New horizons

Feedbacks from the stratosphere

Is that the end of the story? I don't think so. Constantly, in writing this book, I have been struck by how little we know about the way Earth's climate and its attendant systems, feedbacks, and oscillations function. This story contains some heroic guesses, some brilliant intuition, and, no doubt, occasionally some dreadful howlers—because that is where the science currently lies. More questions than answers. Beyond the cautious certainties of the IPCC reports, there is a swath of conjectures and scary scenarios. Some criticize the scientists who talk about these possibilities for failing to stick to certainties, and for rocking the IPCC's boat. But I suspect we still need a good deal more of the same, because we may know much less than we think. I think Wally Broecker and his colleagues deserve praise for developing their scenarios about the global conveyor. They have produced a persuasive narrative that has transformed debate. Of course, producing a persuasive story doesn't make it right, but it does generate new research and new ideas that can be tested. It is time someone in the tropical school produced something comparable.

Equally important, there may be other narratives that need developing. Richard Alley must be right that there are more "inevitable surprises" out there—outcomes that nobody has yet thought of, let alone tested. One area where unconsidered triggers for global climate change may lie is in and around Antarctica. While sinking cores into Antarctica as well as Greenland, the polar school has yet to devote much attention to generating theories about events in the South Atlantic. This may be a mistake. Much of the action in Earth-system science in the next few years will happen there, I am sure. Any place capable of producing something as remarkable as the ozone hole in the stratosphere is surely capable of storing up other surprises.

One new idea emerging from the battle between the polar and tropical schools is that the real driver of climate change up to and including the ice ages may actually lie in the far South. During ice ages, the theory goes, the ocean conveyor did not so much shut down as start getting its new deep water from the Antarctic rather than the Arctic. A certain amount of deep water has always formed around Antarctica, though in recent times it has played second fiddle to the North Atlantic. But, as the ice sheets grew across the Arctic and the chimneys in the North Atlantic shut down, the zone of deepwater formation in the Southern Ocean seems to have strengthened and may have taken charge of the conveyor.

Some go further and say that there must be a "bipolar seesaw," in which warming in the Southern Hemisphere is tied to cooling in the North and vice versa. That would certainly make sense of some of the Antarctic ice cores that show warming while the North was cooling. The question then is: Which pole leads? Does the North Atlantic end of the system shut down, closing off the Gulf Stream's northward flow of warm water and leaving more heat in the South Atlantic? Or does some switch in the South trigger the shutdown of the Gulf Stream and leave the Northern Hemisphere out in the cold, with the North Atlantic freezing over?

The idea that the South may lead in this particular dance gained ground late in 2005, when results were published from new ice cores in Antarctica. A European group found that the tightest "coupling" between temperature and carbon dioxide levels in the atmosphere is to be found in Antarctic cores, rather than their Greenland equivalents. "The way I see things is that the tropics and Antarctica are in phase and lead the North Atlantic," says Peter deMenocal, of Lamont-Doherty. "Even though we may see the largest events in the North Atlantic, they are often responding, not leading." By this reading, the onset of the Northern glaciation may have its origins in the Southern Hemisphere.

This apparently obscure debate could matter a great deal in the twenty-first century. Right now, the world has become worried that melting ice in the Arctic could freshen the far North Atlantic and shut down the Gulf Stream. This is a real fear. But maybe, while we are researching that possibility, we are ignoring the risk that large stores of freshwater in the Antarctic might break out and disrupt deepwater formation there. Arguably, the risks are far greater in the South, where, besides the potential breakout of ice from Pine Island Bay, recent radar mapping studies have revealed a large number of lakes of liquid water beneath the ice sheets of Antarctica. They might set off a cascade of freshwater into the Southern Ocean, similar in scale to the emptying of Lake Agassiz. Yet nobody, so far as I am aware, has studied what the effects of such a breakout might be for deepwater formation and the Southern arm of the ocean conveyor.

Or, rather than shutting down deepwater formation in Antarctica, might we be about to trigger a switch in the bipolar seesaw, so that deepwater formation in the South takes over from that in the far North? Could that switch be flipped in the South, rather than in the North? And if so, how? And what might happen? It would certainly lead to the Southern Hemisphere's hanging on to very large amounts of heat that currently head north on the Gulf Stream. The Southern Ocean might warm dramatically while the North Atlantic froze. And if the Southern Ocean were to warm substantially, says Will Steffen, the former head of the International Geosphere-Biosphere Programme, "it could result in the surging, melting, and collapse of the West Antarctic ice sheet." Ouch.

If anybody doubts that plenty of new surprises are waiting to be discovered, then the work by Drew Shindell, of the Goddard Institute for Space Studies (GISS), should offer food for thought. His story starts with an apparent success for climate modelers. Since the days of Arrhenius, most climate models have predicted that global warming will be greatest at high latitudes, where known feedbacks like ice-albedo are most pronounced. So rises in temperatures of up to 5°F over parts of the Arctic and the Antarctic Peninsula in recent decades have often been taken as the first proof of man-made climate change.

