With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (29 page)

It had been known for a while that the Indian monsoon turned off during the last ice age but probably flickered on briefly during the warm episodes that punctuated the glaciation. The new study showed that the strength of the monsoon also shadowed the flutters of the Atlantic system during the postglacial era, faltering during the Younger Dryas and the chill of 8,200 years ago, for instance. The changes clearly followed Bond's 1,5oo-year solar pulse. Thus the last faltering of the monsoon came during Europe's little ice age, which ended in the final decades of the nineteenth century. Soon, as colonial records confirm, the monsoon was regaining its reliability.

But this pattern, impressive though it is, does not explain how the link with the Atlantic works. Does the Atlantic tell the monsoon what to do? Does the monsoon tell the Atlantic what to do? Does Bond's solar pulse independently determine both? Or is there another element not taken into account? Where does El Nino fit in, for instance?

Jonathan Overpeck, of the University of Arizona, one of the authors of the monsoon history, holds that the Atlantic has the whip hand. He says that a warm North Atlantic sends heat east on the winds, warming Asia in spring, and allowing a rapid melt of the Tibetan plateau and an early start to the rain-giving monsoon winds. But when the Atlantic is cold, he says, "more snow on the Tibetan plateau in spring and early summer uses up all the sun's heating, because it has to be melted and evaporated before the land can warm." If he is right, then should the ocean conveyor falter in the coming years, the effects for Asia could be even more grievous than for Europe. "There could be a weakened monsoon and less water for all the people who depend on it," says Overpeck.

The tropical school disagrees with this analysis. It holds that both the cooling of the Atlantic and the weakening of the monsoon are likely to be triggered by changes in the heating of the tropics. According to this theory, a cooling of the tropics will weaken monsoon winds and rains, while at the same time sending less warm water north in the Gulf Stream. The theories of the polar and tropical schools are on this occasion not mutually exclusive. In fact, they are mutually reinforcing.

But right now, neither theory offers much enlightenment about what might happen to the Asian monsoon in the coming decades. Global warming driven by accumulating greenhouse gases without a solar component may have different features and different outcomes from the solardominated scenarios of the past. The situation is further complicated because across much of monsoon Asia, warming is itself severely compromised and sometimes extinguished by the aerosols in the Asian brown haze. As we have seen, the haze's biggest impact is on the radiation bal ante between the land surface and the air aloft-a vital parameter in determining the strength of the monsoon. The fear is that the haze may break the seasonal heating cycles between land and ocean, and turn off the monsoon. It hasn't yet, but it may. And, valuable though reconstructed histories of the Asian monsoon may be, it is unlikely that they will ever be able to provide a firm prognosis for the monsoon.

AT THE MILLENNIUM

32

THE HEAT WAVE

The year Europe felt the heat of global warming

At a zoo near Versailles, outside Paris, keepers kept twenty-seven polar bears cool by feeding them mackerel-flavored ice. In Alsace, the electricity company trained water cannons on the roof of a nuclear power reactor as temperatures outside soared to i i8°F. In Rome, tourists queued up to pay the fine for bathing in Trevi Fountain. It seemed like a good deal, they said. Crops died; forests burned; power blacked out as office air conditioners were turned to full power; rivers from the Danube to the Po and the Rhine to the Rhone were at or near record lows.

This was by no standards an ordinary summer heat wave. For one thing, it killed at least 35,000 people: 20,000 in Italy and 15,000 in France. Old people, many of them abandoned in apartments without air conditioning as their families took their August holidays, suffered most. Dehydrated and short of breath, they died by the thousands in temperatures that often exceeded 104°F during the day and stuck close to 86°F at night. It was Europe's hottest summer in at least half a millennium. At the heat wave's peak, on August 13, 2003, the twenty-four-hour death toll in Paris was eight times the norm. In parts of the city, there was a three-week wait for funerals. More than 400 bodies were never claimed by relatives.

