With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (10 page)

Nobody is yet saying that the East Antarctic ice sheet is vulnerable in the way that the western sheet appears to be. It remains very big and, by and large, extremely stable. But, as Rignot puts it, "it is not immune." And every new discovery seems to raise the stakes for the fate of the Antarctic ice. As recently as 2001, the IPCC reported a scientific consensus that it was "very unlikely" that Antarctica would produce any significant rise in sea levels during the twenty-first century. Few glaciologists are repeating that claim with any confidence now. Most would agree with Alley that "major changes are taking place in the Antarctic, on much shorter time scales than previously anticipated."

The British Antarctic Survey now employs a mathematician full time to apply chaos and complexity theory to the fate of the continent's ice-a topic once considered to be of the utmost simplicity. The BAS is using the language of fractals, phase space, and bifurcations to work out what might happen next to the ice sheets of the Antarctic Peninsula and the glaciers of Pine Island Bay. Its scientists have seen Larsen B shatter in three days; they believe they are seeing the soft underbelly of the West Antarctic ice sheet ripped open before their eyes. What next?

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RISING TIDES

Saying "toodle-oo" to Tuvalu

The Carteret Islands are to be abandoned. Life is simply too hard for their 2,000 inhabitants, huddled on a clutch of low-lying coral islands in the South Pacific, with a total surface area of just 150 acres, and rising sea levels threatening to wash them away. The islands, named after an eighteenth-century English explorer of the South Seas, Philip Carteret, have been under nearly constant erosion since the 196os, and the current guess is that they will be wholly submerged by 2015. Already their fields have been invaded by salt water, and the breadfruit crops have died. The people, refugees in their own land, depend on handouts.

In 2001, when strong winds and rough seas cut off the atoll and prevented them from going to sea to catch fish, many resorted to eating seaweed. One resident on the island of Han pleaded by radio for rescue: "Erosion is occurring from both sides, and the island is getting narrow. In Piul, many families are leaving. Huene Island is divided in half, and four families only are left. On lolasa, losela, and langain, when high seas occur, they stand below sea level. This is very frightening." Indeed. In November 2005, the central government in Papua New Guinea, of which the Carteret Islands form a part, agreed that the islanders should all be moved to Bougainville, a four-hour boat ride to the southwest. Ten families at a time will journey over the next few years, relinquishing their ancestral homes forever.

For most people around the world, stories of a rise in sea level remain a matter of academic interest, if that. The risks seem remote. But for the inhabitants of low-lying islands like the Carterets, it is happening now and devastating their lives.

The io,ooo citizens of the nine inhabited South Pacific islands of Tuvalu are also abandoning ship. High tides regularly wash across the main street in the capital, Funafuti; sea salt is poisoning their fields and killing their coconuts. Tuvalu is a full-fledged nation-state. Formerly the British Ellice Islands, it won independence in 1975. But just thirty years on, it seems destined to be the first modern nation-state to disappear beneath the waves. A twenty-first-century Atlantis. "In fifty years, Tuvalu will not exist," says the prime minister. His government has signed a deal with New Zealand, i,8oo miles away, that will allow the entire population to move there in the coming years, as rising tides and worsening storms destroy their homes.

One by one, the island nations of the South Pacific are drowning. Kiribati, formerly the British Gilbert Islands, won its independence on the same day as Tuvalu. It, too, is going under. Two uninhabited islands disappeared in 1999. The following year, Nakibae Teuatabo, a resident of Kiribati, explained its plight to me at a climate-change conference in Bonn, where he had been sent to plead for his country's survival. "Eight or nine house plots in the village that my family belongs to have been eroded. I remember there was a coconut tree outside the government quarters where I lived. Then the beach all around it was eroded, and eventually the tree disappeared. It might not sound a lot to you. But the atolls are just rings of narrow islands surrounding a lagoon, with the open ocean on the outside. Some of the islands are only a few yards wide in places. Imagine standing on one of these islands with waves pounding on one side and the lagoon on the other. It's frightening."

