent, northern China is subject to an annual monsoon cycle. In summer, moist air blows in from the ocean, bringing heavy rainfall to the south. In winter, however, the pattern reverses and strong winds sweep down from the north, bringing dust and freezing temperatures. The Lanzhou scientists, using complex techniques to measure particle sizes and magnetic data from the palaeosol section, were able to draw conclusions from their sample about changing monsoon strength 129,000 years ago, as the Eemian climate gradually warmed up. Because of how long it takes the oceans to absorb heat, it appeared, the dry winter monsoon responded much more rapidly than the summer one to the changing conditions. The result was a period of drought and continental-scale dust storms, before the summer monsoon pushed far enough inland to bring significant rainfall to the loess plateau.

So could such a mechanism plausibly repeat in the two-degrees-warmer world? Studies of Pacific sea floor sediments suggest that the height of the Eemian interglacial saw global temperatures about 1°C higher than today's, making this period a potentially useful analogue for a warmer climate in the future-particularly as regional temperatures in a large continent like Asia would in turn have been a degree or so higher than the global average. And if it did take China's monsoon climate longer to make the transition from cool/dry to warm/wet 129,000 years ago, as some scientists believe, this does suggest a possible cause for the droughts and rising temperatures that have struck northern China in recent years. So whilst southern China can expect more flooding as the climate warms, the oceanic time lag means that it may take much longer for the rain-bearing summer monsoon to reach the drought-stricken north. With China split between two extremes, agriculture will inevitably suffer, and water-stressed cities like Beijing and Tianjin will continue to experience shortages-particularly as economic growth spirals upwards and underground aquifers are pumped dry. The Chinese government has begun construction of a massive water transfer project, which aims to take billions of cubic metres of water from the Yangtze River in the south to the thirsty cities in the north. However, even this mega-project-the largest ever constructed on the planet-will (if it works, which many doubt) have difficulty keeping the taps running. With a chronic shortage of water, China will not just struggle to develop a more affluent lifestyle, it will struggle to feed itself too.

Acidic oceans

Greenhouse gases released over the last hundred years or so have not only changed the climate; they have also begun to alter conditions in the largest planetary habitat of all-the oceans. At least half the carbon dioxide released every time you or I jump on a plane or turn up the air-conditioning ends up in the oceans. This may seem like a good place for nature to dump it, but ocean chemistry is a complex and delicate thing. The oceans are naturally slightly alkali, allowing many animals and plants which inhabit the seas to build calcium carbonate shells.

However, carbon dioxide dissolves in water to form carbonic acid, the same weak acid that gives you a fizzy kick every time you swallow a mouthful of carbonated water. That's great for a glass of Perrier, but not so good if it's beginning to happen on a gigantic scale to the whole of the global ocean. And it is indeed beginning: humans have already managed to reduce the alkalinity of the seas by 0.1 pH units. As Professor Ken Caldeira of the Carnegie Institution's global ecology department says: ‘The current rate of carbon dioxide input is nearly 50 times higher than normal. In less than 100 years, the pH of the oceans could drop by as much as half a unit from its natural 8.2 to about 7.7.’ This may not sound like much, but this half point on the pH scale represents a fivefold increase in acidity. And because the oceans circulate only very slowly, even if atmospheric carbon dioxide levels are eventually stabilised-perhaps because humanity wakes up to their warming effect-these changes in ocean chemistry will persist for thousands of years.

This fast-moving area of scientific research was the subject of a major report by the Royal Society in June 2005, which identified some of the main concerns that are increasingly keeping marine biologists awake at night. First and foremost is the possibility that even with relatively low future emissions during this century (equating to two degrees or less of temperature warming), large areas of the Southern Oceans and part of the Pacific will become effectively toxic to organisms with calcium carbonate shells after about 2050. With higher emissions, indeed, most of the entire global ocean will become eventually too acidic to support calcareous marine life.