It wasn't just the mortuaries that were rewriting their record books. This was the first single weather event that climate scientists felt prepared to say was directly attributable to man-made climate change. In the past, the assumption had always been that any individual weather event could be the product of chance. But the 2003 heat wave was different, says the Oxford University climate scientist and statistician Myles Allen. "The immediate cause, I agree, was a series of anticyclones over Europe. They always raise temperatures in summer, and we can't say those were made any more likely by climate change. But we can say that climate change made the background temperatures within which those anticyclones operated that much higher."

There is no doubt that average temperatures have been rising strongly for years. In much of Europe, the summer average at the start of the new century was o.9 to i.8°F warmer than it was in the first half of the twentieth century. In the summer of 2003, temperatures averaged 4.1 degrees warmer. Judging from past averages, the heat wave was probably a oncein-a-thousand-years event. But, says Allen, "small changes in average temperatures make extreme events much more likely."

One of the nicest confirmations of how exceptional the summer of 2003 was came from a study published at the end of 2004. The French mathematician Pascal Yiou, of the Laboratoire des Sciences du Climat et de 1'En- vironnement, had collected more than 6oo years' worth of parish records showing when the Pinot Noir grape harvest began in the Burgundy vineyards of eastern France. There is a clear relationship between summer temperatures and the start of the harvest, so he extrapolated backward to produce a temperature graph from the present to 1370. The results showed that temperatures as high as those typical in the 19gos were unusual, but had happened several times before. "However," Yiou said, "the summer of 2003 appears to have been extraordinary, unique." Temperatures in Burgundy that year were almost i OF above the long-term average. And if Yiou's formula was accurate, the highest previous temperature had been just 7° above the average. That happened in 1523, in a warm interlude during the little ice age.

"The 2003 heat wave was far outside the range of normal climate," says Allen. It was not impossible that it could have happened without global warming, but it was very improbable. "Our best estimates suggest the risk of such a heat wave has increased between four- and sixfold as a result of climate change." Many scientists continue to argue about how we might recognize "dangerous" climate change, he told me. "Well, for the thousands of victims in Europe in the summer of 2003, it is clear we have already passed that threshold."

And the big heat is only just beginning. Allen says that by mid century, if current warming trends persist, the extreme temperatures experienced in 2003 in Europe could occur on average once every two years. Richard Betts, of Britain's Hadley Centre, says that for people living in cities, the risks are even greater. They are already feeling the worst of climate change, because they also suffer the "urban heat island effect." During heat waves, the concrete, bricks, and asphalt of buildings and roads hold on to heat much better than does the natural landscape in the countryside. In the typically windless, anticyclonic conditions of a European heat wave, the effect is even more marked. The air just stays in the streets and cooks. The effect is especially marked at night, which doctors say is a critical time for the human body to recover from daytime heat.

Betts says global warming will push the urban heat island effect into overdrive. Doubling carbon dioxide levels in the air will triple the effect, he calculates.

33

THE HOCKEY STICK

Why now really is different

It was a seductive image. So seductive that the IPCC put it right at the front of its thousand-page assessment of climate change, published in 2001. The panel hoped that it would become as talked about as the Keeling curve. And scientists gave it a snappy caption: this was the graph they called the "hockey stick." As I don't play hockey, I was initially left wondering why. But if you lay a hockey stick on the ground and look at its shape as a graph, you will see that the long, flat shaft has at the end of it a short but sharply upturned blade, the bit you hit the puck with. And that, according to the IPCC authors, is the shape of the world's temperatures over the past thousand years: about goo years of little or no change, followed by a century with a short, sharp upturn.

The assembly of the data behind the hockey stick graph has become a political cause celebre. It began with high hopes: it was to be the first serious attempt to piece together a global picture of climate over the past millennium from a wide variety of different kinds of sources. Rather than carrying on the well-established work of reconstructing past temperatures from analysis of tree rings, it sought to add in other proxy data from ice cores, coral growth rings, and lake sediments. The idea was to lose the built-in bias of tree-ring chronologies, which must rely on trees from Northern Hemisphere regions outside the tropics, because those are the trees with well-defined annual growth rings.