But there has been a persistent and troubling counterargument. The warming in the polar regions appears to be linked to two natural climatic fluctuations, one in the North and one in the South. In the North, the fluctuation is known as the Arctic Oscillation, an extension of the better-known North Atlantic Oscillation. It is the second largest climate cycle on Earth, after El Nino. The oscillation itself, as measured by meteorologists, is a change in relative air pressure, but its main impact is to strengthen or weaken the prevailing westerly winds that circle the Arctic.

Like El Nino, the Arctic Oscillation flips between two modes. In its positive mode, air pressure differences between the polar and extrapolar regions are strong, and winds strengthen. Especially in winter, the winds take heat from the warm oceans and heat the land. So, during a positive phase of the Arctic Oscillation, northern Europe, Svalbard, Siberia, the Atlantic coast of North America, and Alaska all warm strongly. Likewise, when the oscillation is in its negative phase, the winds drop and the land cools.

The strength of this effect depends on the warmth of the oceans, and in particular on the Gulf Stream and the health of the ocean conveyor. But for most of the past thirty-five years, the Arctic Oscillation has been in a strongly positive mode, helping sustain a long period of warming. Modeling studies suggest that at least half of the warming in parts of the Northern Hemisphere is directly due to its influence, leaving global warming itself apparently a bit player. Except that there is growing evidence that global warming is driving the Arctic Oscillation, too. And it does so from a surprising direction.

Enter Shindell. He likes to occupy the unpopular boundaries between scientific disciplines. His particular interest is the little-studied relationship between the stratosphere, the home of the ozone layer, and the troposphere, where our weather happens. He studies this with the aid of the GISS climate model, one of the few that can fully include the stratosphere in its calculations. Most models show little relationship between global warming and the Arctic Oscillation. The GISS model is the same when the stratosphere is not included. But Shindell discovered that when the stratosphere is hooked up, the result is a huge intensification of the Arctic Oscillation and the westerly winds around the Arctic. In fact, with current levels of greenhouse gases, he has reproduced a pattern very similar to the current unusually strong positive state of the oscillation.

What is going on? One of the problems with climate models is that it is not always easy to pinpoint exactly which of the elements in the model is causing the effects that you see in the printout. But here the role of the stratosphere is clear. And Shindell reckons he has the links in the chain explained, at least. As greenhouse gases cool the stratosphere, this cooling alters energy distribution within so as to strengthen stratospheric winds. In particular, a wind called the stratospheric jet, which swirls around the Arctic each winter, picks up speed. This wind, in turn, drives the westerly winds beneath, in the troposphere. So they go faster, too. In this way, a stratospheric feedback is amplifying global warming in the Arctic region by pushing the Arctic Oscillation into overdrive and strengthening the winds that warm the land. It is a brilliant, startling, and, until recently, entirely unexpected feedback.

Might the same apply to events in Antarctica? The GISS model suggests so. There, the dominant climatic oscillation is the Southern Hemisphere annular mode, or SAM. Like the Arctic Oscillation, the SAM is a measure of the air pressure difference between polar and nonpolar air that drives westerly winds sweeping around Antarctica. The geography is somewhat different from the Arctic's. The winds whistle around the Southern Ocean and hit land only on the Antarctic Peninsula, which juts out from the Antarctic mainland toward South America.

The climatologist John King has studied the SAM for the British Antarctic Survey. He says that, like the Arctic Oscillation, it has been in overdrive since the mid-1960s, driving stronger westerly winds. And, again like the Arctic Oscillation, it is amplifying warming along its path. The Antarctic Peninsula has seen air temperatures rise by 5°F since the 1960s—the only spot in the Southern Hemisphere to show warming on this scale. The effects include the melting of the peninsula's glaciers and the dramatic collapse of its floating ice shelves, such as the Larsen B. Additionally, by bringing more warm air farther south, the SAM winds are warming the waters that wash around the edges of Antarctica and beneath its ice—helping destabilize the West Antarctic ice sheet.

Here again, Shindell's model suggests that the strengthening of the SAM is the product of a cooling stratosphere and a strengthening of stratospheric jets. There is an important additional element here in the thinning ozone layer, which makes an additional contribution to stratospheric cooling.

All this is alarming evidence of a new positive feedback that intensifies warming in two particularly sensitive regions of the planet, where that extra warming could unleash further dangerous change. Glaciologists say that the Greenland ice sheet could collapse if warming there reaches 5°F. The huge stores of methane beneath the Siberian permafrost and the Barents Sea could be liberated by similar warming. And "the SAM warming now includes parts of the West Antarctic ice sheet, as well as the Antarctic Peninsula," says Shindell's boss, Jim Hansen. "This is a really urgent issue."

The discovery of the stratospheric feedback also helps answer another question that has long bothered climate scientists: Why do variations in solar output that are probably no more than half a watt per 10.8 square feet cause the big climate fluctuations in the North Atlantic identified by Gerard Bond in his analysis of the 1,500-year solar pulse? Conventional climate models without a stratospheric dimension suggest that such a solar fluctuation shouldn't produce temperature changes of more than 0.35°F. But, although the global temperature change may well have been close to that, in parts of Europe and North America the pulses produce changes ten times as great.