The most important life-forms to be affected are those that form the bedrock of the oceanic food chain: plankton. Although individually tiny (only a few thousandths of a millimetre across), photosynthesising plankton like coccolithophores are perhaps the most important plant resource on Earth. They comprise at least half the biosphere's entire primary production-that's equivalent to all the land plants put together-often forming blooms so extensive that they stain the ocean surface green and can easily be photographed from space. The places where phytoplankton thrive are the breadbaskets of the global oceans: all higher species from mackerel to humpbacked whales ultimately depend on them. Yet coccolithophores have a calcium carbonate structure, and this makes them especially vulnerable to ocean acidification. When scientists simulated the oceans of the future by pumping artificially high levels of dissolved CO₂ into sections of a Norwegian fjord, they watched in dismay as coccolithophore structures first corroded, and then began to disintegrate altogether.

Acidification will also directly affect other ocean creatures. Crabs and sea urchins need their shells to survive, whilst fish gills are extremely sensitive to ocean chemistry-just as our lungs are to the air. Mussels and oysters, vitally important both as economic resources and as part of coastal ecosystems worldwide, will lose their ability to build strong shells by the end of the century-and will dissolve altogether if atmospheric CO₂ levels ever reach as high as 1,800 ppm (parts per million: this ppm measure means, very simply, that for every million litres of air there are 1,800 litres of carbon dioxide). Tropical corals, already badly hit by bleaching, will more and more be corroded by this increasing oceanic acid. Walk out to sea on a reef in 2090, and it may crumble beneath your feet. Ships, rather than being torn apart when they strike rocky coral, may find themselves ploughing through weakened reefs like sponge. Indeed, it's difficult to overstate just how dangerous an experiment we are now conducting with the world's oceans. As one marine biologist says: ‘We're taking a huge risk. Chemical ocean conditions 100 years from now will probably have no equivalent in the geological past, and key organisms may have no mechanisms to adapt to the change.’ Phytoplankton are also crucial to the global carbon cycle. Collectively they are the largest producer of calcium carbonate on Earth, removing billions of tonnes of carbon from circulation as their limestone shells rain down onto the ocean floor. There's nothing new about this process: the chalk in the cliffs and downs of southern England originally formed as the limey sludge from countless billions of dead coccolithophores back in the Cretaceous era. But as the oceans turn more and more acidic, this crucial component of the planetary carbon cycle could slowly grind to a halt. With fewer plankton to fix and remove it, more carbon will remain in the oceans and atmosphere, worsening the problem still further.

Phytoplankton are also hit directly by rising temperatures, because warmer waters on the surface of the ocean shut off the supply of upwelling nutrients that these tiny plants need to grow. As with acidification, changes are already detectable today: in 2006 scientists reported a decline in plankton productivity of 190 mega-tonnes a year as a result of the current warming trend. Together these two factors, warming and acidification, represent a devastating double blow to ocean productivity. As Katherine Richardson, professor of biological oceanography at Aarhus University in Denmark, says: ‘These marine creatures do humanity a great service by absorbing half the carbon dioxide we create. If we wipe them out, that process will stop. We are altering the entire chemistry of the oceans without any idea of the consequences.’

Wiping out phytoplankton by acidifying the oceans is rather like spraying weedkiller over most of the world's land vegetation, from rainforests to prairies to Arctic tundra, and will have equally disastrous effects. Just as deserts will spread on land as global warming accelerates, so marine deserts will spread in the oceans as warming and acidification take their unstoppable toll.

The mercury rises in Europe

Under normal circumstances, the human body is good at dealing with excess heat. Capillaries under the skin flush with blood, allowing the extra warmth to radiate into the air. Sweat glands pump out moisture, disposing of heat through evaporation. Heat can even be lost through panting, and the heart works overtime. During exercise, the normal body temperature of 37 degrees Celsius can rise to 38 or 39°C with no ill effects.