The hockey stick graph was first put together in 1998. The politics soon got going. That year turned out to be the warmest in the instrumental record. So it wasn't much of a stretch to argue that the hockey stick revealed 1998 to be the warmest year in the warmest century of the past millennium. That got headlines. And brought trouble-not least for the voluble, self-confident, and likable collator of the hockey stick data, Mike Mann. Even though the IPCC published other data sets showing much the same, Mann was accused of concocting a spurious case that late-twentiethcentury warming was exceptional and therefore, presumably, a result of man-made pollution.

It probably didn't help that at the time, Mann was based at the University of Virginia, home of the biggest voice among the climate skeptics: Pat Michaels. Soon Mann was fraud-of-the-month on the Web sites of the climate skeptics. But the criticism went beyond the normal community of climate skeptics: some serious climate researchers expressed misgivings about Mann's methods.

When I finally met Mann, he had moved from Virginia to Penn State University, where he is now director of the Earth Science Systems Center. But the flak had followed. Some was fair; some was unfair; some was deployed as political hand grenades; some was just a part of the normal adversarial flow of scientific debate; and some was just plain personal -like Wally Broecker's bad-mouthing of Mann, quoted at the start of Chapter 23. Mann was even damned in Washington, where Senator James Inhofe of Oklahoma accused him of playing fast and loose with the data, and Representative Joe Barton of Texas summoned him to provide his committee with voluminous details about working procedures and funding. Some called it a McCarthyite vendetta. But Mann seemed up for it, dismissing Inhofe as "the single largest Senate recipient of oil industry money."

I will now entertain some of the criticisms that have rained on Mann, because they matter. But it is worth saying first that nothing I have heard impugns Mann's scientific integrity, credentials, or motives. He is just braver than some, and more willing to have his debates in public-and to fight back when the brickbats start flying. (You can read him in action on the Web site he started with scientific colleagues at www.realclimate.org.) Some researchers have suffered real personal and psychological damage from attacks by skeptics. I hope that won't happen to Mann. I wish more scientists were like him.

First, does the hockey stick fairly represent the temperature record? Does Mann's take-home conclusion, that the last century warmed faster and fur ther than any other in the past thousand years, stand up to scrutiny? The short answer is yes-but only just.

The world of proxy data trends is a statistical minefield. This is partly because the physical material that shows past climate loses detail with time. Tree rings, for instance, get smaller as the tree gets older, so annual and even decadal detail gets lost. "You lose roughly 40 percent of the amplitude of changes," says the tree ring specialist Gordon Jacoby, of Lamont-Doherty. But it goes far beyond that. To make any sense, analysis of a single data set-for instance, from the tree rings in a forest-involves smoothing out the data from individual trees to reveal a "signal" behind the "noise" of short-term and random change. The kind of analysis pioneered by Mann, in which a series of different data sets are merged, involves further sorting and aggregating these independently derived signals, and smoothing the result. And Mann's work involves a further stage: meshing that proxy synthesis with the current instrumental record.

Some, including Jacoby, complain that by combining smoothed-out proxy data from past centuries with the recent instrumental record, which preserves many more short-term trends, Mann created a false impression of anomalous recent change. "You just can't do that if you are losing so much of the amplitude of change in the rest of the data," Jacoby told me. Mann argues the contrary-that in fact he was one of the first analysts in the field to include error bars on his graph. "The error bars represent how much variance is lost due to the smoothing," he says.

But the accusation that he has somehow fixed the data analysis continues to dog him. The most persistent line of criticism, and the one most widely championed by anti-IPCC lobbyists, came from two Canadians: Stephen McIntyre, a mathematician and oil industry consultant, and Ross McKitrick, an economist at the University of Guelph. They claimed to have found a fundamental flaw in Mann's statistical methodology that biased the temperature reconstruction toward producing the hockey stick shape.