Researchers have struggled to find amplifying mechanisms that might have caused that. Sea ice, the ocean conveyor, and tropical flips like El Nino have all been suggested, but none seems up to the task. Shindell says the answer is his stratospheric feedback. The heart of the mechanism this time is ultraviolet radiation. While the total solar radiation reaching Earth's surface during Bond's pulses varies by only a tenth of a percentage point, the amount of ultraviolet radiation reaching Earth changes by as much as 10 percent. Most of the ultraviolet radiation is absorbed by the ozone layer in the stratosphere, so its impact at ground level is small. But the process of absorption causes important changes in energy flows in the stratosphere. These eventually change the stratospheric jets, and with them the Arctic Oscillation in the Northern Hemisphere and the SAM in the South.

Shindell modeled the likely effects of the last reduction of solar radiation at the Maunder Minimum in the depths of Europe's little ice age, 350 years ago. The GISS model without the stratosphere was unmoved by the tiny change in solar radiation. But with the stratosphere included, it delivered a drop in temperatures of 1.8 to 2.6°F in Europe, but only a tenth as much globally—results remarkably close to likely events in the real world. The declining flows of ultraviolet radiation into the stratosphere triggered a slowdown in the westerly winds at ground level, says Shindell. That, in turn, caused winter cooling, particularly over land, in the higher latitudes of the Northern Hemisphere.

The stratosphere and its influence on polar and midlatitude winds thus seem to be a hidden amplifier that can turn small changes in solar radiation into larger changes in temperature in the polar regions of the planet. This is not the only amplifier in those regions. Ice and snow are important, along with the ocean conveyor and, maybe, methane. But it appears to be the critical ingredient that turns minor solar cycles into big climatic events. It makes sense of Bond's solar pulse and, perhaps, of tiny short-term variability in solar radiation.

Climate skeptics have sometimes argued that sunspot cycles correlate so well with warming in the twentieth century that greenhouse gases could be irrelevant. Mainstream climate scientists dismissed this idea because they could not see the mechanisms that might make this happen. The changes in solar radiation seemed much too small. Shindell's finding of a powerful stratospheric feedback to the solar signal have forced a rethink. But Shindell has not joined the climate skeptics. Far from it.

His conclusion is that for the first half of the century, the correlation between estimated solar output and Earth's temperature is not bad. And the stratospheric feedback might show how the sun could have driven some warming early in the century, followed by a midcentury cooling that made some fear an oncoming ice age. But since then, there has been no change in the solar signal that could be amplified to explain the recent warming. During the final three decades of the twentieth century, average solar output, if anything, declined, while global temperatures—not just at high latitudes but almost everywhere—surged ahead at what was probably a record rate. So, Shindell says, "although solar variability does impact surface climate indirectly, it was almost certainly not responsible for most of the rapid global warming seen over the past three decades."

For that most recent period, he says, it is clear that rising concentrations of greenhouse gases are the primary driver. But besides producing a general global warming, they have generated changes in the stratosphere that have produced a specific positive feedback to warming in the polar regions and the midlatitudes. The positive feedback has manifested itself through the apparently natural Arctic Oscillation and the SAM—cycles that appear to have gone into overdrive.

Only a fool would conclude from this that we don't need to worry so much about man-made climate change. On the contrary, Shindell's dramatic discovery of the stratospheric feedback suggests that the natural processes of temperature amplification are much stronger than those in most existing climate models. His newly discovered feedback seems set to continue, driving up temperatures in Arctic regions beyond the levels previously forecast. That additional warming is likely to unleash other feedbacks that will melt ice, raise sea levels, release greenhouse gases trapped in permafrost and beneath the ocean bed, and perhaps cause trouble for the ocean conveyor.

Relieved? I don t think so.

conclusion: another planet

Over the past 100,000 years, there have been only two generally stable periods of climate, according to Richard Alley. The first was "when the ice sheets were biggest and the world was coldest," he says. "The second is the period we are living in now." For most of the rest of the time, there has been "a crazily jumping climate." And now, after many generations of experiencing global climatic stability, human society seems in imminent danger of returning to a world of crazy jumps. We really have no idea what it will be like, or how we will cope. There is still a chance that the jumps won't materialize, and that instead the world will warm gradually, even benignly. But the odds are against it. There are numerous feedbacks—waking monsters, in Chris Rapley's words—waiting to provide the crazy jumps. Climatically, we are entering terra incognita.

The current generation of inhabitants of this planet is in all probability the last generation that can rely on anything close to a stable global climate in which to conduct its affairs. Jim Hansen gives us just a decade to change our ways. Beyond that, he says, the last thing we can anticipate is what economists call "business as usual." It will be anything but. "Business as usual will produce basically another planet," says Hansen. "How else can you describe climate change in which the Arctic becomes an open lake in the summer, and most land areas experience average climatic conditions not experienced before in even the most extreme years?"