But 2003 was not a normal summer, and the heatwave experienced in Europe during the three months of June, July and August did not produce normal circumstances. In Switzerland the mercury climbed above 30°C as early as 4 June, rising to a maximum of 41.1°C in the south-east of the country on 2 August-the sort of searing temperature more often associated with the Arabian Desert than temperate central Europe. Across the continent, records tumbled: in Britain temperatures reached 100° Fahrenheit for the first time. Beaches were packed as holidaymakers enjoyed the summer heat, but in big cities like Paris, a hidden disaster was unfolding.

The first symptoms of heat stress may be minor. An affected individual will feel slightly nauseous and dizzy, and perhaps get irritable with those around. This needn't yet be an emergency: an hour or so lying down in a cooler area, sipping water, will cure early heat exhaustion with no longer-lasting symptoms. But in Paris, August 2003 there were no cooler areas, especially for elderly people cooped up in their airless apartments. It wasn't so much the high temperatures of the day, but the fact that things didn't cool down enough at night to give the body time to recover. The effects were cumulative, and the most dangerous-and often fatal-form of heat stress then became much more likely: hyperthermia or heatstroke.

Once human body temperature reaches 41°C (104°F) its ther-moregulatory system begins to break down. Sweating ceases, and breathing becomes shallow and rapid. The pulse quickens, and the victim may rapidly lapse into a coma. Unless drastic measures are taken to reduce the body's core temperature, the brain is starved of oxygen and vital organs begin to fail. Death will be only minutes away unless the emergency services can quickly get the victim into intensive care.

These emergency services failed to save over 10,000 Parisian heatstroke victims in the summer of 2003. Mortuaries quickly ran out of space as hundreds of dead bodies, mainly of elderly and marginalised people, were brought in each night. The crisis caused a political furore as people accused politicians and municipal administrators of being more concerned with their long August holidays than with saving lives in the capital. Estimates vary, but across Europe as a whole, between 22,000 and 35,000 people are thought to have died.

The heatwave and drought also devastated the agricultural sector: crop losses totalled around $12 billion, whilst forest fires in Portugal caused another $1.5 billion of damage. Major rivers such as the Po in Italy, the Rhine in Germany and the Loire in France ran at record low levels, grounding barge traffic and causing water shortages for irrigation and hydroelectric production. Toxic algal blooms proliferated in the denuded rivers and lakes. Melt rates on mountain glaciers in the Alps were double the previous record set in 1998, and some glaciers lost 10 per cent of their entire mass during the heat of that one summer. Meanwhile-as described in chapter 1-melting permafrost caused rockfalls in mountain areas like the Matterhorn.

It wasn't long before questions were asked about global warming's possible contribution to the disaster. Meteorologists who investigated past hot spells found that the 2003 heatwave was off the statistical scale-a one-in-several-thousand-year event. According to an analysis by UK-based climatologists, twentieth-century global warming has already doubled the risk of such a heatwave occurring. Right across Europe, according to research published in 2007, the frequency of extremely hot days has tripled over the last century, and the length of heatwaves on the Continent has doubled. The conclusion is stark: the 2003 summer hot spell was not a natural disaster.

The intensity of the heatwave also tells us something about the future. Averaged across the whole continent, temperatures were 2.3°C above the norm. So does that mean that in the two-degree world, summers like 2003 will be annual events? It seems so: in the UK-based study mentioned above, scientists used the Met Office's Hadley Centre computer model to project future climate change with increasing greenhouse gas emissions, and concluded that by the 2040s—when temperatures globally in their model are still below two degrees-more than half the summers will actually be warmer than 2003.

That means that extreme summers in 2040 will be much hotter than 2003-and the death toll will rise in consequence-perhaps reaching the hundreds of thousands. Elderly people may have to be evacuated for months at a time to air-conditioned shelters, and outside movement during the hottest part of the day will become increasingly dangerous. Temperatures may soar to highs commonly experienced today only in North Africa, as rivers and lakes dry up and vegetation withers across the entire continent. Crops which require summer rainfall will bake in the fields, and forests which are more accustomed to cooler climes will die off and burn. As a result, catastrophic wildfires may penetrate north into new areas, torching broadleaved forests from Germany to Estonia.