Villagers on some outer islands have already moved away as the sea gobbles up their land, he said. "Apart from causing coastal erosion, higher tides are pushing salt water into the fields and into underground freshwater reservoirs. In some places, it just bubbles up from the ground." It was a heart-rending story-good for journalists, but of no interest to most government negotiators at the conference. Such nations, it seems, are expendable.

The world's sea levels have been largely stable for the past 5,000 years, since the main phase of melting of ice sheets after the end of the last ice age abated. Some residual ice loss continued to raise sea levels at less than one hundredth of an inch a year. But around 19oo, the rise began to increase. At first, this was most likely owing to the melting of glaciers after the little ice age ended, in the mid-nineteenth century. That should have diminished during the twentieth century. But instead it has accelerated in the past fifty years, to around o.o8 inches a year. About half of this increase is probably due to the process known to physicists as thermal expansion. And the rest is probably due to the resumed melting of the world's glaciers and ice caps, doubtless largely a result of man-made climate change.

The first signs of a further acceleration emerged in the early 199os, when satellite data suggested a sudden rise of o.1I inches a year. Since 1999, it may have risen further, to 0.14 inches. At the time of this writing, these figures had failed to gain much attention, because glaciologists remained worried about their reliability. Some think there may be a problem calibrating the satellite data; others that it may simply be a natural fluctuation. But, with every year that passes, more researchers are concluding that we are seeing the first effects of the dramatic changes apparently under way on the ice sheets of Greenland and Antarctica.

The planet has a history of startling sea level rise that cannot be explained by the conventional models used by glaciologists to predict future change. Consider events toward the end of the last ice age. Around 20,000 years ago, at what glaciologists call the "glacial maximum," so much water was tied up in ice on land that sea levels were around 400 feet lower than they are today. Then a thaw began. Sea levels initially rose by around 0.4 inches a year. That is four or five times faster than today, but within the traditional expectations of glaciologists. Then something happened. About 14,500 years ago, the tides went haywire. Within 400 years, sea levels rose by 65 feet. That's an average rate of just over a yard every twenty years.

It is worth thinking about those numbers. If such a rise happened today, you could say "toodle-oo" to Tuvalu by 2010; most of Bangladesh would be under water by 2020; millions of people on the Nile Delta would be looking for new homes by 2025; London would need a new Thames Barrier immediately. New Orleans? Well, forget New Orleans, and Florida, and most of the rest of the U.S. seaboard, too. Lagos, Karachi, Sydney, New York, Tokyo, Bangkok: you name your coastal megacity, and it would be abandoned by midcentury. It sounds unbelievable, but we know the rise happened. The evidence is in tidemarks on ancient cliffs and in the remains of coral that can live only close to sea level.

How could such a thing have happened? It required the transfer into the oceans of about 13 billion acre-feet of ice every year throughout the 400-year period. That is a huge amount of ice. Glaciologists believe that the West Antarctic ice sheet, which was much larger then, was the most likely source. But wherever it came from, it could have reached the oceans in such quantities and at such speed only by some process in addition to melting. Such discharges required the physical collapse of ice sheets on a grand scale. That can have happened only if the ice sheets were lubricated at their base by great rivers of meltwater, and destabilized at the coasts by the shattering of ice shelves.

Go back further. In the last interglacial period, about 120,000 years ago, evidence such as wave-cut notches along cliffs in the Bahamas show sea levels 20 feet higher than they are today. During a previous interglacial, some 400,000 years ago, they may have been even higher. In neither period were temperatures significantly higher than they are today. On the face of it, either the West Antarctic ice sheet, or the Greenland ice sheets, or both, succumbed at temperatures close to our own. We can expect that temperatures will rise by about 3 to 5 degrees within the coming century. That, says Hansen, would make them as high as they were 3 million years ago, before the era of ice ages started. What were sea levels then? About 25 yards higher than today, plus or minus ro yards, he says.