I am sorry if you have got this far hoping for a definitive prognosis for our planet. Right now, the only such prognosis is uncertainty. The Earth system seems chaotic, with the potential to head off in many different directions. If there is order, we don't yet know where it lies. No scenario has the ring of certainty. No part of the planet has yet been identified as holding an exclusive key to our future. No feedback is predestined to prevail. On past evidence, some areas may continue to matter more than others. But "the story of abrupt climate change will become more complicated before it is finished," as Alley puts it. "We have to go looking for dangerous thresholds, wherever they may be."

For now, we have checklists of concerns. Melting Arctic ice, whether at sea or on land, could have huge impacts, both by raising sea levels and by amplifying global warming. Glaciological "monsters" could be lurking in Pine Island Bay or the Totten glacier. The whole West Antarctic ice sheet could just fall apart one day. El Nino may get stuck on or off, triggering megadroughts or superhurricanes. The Amazon rainforest may be close to disappearing in a rage of drought and fire that would impact weather systems around the world. The oceans may turn into a giant lifeless acid bath. Smog may cripple the hydroxyl cleaning service or shut down the Asian monsoon. And the stratosphere may contain yet more surprises.

Methane is always lurking in the background, ready to repeat the great fart of 55 million years ago, if we allow it out of its various lairs. And the North Atlantic seems to hold a particular fascination. I keep coming back to Alley's disturbingly simple choice for the Gulf Stream as it surges north: sink or freeze? And to Peter Wadhams's lonely chimney, stuck out off Greenland somewhere northeast of Scoresby Sound, endlessly delivering water to the ocean floor. Until it stops. Who knows when? And who knows what will follow?

Quite a lot of this book has been taken up with climatic history. This is deliberate. The past shows more clearly than any computer model how the climate system works. It works not, generally, through gradual change but through periods of stability broken by sudden drunken lurches. And the past operation of the climate system reveals in their fully conscious state the monsters we may be in danger of waking.

But past climate does not provide a blueprint for the future. There are no easy analogues out there. We have already strayed too far from the tracks created by Bond's solar cycles and the other natural oscillations of the Earth system. Greenhouse gas concentrations are already probably at their highest level in millions of years; temperatures will soon join them. But the distinctive nature of our predicament goes a long way beyond that. Give or take the occasional asteroid impact, past changes have almost all been driven by changes in solar radiation, beamed down to us through the stratosphere. Earthly feedbacks such as biological pumps and spreading ice sheets, and threshold changes to marine currents and terrestrial vegetation, followed on the solar signal. This time, we are starting from the ground up, with a bonfire of fossil fuels that has shaken the carbon cycle to its core. Not only that: we are simultaneously filling the atmosphere with aerosols and assaulting key planetary features like the rainforests and the ozone layer. There can be no certainty about how the monsters of the Earth system will respond. We can still learn from the past, but we cannot expect the past to repeat itself.

When I first wrote at length about climate change, back in 1989, in a book called Turning Up the Heat, I warned that we passengers on Spaceship Earth could no longer sit back for the ride. We needed to get hold of the controls or risk disaster. But it was at heart an optimistic book. I figured that if Homo sapiens had come through the last ice age as a mere novice on the planet, then we could make it this time, too. We had the technology; and the economics of solving the problems wouldn't be crippling. I compared the task to getting rid of the old London pea-soupers of half a century ago. Once the decision was taken to act, the delivery would be relatively easy. We'd soon be wondering why we had dawdled for so long.

Fifteen years on, the urgency of the climate crisis is much clearer, even if the story has grown a little more complicated. But we are showing no signs yet of acting on the scale necessary. The technology is still straightforward, and the economics is only easier, but we can't get the politics right. Even at this late hour, I do believe we have it in our power to set Spaceship Earth back on the right course. But time is short. The ship is already starting to spin out of control. We may soon lose all chance of grabbing the wheel.

Humanity faces a genuinely new situation. It is not an environmental crisis in the accepted sense. It is a crisis for the entire life-support system of our civilization and our species. During the past 10,000 years, since the close of the last ice age, human civilizations have plundered and destroyed their local environments, wrecking the natural fecundity of sizable areas of the planet. Nevertheless, the planet's life-support system as a whole has until now remained stable. As one civilization fell, another rose. But the rules of the game have changed. In the Anthropocene, human influences on planetary systems are global and pervasive.

In the past, if we got things wrong and wrecked our environment, we could pack up and move somewhere else. Migration has always been one of our species' great survival strategies. Now we have nowhere else to go. No new frontier. We have only one atmosphere; only one planet.

appendix: the trillion-ton challenge

All the world's governments are committed to preventing "dangerous" climate change. They made that pledge at the Earth Summit in Rio de Janeiro in 1992. (The signatories included the U.S. and Australia, which both refused to ratify the subsequent Kyoto Protocol and its national targets for emissions reductions.) But what constitutes dangerous climate change? And how, in practice, can we prevent it?

