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In 2006 James Lovelock published a book that bluntly laid before us the consequences of the carbon imbalance. The Revenge of Gaia was published in its author's eighty-seventh year, and it is as bleak and penetrating a perspective on human folly in regard to the environment as has ever been written. Lovelock argues that Gaia's climate system is far more sensitive to greenhouse-gas pollution than we imagine, and that the system is already trapped in a vicious circle of positive feedback. "It is almost as if we had lit a fire to keep warm," Lovelock opines, "and failed to notice, as we piled on fuel, that the fire was out of control and the furniture had ignited." Although there is still time to avert a catastrophe, Lovelock believes that humans lack the foresight, wisdom, and political energy required to do so. Instead, he predicts, before the twenty-first century is out our global civilization will have collapsed and a new dark age will have descended on us. Only a few survivors (perhaps just one out of every ten alive today) will cling to the few remaining habitable regions, such as Greenland and the Antarctic Peninsula.

The events likely to destroy our civilization include dramatic rises in sea level, which will flood coastal cities and some of the best agricultural land; changes in rainfall; extreme weather; and the disappearance of the glaciers that act as dams and whose meltwaters provide our most productive agricultural regions with water in the growing season. The ensuing starvation, warfare, and chaos will be the greatest scourge, for in Lovelock's projected dark age the warlords will be armed with nuclear weapons.

How probable is it that this bleak vision will come to pass? Because of new scientific data and technological analysis we are better placed than ever before to determine the scale of the threat and its imminence. Let's begin with a new analysis of work done by the Intergovernmental Panel on Climate Change (IPCC) in 2001. In its Third Assessment Report, the IPCC published a series of projections concerning key indicators of Earth's climate system. These included estimates of how swiftly Earth's average temperatures might increase over the course of the twenty-first century, how much the oceans would rise, and how quickly CO2 would accumulate in the atmosphere. The projections had an upper and a lower limit, and they encompassed quite a wide range of possibilities. The projection concerning temperature, for example, indicated that the increase might be as little as 2.5 degrees Fahrenheit, or as much as 10.4 degrees. From the perspective of human survival, the difference between 2.5 degrees and 10.4 is profound. Humanity can probably cope with a warming of less than 3 degrees, but a 10.4-degree warming would be truly catastrophic, heralding an ice-free world, and most likely human tragedy on the scale envisaged by Lovelock.

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At the time these projections were published, skeptics described them as unbelievable and grossly inflated, and widely proclaimed in the popular press that they amounted to scientific scaremongering. By 2007, however, scientists had five to six years' worth of real-world data under their belts, allowing them to revisit the projections to determine their accuracy, at least over the near-term, early portion of the curve. What they discovered should have been reported the front page of every newspaper on the planet. Astonishingly, in every instance the real-world changes were at the upper limit, or worse than even the worst-case scenario presented by the IPCC. The full implications of these new studies have yet to sink in among those negotiating the global treaty that is supposed to protect humanity from dangerous climate change. The negotiators continue to argue on the basis of the old projections, which call for action far less urgent than what is actually required. Worse, the negotiations grind on as if we had an eternity to achieve outcomes. Lovelock, who seemed like just another prophet of doom just two years ago, appears to have been right after all-unless, that is, we can rouse ourselves to take immediate action.

These changes in the Arctic have left many scientists worried that the region is already in the grip of an irreversible transition

From mid-2007 onward I've found it increasingly difficult to read the scientific findings on climate change without despairing. Perhaps the most dispiriting changes are occurring at the north pole. The sea ice that covers the Arctic Ocean is an ancient feature of our planet. It has glistened brightly into space for at least 3 million years, and over that time a host of organisms, from plankton to walruses and narwals, have adapted to life on and under it. But its importance to Gaia is far greater than as a home for an unusual fauna: the northern ice acts as a refrigerator that cools the entire planet. It does this by reflecting the sun's energy away from Earth. During the summer, the sun's rays beat down upon it twenty-four hours a day, but because the ice is bright, 90 percent of that energy (which averages 22 watts per square foot) is deflected back into space. Where the ice is absent, however, the dark ocean is revealed, and it soaks up all that solar energy and turns it into heat.