A first guess is that we will very soon have set the world on a course for reaching such levels again. The models ofglaciologists suggest that, if this happens, it will take thousands of years. Jim Hansen doesn't believe it. "I'm a modeler, too, but I rate data higher than models," he says. He already sees evidence of the start of runaway melting in Greenland and Antarctica, and anticipates that "sea levels might rise by a couple of yards this century, and several more the next century."

Some see this prognosis as alarmist. Where, they say, is the evidence of big sea level rises so far? Hansen says that much of the extra melting has been camouflaged by increased snowfall on the ice sheets: "Because of this, sea level changes slowly at first, but as global warming gets larger, as summer melt extends higher up the ice sheet, and as buttressing ice shelves melt away, multiple positive feedbacks come into play, and the nonlinear disintegration wins the competition, hands down."

The world's ice sheets are "a ticking time bomb," he says. There is no reason why the events of 14,000 years ago should not be repeated in the twenty-first century. "The current planetary energy imbalance is now pouring energy into the Earth system at a rate sufficient to fuel rapid deglaciation." Hansen's hunch is that an increasing amount of global warming will be harnessed to melting the ice sheets. That could slow the heating of the atmosphere, but at the price of faster-rising sea levels. Within a few decades, vast armadas of icebergs could be breaking off the Greenland ice sheet, making shipping lanes impassable and cooling ocean surfaces like the ice in a gin and tonic. Sea level rise, he concludes, is "the big global issue." He believes it will transcend all others in the coming century.

It is easy to forget the plight of the people of the Carteret Islands and Tuvalu. Few of us could even find these places on the map. But as the tides rise ever higher, and as the precarious state of the big ice sheets becomes more apparent, we might want to heed those people's fate. It could be that of our own children.

RIDING THE CARBON CYCLE

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IN THE JUNGLE

Would we notice if the Amazon went up in smoke?

The Amazon rainforest is the largest living reservoir of carbon dioxide on the land surface of Earth. Its trees contain some 77 billion tons of carbon, and its soils perhaps as much again. That is about twenty years' worth of man-made emissions from burning fossil fuels. The rainforest is also an engine of the world's climate system, recycling both heat and moisture. More than half of the raindrops that fall on the forest canopy never reach the ground; instead they evaporate back into the air to produce more rain downwind. The forest needs the rain, but the rain also needs the forest.

But as scientists come to understand the importance of the Amazon for maintaining climate, they are also discovering that it may itself be under threat from climate change. We are familiar enough with the damage done to the world's biggest and lushest jungle by farmers armed with chain saws and firebrands. But, hard as they try, they can destroy the rainforest only slowly. Despite many decades of effort, most of this jungle, the size of western Europe, remains intact. Climate change, on the other hand, could overwhelm it in a few years.

Until recently, many ecologists have thought of the Amazon rainforest much as their glaciologist colleagues conceived of the Greenland ice sheet: as big and extremely stable. The Greenland ice maintained the climate that kept the ice securely frozen, while the Amazon rainforest maintained the rains that watered the forest. But, just as with the Greenland ice sheet, the idea that the Amazon is stable has taken a knock: some researchers believe that it is in reality a very dynamic place, and that the entire ecosystem may be close to a tipping point beyond which it will suffer runaway destruction in an orgy of fire and drought. Nobody is quite sure what would happen if the Amazon rainforest disappeared. It would certainly give an extra kick to climate change by releasing its stores of carbon dioxide. It would most likely diminish rainfall in Brazil. It might also change weather systems right across the Northern Hemisphere.

One man who is trying to find out how unstable the Amazon rainforest might be is Dan Nepstad, a forest ecologist nominally attached to the Woods Hole Research Center, in Massachusetts, but based for more than two decades in the Amazon. He doesn't just watch the forest: he conducts large experiments within it. In 200 1, Nepstad began creating a man-made drought in a small patch of jungle in the Tapajos National Forest, outside the river port of Santarem. Although in most years much of the Amazon has rain virtually every day, Tapajos is on the eastern fringe of the rainforest proper, where weather cycles can shut down the rains for months. The forest here is, to some extent, adapted to drought. But there are limits, and Nepstad has been trying to find out where they lie.