For some people, dangerous climate change is already a reality. Many victims of recent hurricanes, floods, and droughts blame climate change. Such claims are usually impossible to prove. But that doesn't mean that our weather is not changing, says Myles Allen, of Oxford University. In essence, climate change is loading the dice in favor of weird and dangerous weather. "The danger zone is not something we are going to reach in the middle of this century," Allen says. "We are in it now." The 35,000 Europeans who died in the heat wave of 2003 were victims of an event that would almost certainly not have happened without the insidious increase in background temperatures that turned a warm summer into a killer.

But, despite such local disasters, most would argue that the critical aim in the quest to prevent dangerous climate change is to avoid crossing thresholds in the climate system where irreversible global changes occur—especially changes that themselves trigger further warming. There is no certainty about where such "tipping points" lie. But there is a growing consensus, especially in Europe, that the world should try to prevent global average temperatures from rising by more than 3-6°F above pre-industrial levels, or about 2.5 degrees above current levels.

Unfortunately, there is no certainty either about what limits on greenhouse gases will achieve that temperature target. We don't yet know how sensitive the climate system is. Current estimates suggest that to stack the odds in favor of staying below a 3.6-degree warming, we probably need to keep concentrations of man-made greenhouse gases below the heating equivalent of 450 parts per million of carbon dioxide. In practice, that probably means keeping carbon dioxide levels themselves below about 400 ppm. Let's call this the "safety-first" option.

Forgive me if I now abandon this language of parts per million. I find it an irritating and unnecessary abstraction. It seems to me much more sensible to talk in terms of tons of carbon instead. Then we can establish how much there is in the atmosphere and see more clearly how much we can afford to add before the climate goes pear-shaped.

The simple figures are these. At the depths of the last ice age, there were about 440 billion tons of carbon dioxide in the atmosphere. As the ice age closed, some 220 billion tons switched from the oceans to the atmosphere, raising the level there to about 660 billion tons. That's where things rested at the start of the Industrial Revolution, when humans began the large-scale burning of carbon fuels. Today, after a couple of centuries of rising emissions, we have added another 220 billion tons to the atmospheric burden, making it about 880 billion tons. If we want to keep below the safety-first concentration, we have to keep below 935 billion tons. So we have only about another 5 5 billion tons to

Currently, we pour about 8.2 billion tons of carbon into the atmosphere annually. Of this, a bit over 40 percent is quickly taken up by the oceans and by vegetation on land. The rest stays in the air, where its life expectancy is more than a century. So, for practical purposes, we are adding about 4.4 billion tons of carbon dioxide a year to the atmosphere. Even at current rates of emissions, that means that we will be above our 93 5-billion-ton safety-first target before 2020; and assuming that emissions continue to rise at the current rate, we will be there in less than a decade. Frankly, barring some global economic meltdown, there is now very little prospect of not exceeding 935 billion tons. If we had acted quickly after 1992, we could have done it. But the world failed.

If we are lucky—if climate sensitivity turns out to be a little lower than the gloomier predictions suggest—the 3.6-degree target may still be achieved while we allow carbon dioxide levels to rise significantly above 935 billion tons. We cannot be sure. There is already about 1 degree of warming "in the pipeline" that we can no longer prevent. But if we are feeling lucky—and with a nod to both round numbers and political reality—we might allow ourselves a ceiling of a trillion tons. Some would call that a "realistic" target, though others would brand it a foolish bet on a climate system we know little about.

The "trillion-ton challenge" is still a tough call. Literally, whatever target we set will require drastic cuts in emissions. Nature will probably continue to remove a certain amount of our emissions. But experts on the carbon cycle say that we must reduce emissions to around a quarter of today's levels before nature can remove what we add each year. Only then will atmospheric levels stabilize; only then will climate start to stabilize. The quicker we can do it, the lower the level at which carbon concentrations in the air will flatten out. Reaching the safety-first target of 935 billion tons of carbon dioxide would require an immediate and dramatic ditching of business as usual in the energy industry worldwide. Global emissions would need to peak within five years or so, to fall by at least 50 percent within the next half century, and to carry on down after that. A trillion-ton target could be achieved with more modest early cuts and greater reductions later.

Another consideration is the danger posed by the sheer speed of warming. Many climate scientists say that rapid warming may be more destabilizing to vulnerable systems like carbon stores and ice caps than slower warming. For this reason, it could be important to take some urgent steps to limit short-term warming while we get carbon dioxide emissions under control. And there is a way to do that—through a concerted assault on emissions of gases other than carbon dioxide that have a big short-term "hit" on climate.

Let me explain. Different greenhouse gases have different lifetimes in the atmosphere, ranging from thousands of years to less than a decade. For convenience, climate scientists usually assess their warming impact as if it operated over a century—carbon dioxide's average lifetime in the atmosphere. But this is rather arbitrary. And it has the effect of "tuning" the calculations to make carbon dioxide seem more important, and other gases less so. Most significant here is methane, which, however you measure it, is the second most important man-made greenhouse gas after carbon dioxide. Measured over a century, the warming caused by a molecule of methane is about twenty times as great as that caused by a molecule of carbon dioxide. But methane does most of its warming in the first decade, its typical lifetime in the atmosphere. It has a quick hit. Measured over the first decade after its release, a molecule of methane causes a hundred times as much warming as a molecule of carbon dioxide.