Around 1975, scientists noticed that the Arctic ice had begun to melt away. At first the rate was hardly worrying, and indeed many thought that it might just be part of a long-term cycle. But the trend continued, so that by 2005 the Arctic ice cap had been melting at a rate of around 8 percent per decade for thirty years. At that rate, it would have taken until 2100 or thereabouts for the ice cap to disappear altogether, and to many people, that was a comfortably distant date. But then, in the summer of 2005, a dramatic change occurred. The rate of melting accelerated, so that about four times as much ice melted, compared with previous summers. As at the onset of the melting trend, scientists were hoping that this was a freak or cyclic event, and that in a subsequent summer the melting would once again slow. But the summer of 2006 saw almost as much ice lost as in 2005. Then, during the summer of 2007, the very worst loss of Arctic ice ever witnessed occurred.

These changes in the Arctic have left many scientists worried that the region is already in the grip of an irreversible transition. During the winter months, the Arctic is now warming four times faster than the global average, and the existing temperature increase year-round already exceeds 3.5 degrees Fahrenheit. As a result, profound shifts are occurring in species distribution: some fish stocks in the Bering Sea, for example, have already moved by 500 miles. None of the models used to predict how the Arctic will alter as it warms has been able to replicate any of these changes. None, indeed, is even remotely accurate, so as we try to predict the region's future, we are truly flying blind.

The extent of uncertainty prevailing among scientists is illustrated by a straw poll conducted among experts on the Arctic in March 2008. They were asked whether they thought that summer 2008 would see a regrowth of the Arctic ice. The winter had been a cold one, and the great loss of ice the previous summer had been exceptional, leading the majority to say that a regrowth of the ice cap was likely. Yet by May 2008 the melting had begun once more, and the average daily loss of Arctic sea ice was, on average, 2,300 square miles per week greater than for the same period of 2007. By June the losses had become so severe that one Norwegian expert was saying that 2008 might see the Arctic's first ice-free summer. As it happened, 2008 saw a slight improvement in the extent of the ice (about 115,000 square miles) over the previous year. But the following winter brought extremely poor ice formation, with the area of ice the same as the all-time record low of 2007. It's now clear that the Arctic's first ice-free summer can be no more than a few decades away, and indeed may come to pass in just a few years.

In 2006 scientists realized that the sea can die as a result of severe global warming

What will happen during that first iceless summer? Most likely, not much at all, for it will take several summers' worth of energy to warm the surface of the Arctic sea to a point where dangerous changes are generated farther south. If recent history is anything to go by, during that first iceless summer the skeptics will say, "See, we told you that there was nothing to fear from an ice-free Arctic," and those who don't know any better will grasp at the reassurance. But each year thereafter, the ocean at the top of the world will inexorably warm, and the temperature gradient that controls climatic zones across the northern hemisphere will shift. It's difficult to know precisely how that will affect humanity, but if we look back at the last time in Earth's history such a great warming occurred-55 million years ago-we see an ominously different world. Back then, lemurs proliferated in the rain forests of Greenland, and the tropics were covered by a spiny, thin, alien-looking vegetation, which is today extinct. No one knows how quickly the world's climate altered then, but one cannot help fearing what a similar change might mean for humanity today.

So swift are the changes already occurring in the Arctic that much of the human response to the crisis thus far has been rendered hopelessly inadequate. The warming, for example, has accelerated the rate of melting of the Greenland ice cap, which is now vanishing by between sixty and seventy cubic miles per year. Public policy responses and political discourse, meanwhile, are based on a previous rate of loss of just twelve cubic miles per year. And this melting really does have immediate relevance, for the Greenland ice cap sits on land, and as it melts, it contributes to a rise in sea level. Even the most committed conservationists have been forced to rethink their strategy. Neil Hamilton, director of the WWF International Arctic Programme, said in May 2008, "We [WWF]are no longer trying to protect the Arctic," because it is too late. He believes that the region's first ice-free summer may arrive before 2013, and admits that he has no idea what the Arctic might look like in 2050.