Here again the summer of 2003 gives us a glimpse of things to come. Europe-wide monitoring systems showed a 30 per cent drop in plant growth across the continent, as photosynthesis began to shut down in response to the twin stresses of high temperatures and crippling drought. From the deciduous beech forests of northern Europe to the evergreen pines and oaks of the Mediterranean rim, plant growth across the whole landmass slowed and then stopped. Instead of absorbing carbon dioxide from the air, the stressed plants instead began to emit it; around half a billion tonnes of carbon was added to the atmosphere from European plants, equivalent to a twelfth of total global emissions from fossil fuels. This is a positive feedback of critical importance, because it suggests that as temperatures rise-particularly during extreme heatwave events-carbon emissions from forests and soils will also rise, giving a further boost to global warming. And if these land-based emissions are sustained over long time periods and large areas of the Earth's surface, global warming could begin to spiral out of control, as the next chapter shows.

We may have come dangerously close to that point during the 1998-2002 mid-latitudinal drought in the northern hemisphere, which left plants withering through regions as far afield as the western US, southern Europe and eastern Asia. One study showed that carbon emissions which would normally have been taken up by plants instead accumulated in the atmosphere, explaining the abnormally large jumps in the atmospheric CO₂ concentration in following years. (Jumps which caused jitters among many climate change watchers about whether runaway positive feedbacks might have already begun.) Over a billion tonnes of extra carbon poured out of plants and soils in response to the drought and heat.

At the time of writing, the heatwave of 2003 has already begun to fade in people's memories, and the ‘normal’ summers of the following two years will have begun to soak up some of the extra carbon that entered the atmosphere during that deadly hot spell.

But we forget at our peril. The summer of 2003 was a ‘natural experiment’ whose conclusions should be taken very seriously. This wasn't just some output from a computer model, whose assumptions and projections can be legitimately challenged. It actually happened. Moreover, the near-repeat of the 2003 heatwave in the summer of 2006 suggests that if anything the models are underestimating the likely frequency and severity of future heatwaves.

We have been warned.

Mediterranean sunburn

Perhaps the most striking images from 2003's hot summer came from Portugal, where gigantic forest fires swept through the tinder-dry landscape, destroying orchards, torching houses and killing eighteen people. In total an area almost the size of Luxembourg was devastated. The conflagrations were so huge that they cast palls of smoke right over the North Atlantic, with both fires and smoke easily visible from space. The fires must have been particularly shocking for tourists, many of whom flock to southern Portugal from northern Europe-more in search of the sun than several days of smoke inhalation.

However, one study shows that such wildfires are going to be an increasingly common sight for holidaymakers to southern Europe and the Mediterranean. Climate change simulations show the region getting drier and hotter as the subtropical arid belt moves northward from the Sahara. In the two-degree world, two to six weeks of additional fire risk can be expected in all countries around the Mediterranean rim, with the worst-hit regions being inland from the coast where the temperatures are highest. In North Africa and the Middle East virtually the whole year will be classified as ‘fire risk’.

These fires will be driven on by scorchingly hot temperatures.

The number of days when the mercury climbs over 30°C is expected to increase by five to six weeks in inland Spain, southern France, Turkey, northern Africa and the Balkans. The number of ‘tropical nights’, when temperatures don't cool off past 20°C, will increase by a month, and the entire region can expect an additional 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 regionally, for example, if greenhouse gas emissions continue to rise unabated) most of the ice sheet will disappear over the next 1,000 years, still giving humanity plenty of time to prepare for the full 7 metres of inundation-even though lower-lying areas would go under much sooner.