By following the scientists' conventional time frame, Kyoto Protocol emissions targets have underplayed the potential short-term benefits of tackling methane emissions. It is unlikely that the politicians who signed the protocol were even aware of this.

But underplaying the benefits has had an important effect on policy priorities. To take one example, if the British government decided today to eliminate all methane emissions from landfill sites, it would meet only a fraction of the country's Kyoto targets, because the Kyoto rules measure the impact of foregone emissions over the whole of the coming century. If the initiative were measured instead on its impact over the first decade, the benefits would be five times as great. The methane specialist Euan Nisbet, of London's Royal Holloway College, reckons that the short-term hit would be almost as great as banning all cars on the streets of Britain. And, if the rules had been drawn up differently, it would have been enough to entirely meet Britain's Kyoto target.

If the world is mainly concerned about the effect of greenhouse gases in fifty to a hundred years' time, then we should probably stick with the existing formula. But if we are also concerned about quickly reducing global warming to stave off more immediate disaster, then there is a strong case for coming down hard on methane now—on leaks from landfills, gas pipelines, coal mines, the guts of ruminants, and much else. "Cutting carbon dioxide emissions is essential, but we have neglected methane and the near-term benefits [acting on] it could bring," says Nisbet. He wants the Kyoto Protocol rules narrowed to a twenty-year time horizon. Jim Hansen takes a similar view. "It makes a lot of sense to try to reduce methane, because in some ways it's easier," he says.

Hansen also advocates action on soot, which he calculates to be the third biggest man-made heating force in the atmosphere. Soot, as we saw in Chapter 18, has a local cooling effect but a wider and more considerable warming effect. It sticks around in the atmosphere for only a few days, but while it is there, its effects are large. Action against soot and methane would not stop global warming. But it would give the world time to introduce measures against the chief culprit: carbon dioxide.

Kyoto Politics

The Kyoto Protocol, signed in 1997, was the first, tentative step toward implementing the Rio pledge to prevent dangerous climate change. Some forty industrialized nations promised to make cuts in their emissions of six greenhouse gases, including the "big two": carbon dioxide and methane. Different countries accepted different targets, and the countries of the European Union later internally reallocated theirs. Those cuts averaged about 5 percent, measured between 1990 and the first "compliance period," which runs from 2008 to 2012. The protocol included various "flexibility mechanisms" aimed at making more effective use of cleanup investment funds. They allow countries to offset emissions by investing in cleanup technology abroad and in planting trees to soak up carbon dioxide from the air, and to trade directly in pollution permits.

The protocol did not impose targets on developing countries, because their emissions per resident are mostly much lower than those of the rich industrialized world (some conspicuous exceptions include South Korea,

Singapore, and several oil-rich Gulf states). The U.S. and Australia originally signed up to Kyoto targets, but then pulled out. The protocol came into force in 2005, and at the end of that year, its signatories agreed to start negotiations on tougher cuts to come into force after 2012.

So far, so good. But the current Kyoto targets are very small compared with the cuts in emissions that will eventually be needed. And the delay has effectively shut off the option of a safety-first limit on carbon concentrations in the atmosphere. Some European countries have set themselves informal targets of a 60 percent emissions reduction by midcentury, which is closer to what is needed. But even if all the Kyoto nations did likewise, they are responsible for only a minority of emissions today. So more cuts by other nations would still be needed.

Eventually, if the climate regime develops as many hope, every country and every major energy and manufacturing company will need a license to emit greenhouse gases. The system, some say, could even be extended to individuals. If we are to stop dangerous climate change, the number of licenses available will have to be very limited. So the question of how they should be shared out becomes critical. It is political dynamite. The very suggestion sets the industrialized and developing worlds at loggerheads. This is partly because the industrialized countries of Europe and North America have already used up something like half of the atmospheric "space" available for emissions, and partly because developing nations are coming under pressure to reduce their emissions before they have had a chance to industrialize.

Big developing nations like China and India may have high national emissions. But measured in ratio to population, their emissions remain low.

While the U.S. and Australia emit around 5.5 tons of carbon a year for every citizen, and European countries average around 3 tons, China is still around 1 ton, and India below half a ton. Developing countries feel they are being asked to forego economic development to help clean up a mess they did not create. On the other hand, they increasingly see that climate change threatens their prospects for economic development. The only solution is to institute a rationing system for pollution entitlements, based on a shared view of fairness.

Perhaps the simplest blueprint is "contraction and convergence." Developed by a small British group called the Global Commons Institute, it is attracting support around the world. The contraction half of the formula would establish a rolling program of annual targets for global emissions. The targets would begin roughly where we are today, and would fall over the coming decades. They would be set so as to ensure that the atmosphere never passed whatever limit on carbon dioxide concentrations the world chose.