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New ramifications of rapid warming are continually being discovered. In 2006 scientists realized that the sea can die as a result of severe global warming. Indeed, it has died, several times during Earth's history, and when it dies, it takes most life on land with it. Evidence of a dying sea comes from the sediments laid down on its floor. At different times enormous deposits of black shale have formed, and these are the source of much of Earth's oil. Oil, of course, is derived from living things, and it can form only when the organic matter that gives rise to it doesn't rot. Very little, if any, oil is forming in the oceans today because their depths are so filled with oxygen that living things can exist there; and life in the abyss, as life always does, efficiently uses and recycles whatever organic matter rains down on it. It therefore takes a dead ocean-or at least one whose depths are dead-to make oil, and oceans begin to die when the abyss is starved of oxygen.

The ocean circulation is vigorous today because the poles are cold and the equator is warm. The source of most of the deep ocean water is around the Antarctic; that is why ocean water is so cool, about 34-35 degrees Fahrenheit. This cold water (which can hold lots of oxygen) permits life in the depths. The cold poles and warm equator also cause the winds that drive surface currents, and the resultant surface mixing helps oxygenate the waters.

The most devastating example of oceanic death occurred 250 million years ago, when 95 percent of all life perished. Just what occurred then is only now beginning to be understood, largely because of a breakthrough in geochemistry. It was realized that living things such as bacteria, which rarely leave conventional fossils, nevertheless leave a chemical signature of their existence in rocks. Geologists studying rocks in Western Australia that dated to the Permian-Triassic extinction of 250 million years ago discovered traces of the unique lipids (fatty molecules) made by strange kinds of bacteria known as purple bacteria and green sulfur bacteria. These bacteria thrive only in waters that are well lit by the sun, yet are low in oxygen and high in hydrogen sulfide. Today, such conditions exist only in very restricted and unusual environments, such as the "jellyfish lakes" of Palau. Yet the story preserved in the rocks reveals that most if not all of Earth's oceans resembled this environment 250 million years ago.

You can think of ice as a kind of battery that stores cold, and the rate at which ice vanishes from a warming world is a key factor in determining when the full warming impact will be felt

The steps leading to the death of the oceans have been reconstructed as follows. First, a sudden increase of CO2 and methane in the atmosphere causes rapid warming of air and sea, which disrupts ocean currents and warms the depths. Increased warming of the poles brings winds and surface currents nearly to a standstill; and because of slowed circulation, and the fact that warm water holds less oxygen than cold water does, the ocean depths become deprived of oxygen. In this environment, bacteria that don't require oxygen multiply, and they emit huge volumes of sulfur. Eventually, the sulfurous, oxygen-starved water reaches the sunlit zone, and then the green sulfur bacteria flourish, producing huge volumes of toxic hydrogen sulfide, which enters the atmosphere in great belched bubbles, destroying much life on land. The gas rises high into the atmosphere, where it destroys the ozone layer, and the increased ultraviolet (UV) radiation devastates what is left of life on Earth. What does an Earth with a dead ocean look like? Peter Ward, a palaeontologist and expert in his field, imagines it as follows:

Look out on the surface of the great sea itself, and as far as the eye can see there is a mirrored flatness, an ocean without whitecaps. Yet that is not the biggest surprise. From shore to the horizon, there is but an unending purple colour-a vast, flat, oily purple, not looking at all like water. . . . The colour comes from a vast concentration of purple bacteria. . . . At last there is motion on the sea, yet it is not life, but antilife. Not far from the fetid shore, a large bubble of gas belches from the viscous oil slick-like surface. . . . It is hydrogen sulphide, produced by green sulphur bacteria growing amid their purple cousins. There is one final surprise. We look upward, to the sky. High, vastly high overhead, there are thin clouds, clouds existing far in excess of the highest clouds found on our Earth. They exist in a place that changes the very colour of the sky itself. We are under a pale green sky, and it has the smell of death and poison.

How much time, exactly, do we have to prove Lovelock wrong? In October 2008, Dr. James Hansen (who is arguably the world's leading climate scientist) and eight of his colleagues provided a new, alarming, though still partial, answer to this question. They looked back over the increasingly complete ice-core record, which documents the last 750,000 years of Earth's climatic history, and tried to determine how much warming a given amount of atmospheric CO2 pollution would produce, and how long it would take to produce this warming. Their most alarming discovery was that, when viewed over the long term, Earth's climate system is about twice as sensitive to CO2 pollution as the IPCC's century-long projections would indicate. This implies that there is already enough greenhouse-gas pollution in the atmosphere to cause 3.5 degrees Fahrenheit of warming, bringing about conditions not seen on Earth for 2 million to 3 million years, and constituting, according to the authors, "a degree of warming that would surely yield 'dangerous' climate impacts."