In addition, some of the melting will be offset by increased snowfall, leading to thicker ice in the centre. This is another result of rising temperatures, as a warmer atmosphere can hold more water vapour. Much of Antarctica and the interior of Greenland is classed as ‘polar desert’ because it is simply too cold for snow to fall in significant amounts. Already evidence suggests that areas of the Greenland ice sheet above the 1,500-metre contour line are accumulating snow and new ice (at 6 cm a year according to one study). It has even been suggested that a thicker Greenland ice sheet could offset rising sea levels.

But real-world evidence runs counter to these optimistic scenarios, suggesting that James Hansen may be right after all. The models on which the predictions of Greenland ice melt are based operate by estimating the difference between water loss from future melting and ice accumulation from future snowfall. There is much more to ice sheet dynamics than just melting and snowfall, however. Vast quantities of ice are constantly flowing away from Greenland's centre in gigantic glaciers, which surge through fjords and discharge icebergs into the sea. These glaciers can have a rapid effect on the stability of the ice sheet, yet they aren't properly accounted for in the models. ‘Current models treat the ice sheet like it's just an ice cube sitting up there melting, and we're finding out it's not that simple,’ says Ian Howat, an expert on Greenland's glaciers.

In particular, as melting proceeds on the surface, whole rivers plunge down through icy sink-holes called moulins onto the bedrock beneath the ice sheet. This meltwater then acts as a lubricant underneath the ice, speeding up the glaciers as they proceed towards the sea. As one glaciologist told Nature: Along the coasts, all the glaciers are thinning like mad, and they're also flowing faster than they ought to. Changes initiated in coastal regions will propagate inland very quickly.' When Byron Parizek and Richard Alley, two glaciologists at Penn State University in the US, had a first stab at including meltwater lubrication in an ice sheet model, they found that this did indeed lead to a thinner ice cap over Greenland, and a greater contribution to sea level rise.

Greenland's glaciers are also changing much quicker than anyone expected. The largest outflow glacier on the whole landmass, Jacobshavn Isbrae in the south-west, is so huge that it alone has a measurable impact on global sea levels, accounting for 4 per cent of the twentieth-century rise. Not only has the gigantic river of ice thinned by a phenomenal 15 metres annually since 1997 (that's about four office block storeys every year), but its flow rate more than doubled between 1997 and 2003, suggesting that an increased quantity of Greenland's ice is being sucked out into the sea. As if to emphasise the abnormal shift, Jacobshavn Isbrae's floating ice shelf has now suffered almost complete disintegration, spawning an armada of icebergs along the coast.

On the eastern side of the ice cap, a second glacier has also suffered dramatic changes. A US-based research team led by Ian Howat studied satellite photos of the Helheim Glacier's behaviour between 2000 and 2005, and was stunned to discover that not only had the ice flow speeded up, but the glacier had thinned by over 40 metres and retreated several kilometres up its fjord. About half of the thinning is due to increased surface melt: in recent years ever-wider areas of Greenland have been rising above freezing, and thousands of blue meltwater pools now pepper the ice surface in the summer. But the remainder is due to the speeding-up of glacial flow rates, to the distinctly unglacial pace of 11 kilometres per year.

This faster flow draws more ice down the valley, thinning the glacier just as a rubber band gets thinner when you stretch it. In the process, now being repeated in glaciers right around the ice cap, billions of tonnes more ice are dumped in the North Atlantic, raising sea levels still further. According to Howat, the thinning has reached a ‘critical point’ which has begun ‘drastically changing the glacier's dynamics’. His conclusion is devastating: ‘If other glaciers in Greenland are responding like Helheim, it could easily cut in half the time it will take to destroy the Greenland ice sheet.’

Other scientists concur. Speaking at the Fall Meeting of the American Geophysical Union in December 2005, the University of Maine's Dr Gordon Hamilton also noted ‘very dramatic changes’ on eastern Greenland's Kangerdlugssuaq Glacier. In just one year, between April 2004 and April 2005, this enormous glacier both doubled in speed and simultaneously retreated by 4 kilometres. If other large glaciers begin to go the same way as Helheim and Kangerdlugssuaq-which have now doubled the rate at which they together dump ice in the ocean from 50 to 100 cubic kilometres a year-it could ‘pull the plug’ on Greenland, Dr Hamilton warned.