The convergence half of the formula would share out those allowable global emissions each year according to population size. So national targets might begin at about 1 ton of carbon per person and then fall to maybe half a ton by 2050 and to that much less again by 2 100, depending on the global target chosen. Of course, at the start that would leave rich nations with too few permits and many poor nations with more than they needed. So they would trade. The costs of buying and selling pollution licenses would be a powerful incentive for a global cleanup.

Fantasy politics? Maybe. But something on this scale will be needed if we are to prevent climatic disaster. And if the rich world wants the poor world to help clean up its mess, and save us all from dangerous climate change, then some such formula will be needed.

Technological Fixes

Politics aside, what are the practicalities of stabilizing climate? President George W. Bush may have become a pariah in environmental circles for refusing to sign the Kyoto Protocol, but he is right on one thing: ultimately, it will be technologies, rather than politics, that solve the problem. The only question is what politics will best deliver the technologies that will allow us to "decarbonize" the world energy system. Those technologies fall into four categories: much more efficient use of energy; a switch to low- carbon and carbon-free fuels; capturing and storing or recycling some of the emissions that cannot be prevented; and finding new methods of storing energy, such as hydrogen fuel cells.

The task sounds daunting. But, in truth, much of it goes with the grain of recent economic and industrial development. In the past thirty years, global carbon dioxide emissions have grown only half as fast as the global economy—thanks mostly to improved energy efficiency. And many of the new energy technologies we will need are already in use, offering benefits such as cheaper or more secure energy. The replacement of coal with lower-carbon natural gas, oil with ethanol made from biofuels, the development of wind and solar power, the proposed expansion of nuclear energy, and investment in energy efficiency all fall into this category. What is needed first is faster progress in a direction in which we are already headed.

The top priority should be energy efficiency. More than half of the immediate cheap potential for reducing carbon dioxide emissions lies in improving energy efficiency in buildings, transport, and industry. Much of it could be done at zero or even negative cost, because the cost savings would outweigh the investment. This is also the area where we as individuals can most easily make a difference—by buying energy-efficient light bulbs and appliances, insulating our homes properly, cutting down on car use, and choosing energy-efficient models such as hybrids.

Also in the short term, there is huge potential to equip the world's fossil-fuel-burning power stations with "scrubbers" to remove carbon dioxide and deliver it via pipelines for burial underground. The technology is already developed and only needs scaling up. The potential global storage capacity in old oil and gas wells alone approaches a trillion tons of carbon. The British government's chief scientist, David King, says that by 2020 Britain could be burying a quarter of its power-station carbon dioxide emissions in old oil fields beneath the North Sea.

Other technologies will take more development before they become cost-effective on a large scale. These include solar power, which is available but currently too expensive for widespread use, and turning hydrogen into the fuel of the future for transport. The idea here would be to manufacture hydrogen in vast quantities for use in batteries, known as fuel cells, to power cars. Hydrogen would become the "new oil." Hydrogen is manufactured by splitting water into hydrogen and oxygen, which is a very energy-intensive process. So if the energy for splitting water were generated by burning fossil fuel, there would be little environmental gain; but if the energy came from renewables, such as solar or wind power, that would change everything.

The hydrogen fuel cell is not so much a new source of energy as a new way of storing energy. It could be the only way to make cars truly greenhouse-friendly. And it may turn out to be the best way of utilizing fickle renewable energy sources like wind and the sun. The big problem with these energy sources is that wind cannot be guaranteed to blow (nor the sun to shine) when the energy is needed. But if the energy is converted into hydrogen, it can be kept for future use.

So what, exactly, would it take to deploy all these technologies in order to bring climate change under control? The most ambitious attempt so far to produce a simple global blueprint comes from Robert Socolow, an engineer at Princeton University. He admits that when he checked out the plethora of options for cutting greenhouse gases, he was overwhelmed, and figured that most politicians and industrialists would be, too. So he decided to break the task down into a series of technological changes that would each cut global emissions of carbon dioxide by about 25 billion tons over the coming fifty years. He called them "wedges," because the impact of each would grow gradually, from nothing in the first year to a billion-ton emissions cut in the fiftieth year. They would each cut a "wedge" out of the graph of rising carbon dioxide emissions.

Socolow proposed more than a dozen possible wedges, but said that seven would be necessary to stabilize emissions at current levels. But we need to do more than that: we need to stabilize actual concentrations of greenhouse gases in the atmosphere, and that would require reducing emissions from their current 8.2 billion tons a year to around 2.2 billion tons. So I have adapted Socolow's blueprint to allow for that tougher target. We might choose the following twelve wedges, each of which could cut emissions by about 25 billion tons over the coming half century, and reduce global emissions from the projected 15.4 billion tons a year by 2060 to 2.2 billion tons:

0 universally adopt efficient lighting and electrical appliances in homes and offices;

° double the energy efficiency of 2 billion cars;

° build compact urban areas served by efficient public transport, halving future car use;

0 effect a fiftyfold worldwide expansion of wind power, equivalent to 2 million 1-megawatt turbines;

0 effect a fiftyfold worldwide expansion in the use of biofuels for vehicles;

0 embark on a global program of insulating buildings;

° cover an area of land the size of New Jersey (Socolow's home state) with solar panels;

° quadruple current electricity production from natural gas by converting coal-fired power stations;

° capture and store carbon dioxide from 1,600 gigawatts of natural gas power plants;

° halt global deforestation and plant an area of land the size of India with new forests;

° double nuclear power capacity;

° increase tenfold the global use of low-tillage farming methods to increase soil storage of carbon.