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Fortunately for us, some-perhaps half-of that warming is currently masked by other pollutants, known collectively as agents of global dimming, which reflect sunlight into space, thus cooling Earth. These include sulfur dioxide (the cause of acid rain), photochemical smog, and tiny particles of carbon called aerosols. All these pollutants are dangerous to human health, and it was for this reason in part that governments in Europe and the United States moved to regulate them long before tackling the greenhouse gases. They are also very short-lived in the atmosphere, lasting only hours to weeks.

Today, China, India, and other rapidly industrializing economies are releasing these agents of global dimming in ever-increasing quantities. Yet because of their effect on visibility and their serious impact on human health, there's good reason to believe that in the near future such nations will move to curb their release. Indeed, in the period leading up to the Beijing Olympics, heroic efforts were being made to do just that over large parts of northeastern China. One particularly effective instrument used to achieve this is a government subsidy for every kilowatt of electricity generated at plants that do not emit sulfur dioxide.

As a result of this scheme, one of China's "big three" providers of electricity-Datang International-had all of its generating plants fitted with sulfur-dioxide scrubbers by the end of 2009, and the competition is not far behind. If no attempt is made to reduce the agents of global warming concurrently with such cleanups of the agents of global dimming, humanity could experience a nearly instantaneous increase in warming that might have catastrophic consequences.

Hansen and his colleagues have arrived at a new understanding of how long it takes for the full warming consequences of a given amount of greenhouse gas to be felt. Two major factors cause a delay in the warming. The first of these, the rate at which the oceans are able to absorb the extra heat trapped in the atmosphere is perhaps the more important, and certainly the more easily determined. According to Hansen, if the delay caused by the oceans alone is considered, then we could expect to feel one-third of any warming caused by a given amount of greenhouse gas in the first few years after the gas is released. Three-quarters of the full warming effect would be felt within 250 years, and all of it within a millennium.

There is a second factor that causes a delay in the warming impact: Earth's ice, which currently covers 10.4 percent of the planet. You can think of ice as a kind of battery that stores cold, and the rate at which ice vanishes from a warming world is a key factor in determining when the full warming impact will be felt. Unfortunately, it is extremely difficult, if not impossible, to predict the decay of Earth's ice fields, mainly because they don't simply melt away like an ice cube. Rather, large portions can collapse spectacularly, spilling into the sea in fragments, where they rapidly melt. Such phenomena cannot be replicated in any of the models used to predict climate change. That is a tragedy, for in the real world the polar and glacial ice caps are altering profoundly and rapidly.

The rapid surface melting of the Greenland ice cap, the collapse of coastal ice shelves that hold back glaciers, a marked speeding of the ice streams that flow through great ice shelves such as the West Antarctic Ice Sheet, and an alarming overall loss of ice are all being observed in the real world, yet we are at a loss to determine how quickly, or how much, they will add to a rising ocean. But one can reasonably speculate. As Hansen and his colleagues put it, "Sea-level changes of several meters per century occur in the palaeoclimate record, in response to forcings slower and weaker than the present human-made forcing. This indicates that the ice may disintegrate and melt faster than previously assumed, and that the warming may be delayed less by the ice than assumed."

In their landmark paper, Hansen and his colleagues make a useful distinction between climatic "tipping points" and "the point of no return." The climatic tipping point is the point at which the greenhouse gas concentration reaches a level sufficient to cause catastrophic climate change. The point of no return is reached when that concentration of greenhouse gas has been in place sufficiently long to give rise to an irreversible process. Humanity is now between a tipping point and a point of no return, and only the most strenuous efforts on our part are capable of returning us to safe ground. The work of Hansen and his colleagues indicates that we still have a few years before we reach the point of no return, but that there is not a second to waste. This is our greatest challenge, and clearly the path forward involves a drastic change in energy use. It also means making full use of the tools we have at our disposal-and inventing new tools-to draw the pollution out of the air and save us from Lovelock's new dark age.

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