The newest evidence suggests that all is not yet lost however. In March 2007 Ian Howat and colleagues reported in Science magazine that results from their latest survey work might be more reassuring. Although both the Helheim and Kangerdlugssuaq glaciers had indeed doubled their rate of mass loss in 2004, as previously reported, two years later-by 2006-they had returned to something more like normality. However, the glaciologists Martin Truffer and Mark Fahnestock, writing in Science magazine in March 2007, are at pains to point out that this most recent change ‘does not mean that [the glaciers] have stabilised’. Instead, ‘the question remains whether changes in the past five years have left the system as a whole more vulnerable’. Clearly these giant rivers of ice are complex beasts that scientists are still struggling to understand.

But whatever the behaviour of individual outlet glaciers, widerscale satellite studies of Greenland's entire ice cap do suggest that major changes are afoot. Scientists working on the GRACE satellite programme (Gravity Recovery and Climate Experiment) reported in November 2006 that big losses are now under way. Whilst the ice sheet was probably in balance for much of the 1990s, between 2003 and 2005 it shed about 100 billion tonnes a year of ice, enough to raise global sea levels by 0.3 mm a year.

Jim Hansen, for one, is continuing his battle to get the world to wake up to the threat of melting ice caps, and attempts by the Bush administration and Hansen's bosses in NASA to silence him have been given characteristically short shrift. (When NASA public relations staff ordered him to stop giving lectures or talking to journalists without clearing material with them first, Hansen went straight to the press with the story, sparking embarrassing ‘NASA censorship’ headlines across the US media.) His publications-even in weighty scientific journals-are now increasingly peppered with strong words such as ‘dangerous’ and ‘cataclysm’, ignoring the usual convention that scientists must muzzle themselves with emotionless jargon. One Hansen paper published (with five leading co-authors) in May 2007 warns bluntly in the abstract that ‘recent greenhouse gas emissions place the Earth perilously close to dramatic climate change that could run out of our control, with great dangers for humans and other creatures’-a clear statement of fact, but one which must have raised disapproving eyebrows throughout the halls of academe.

However, Hansen's contention that the world's ice sheets could collapse much more rapidly than the IPCC suggests does have a solid base in physics. In order to explain the rapid real-world fragmentation of ice sheets at the end of the last ice age, Hansen outlines a process called the ‘albedo-flip’-something which, if repeated today, could destroy the remaining ice sheets much faster than conventional projections suggest. This albedo-flip is worryingly simple: as snow and ice melt they become wet, making the surface darker and therefore more able to absorb sunlight. This raises temperatures further, sparking wider melting, in a classic positive feedback. The albedo-flip is why, Hansen suggests, ice sheet disintegration can be ‘explosively rapid’ rather than a more stately process taking millennia to play out. And given that large areas of Greenland and west Antarctica are already bathed in summer meltwater, Hansen suggests that this ‘trigger mechanism’ of darker, wet snow is already engaged today.

So how fast might sea levels rise? The IPCC's 2007 report suggests only 18 to 59 cm-reassuring figures to those who live close to the coast. However, it also introduces a caveat, admitting that uncertainties about ice sheet response times could make this figure higher. But it doesn't say how much higher, and no one else in the scientific community has ventured to offer an estimate-except, once again, James Hansen whose warning about global warming as long ago as 1988 suggests remarkable foresight. In a paper entitled ‘Scientific reticence and sea level rise’ and published in 2007 in the online free journal Environmental Research Letters, Hansen takes his colleagues to task for staying ‘within a comfort zone’ and refusing to say anything which may prove to ‘be slightly wrong’. Instead, he argues, ‘There is enough information now, in my opinion, to make it a near certainty that IPCC business-as-usual climate forcing scenarios would lead to a disastrous multi-metre sea level rise on the century time scale.’ If the ice sheet melt rate doubles each decade, a serious possibility, the resulting sea level rise will total 5 metres by 2100, he points out.