Economics of the Greenhouse

How much might all this cost? In 2001, a team of environmental economists assembled by the IPCC reviewed estimates for stabilizing atmospheric concentrations of carbon dioxide by 2100. They ranged from a low of $200 billion to a high of $17 trillion—almost a hundred times as much. It seems extraordinary that estimates could range so widely. But, when these are boiled down to their basics, it appears that much of the difference depends on whether the modelers assumed that the necessary technical and social changes would "go with the flow" of future change, or that everything would have to be grafted onto a society and an economy heading fast in a different direction.

Put simply, the high estimates guessed that, under business as usual, rising wealth would produce and require almost equally fast rises in emissions from burning cheap carbon fuels. Diverting from that path would thus require preventing emissions of trillions of tons of carbon using expensive technologies that would not otherwise have been developed. The lower estimates assumed that the world was already slowly losing its addiction to carbon fuels, and that all we would need to do is make the switch faster. They also took a rather different view of technological development, seeing it as molded by a range of economic incentives. In this version, governments could shape technological development by stimulating markets. Once the process was under way, innovation would go into overdrive, and prices would fall away.

Some of the people involved in the IPCC study were instinctively hostile to major efforts to cut carbon dioxide emissions. The Yale environmental economist William Nordhaus suggests that "a vague premonition of potential disaster is insufficient grounds to plunge the world into depression." But let us assume that the real costs will be toward the top end of the range. Would their adoption really push the world into recession?

The veteran climate scientist Stephen Schneider, of Stanford University, redid the arithmetic in 2002, assuming it would cost $8 trillion to stabilize carbon dioxide concentrations by 2100. He found that the same economists who predict doomsday if we try to tackle climate change also believe that citizens of the world will be, on average, five times richer in a hundred years than they are today. So he took the economists at their word and asked: How much would the $8 trillion bill for halting climate change delay those riches? The answer was just two years.

"The wild rhetoric about enslaving the poor and bankrupting the economy to do climate policy is fallacious, even if one accepts the conventional economic models," he told me when his analysis was published. Coincidentally, that was the week that Australia's prime minister, John Howard, announced that his country would not ratify the Kyoto Protocol because it would "cost jobs and damage our industry." Poppycock, said Schneider. "To be five times richer in 2100 versus 2 102 would hardly be noticed." It was a small price to pay.

A small price to pay for what? What would we be buying with this trillion-dollar investment in a stable climate? That, of course, is impossible to answer, because we don't know the extent of what would be avoided. But we can easily see the scale of things, even today. Evidence of the cost of extreme weather is everywhere. The 1998 El Nino cost Asia at least $20 billion. Insured losses from extreme weather in 2004 hit a record $55 billion, which was promptly exceeded by an estimated $70 billion for 2005. Total economic losses for 2005, including uninsured losses, are expected to be three times higher: cleaning up after Hurricane Katrina alone may eventually cost $ 100 billion. Incidentally, a simple extrapolation of trends in insurance claims stemming from extreme weather in recent years suggests that they will exceed total global economic activity by 2060. That may be slightly wacky math, but it is sobering nonetheless.

Not surprisingly, economists disagree about the cost of inaction on climate change as much as they do about the cost of action. Some have attempted to assess the "social cost" of every ton of carbon put into the air. One recent review found a range from approaching $1,700 per ton down to zero. The British government, which commissioned the review, settled on a figure of $70 per ton. One reason for the wide range is accounting practices. Economists routinely apply a discount to the cost of anything that has to be paid for in the future. Dealing with climate change that may happen decades or even centuries ahead allows for huge discounts. Some economists say that very long-term impacts—such as the rise of sea levels as ice caps melt—should be discounted to zero.

This discounting of the future may be a convenient device for corporations, or even governments in their day-to-day business. But it is less clear how sensible it is for the management of a planet. If corporate finances or a nation's economy go wrong, shareholders can sell their shares and governments can print money or go cap in hand to the International Monetary Fund. But the planet, our only planet, is rather different.

Moreover, the existing estimates of social cost are based on IPCC studies that so far have not included many of the irreversible positive feedbacks to climate change that this book has concentrated on. So nobody has yet even asked what price should be attached to a century-long drought in the American West, or an enfeebled Asian monsoon, or a permanent El Nino in the Pacific, or a shutdown of the ocean conveyor, or the acidification of the oceans, or a methane belch from the ocean depths, or a collapse of the West Antarctic ice sheet, or sea levels rising by half a yard in a decade. Though, on reflection, these are perhaps questions best not answered by accountants.

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The Basic Survival Guide

The Basic Survival Guide

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