So perhaps James Hansen's warnings should be taken more seriously, particularly his concern that melt rates and sea level rise could speed up dramatically over the century to come. An early warning is already out there: sea levels are currently rising at 3.3 mm a year-much faster than the 2.2 mm projected by the IPCC's 2001 report. If, as Hansen suggests is likely, melt rates as rapid as those at the end of the last ice age begin to happen again this century, the whole Greenland ice sheet could disappear within 140 years. The geography of the world's coastlines would then look radically different. Miami would disappear entirely, as would most of Manhattan. Central London would be flooded. Bangkok, Bombay and Shanghai would also lose most of their area. In all, half of humanity would have to move to higher ground, leaving landscapes, buildings and monuments that have been central to civilisation for over a thousand years to be gradually consumed by the sea.

Last stand of the polar bear

Not everyone sees the transformation of the Arctic as a bad thing. Even as water sluices off the melting Greenland ice sheet, climate change at the top of the world could be making some people very rich. Pat Broe hopes to be one of them. An American entrepreneur, in 1997 Broe bought the run-down north Canadian port of Churchill for the princely sum of $7. For the fewer than a thousand residents who live in this drab-looking town, life has been tough-even in a place which has rebranded itself as the ‘Polar Bear Capital of the World’.

But according to Broe, boom times lie just around the corner. As the polar ice melts, humble little Churchill could become an important hub in lucrative shipping routes opening up between Asia, Europe and North America across waters that used to be permanently frozen. Which is just as well, because by the time these shipping routes open, the Polar Bear Capital of the World will be looking for another raison d'être-for the simple reason that as the sea ice disappears, so will the polar bears.

One rather unamusing irony of global warming is that the retreat of the northern polar ice cap is sparking a new petroleum gold rush, bringing further fossil fuels onto world markets which-when burned-will inevitably make the climate change problem worse. According to some estimates, a quarter of the world's undiscovered oil and gas reserves lie under the Arctic Ocean, in areas which have historically been seen as undrillable because of thick drifting ice floes. Massive investments are already being made to tap into this economically valuable resource: the Norwegian government is spending billions of dollars building a liquefied natural gas terminal at the far northern port of Hammerfest, whilst a massive gas find in Russian Arctic waters-estimated to contain double Canada's entire reserves-has sparked an unseemly scramble amongst oil majors to partner with Russia's giant Gazprom corporation to exploit it. According to one energy analyst quoted in the New York Times, this new Arctic rush is ‘the Great Game in a cold climate’.

Internationally, Arctic Ocean nations Canada, Denmark, the United States, Norway and Russia are battling to establish undersea mineral rights to ‘their’ sections of the seabed. In August 2007 Russian explorers mounted a particularly audacious land grab by piloting a submersible under the ice and planting a rustproof metal flag on the seabed 4,000 metres below the North Pole-thereby claiming the entire area, and its fossil fuel riches, for the motherland. ‘We are happy that we placed a Russian flag on the ocean bed, and I don't give a damn what some foreign individuals think about that,’ declared expedition leader Artur Chilingarov haughtily as he returned to a hero's welcome-complete with champagne and ranting pro-Kremlin youth groups-in Moscow. Stretching unintended irony to breaking point, the triumphant Chilingarov was then handed a large furry toy polar bear-the emblem of the pro-Putin party United Russia (of which Chilingarov is a parliamentary deputy)-as a military brass band played. The US State Department was studiously unmoved, however: ‘A metal flag, a rubber flag or a bedsheet on the ocean floor… doesn't have any legal standing,’ a spokesman told Reuters. ‘You can't go around the world and just plant flags and say “We're claiming this territory,”’ complained an annoyed Canadian foreign minister, accusing Russia of behaving like a fifteenth-century colonial explorer.