With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (30 page)

The argument is a technical one that hangs on how Mann used wellestablished mathematical techniques for classifying data called principal component analysis. McIntyre and McKitrick claimed that Mann's method had the effect of damping down unwanted natural variability, straightening the shaft of the hockey stick and accentuating twentieth century warming. Mann agrees there was some truth in this charge. He analyzed the data in terms of their divergence from twentieth-century levels, and this had the inevitable effect of giving greater significance to data showing the biggest differences from that period.

But the critical charge was that he had somehow created the hockey stick out of nothing-"mining" the data for hockey-stick-shaped trends, as his critics put it. McIntyre and McKitrick produced their own analysis, showing an apparent rise in temperatures in the fifteenth century, which, they claimed, may have been as warm as the twentieth century. The shaft of the hockey stick had a big kink in it. When it was published, in 2005, this analysis was hailed by some as a refutation of Mann's study.

But while Mann was open to attack, so were McIntyre and McKitrick. Would their "refutation" of Mann stand up to critical attention? During 2005, three different research groups concluded that Mann's findings bear scrutiny much better than do those of his critics. They had bent the statistics more than he had, arbitrarily leaving out certain sets of data to reach their conclusion. Remove all the biases, and the real data looked more like Mann's-a conclusion underlined in early 2006, when Keith Briffa, a respected British tree-ring analyst at the University of East Anglia, published the most complete analysis to date, showing the twentieth century to have been the warmest era for at least the past 1,200 years. Briffa's take was confirmed in June 2006 by the U.S. National Academy of Sciences, which, in a long-awaited review of the hockey-stick debate, endorsed Mann's work. The analysts expressed a "high degree of confidence" that the second half of the twentieth century was warmer than any other period in the previous four centuries. But they said that although many places were clearly warmer now than at any other time since 900, there was simply not enough data to be quite so sure about the period before 16oo.

If the key to successful science is producing findings that can be replicated by other groups using different methodologies, then Mann is on a winning streak. Upward of a dozen studies, using both different collections of proxy data and different analytical techniques, have now produced graphs similar to Mann's original hockey stick. Not identical, for sure, but with the same basic features of unremarkable variability for goo years followed by a sharp upturn in temperatures in the final decades.

The one unexplained factor is that most of these studies show paltry ev idence for the medieval warm period and the little ice age. But an answer to that conundrum now seems at hand. There is growing agreement that the most substantial evidence for the existence of both a medieval warm period and a little ice age comes from the northern latitudes. "What we know about the cold in the little-ice-age era is primarily a European and North Atlantic phenomenon," says Keith Briffa. Most interesting, there is growing evidence from a range of new proxy data that other parts of the world were seeing climate trends opposite to those going on in Europe. The tropical Pacific appears to have cooled during the medieval warm period and warmed during the little ice age. One ice core from the Antarctic shows temperatures during the medieval warm period that were 5"F colder than those in the little ice age. Under the circumstances, says Mann, it is not surprising that his more global assessment of temperatures does not spot much difference during these earlier climatic shifts. They undoubtedly had major influences on regional climates, but the cumulative effect on global temperature was small.

It is no part of this book's case that climate didn't change in the past. Parts of the world clearly saw substantial warming and cooling during the medieval warm period and the little ice age. Other parts saw other changes. In the American West, there were huge, century-long droughts during the medieval warm period. Even Broecker, who holds that the little ice age was global, admits that the evidence of a global medieval warm period is "spotty and circumstantial." But there is a good case for saying that over the millennium until the mid-twentieth century, most climate change concerned the redistribution of heat and moisture around the globe rather than big changes in overall heating. Only recently has there been a major additional "forcing," caused by the introduction of hundreds of billions of tons of greenhouse gases into the atmosphere. Recent warming may be the first global warming since the closure of the ice age itself.

The argument over the hockey stick is an interesting sideshow in the debate about climate change. But it remains a sideshow. Right now, it matters little for the planet as a whole whether the medieval warm period was or was not warmer than temperatures today-or, indeed, whether it was a warm period at all. The subtext of the climate skeptics' assault on Mann's hockey stick has always been that if the current warming is shown not to be unique, then somehow the case that man-made global warming is happening evaporates. But this is a spurious argument. Briffa is not alone in arguing precisely the opposite. If it was indeed very warm globally in the medieval warm period, that is truly worrying, he says. "Greater longterm [natural] climatic variability implies a greater sensitivity of climate to forcing, whether from the sun or greenhouse gases. So greater past climate variations imply greater future climate change."

34

HURRICANE SEASON

Raising the storm cones after Katrina

Corky Perret lost everything when Hurricane Katrina hit. His house on the beachfront out on Highway 9o between Gulfport and Biloxi, Mississippi, was reduced to matchwood by 130-mile-an-hour winds, and sucked away by a 3o-foot storm surge that washed up the beach and over the highway. "Nothing is left; it was totally destroyed," he told me weeks later. Out in the Gulf of Mexico, barrier islands that once provided protection against storms had also succumbed. Perret didn't know if hurricanes would be worse in the future, but without the islands, the effects would probably be worse anyway.

The houses along the section of Highway 9o where Perret lived, along with the hotels and resorts, had been built mostly between the 197os and the i 99os, a period of quiet in the Gulf when there were few hurricanes. Hearing reports that no letup is likely anytime soon, some of his neighbors were going for good. They could see only more hurricanes and more havoc. They were off to Jackson or Dallas or Memphis, or anywhere inland. But when we spoke in late 2005, Perret still had his job as director of marine fisheries for the state of Mississippi, and was unsure what to do. He wanted to stay and rebuild, but was that wise?

The year 2005 had been an extraordinary one in the Atlantic. There were so many tropical storms that for the first time meteorologists ran out of names for them. Wilma became the most powerful Atlantic storm ever recorded. Katrina brought an entire U.S. city to its knees. It was the second hurricane year in a row to be described by meteorologists as "exceptional" and "unprecedented," and it came after a decade of rising hurricane activity that stretched the bounds of what had previously been regarded as natural. So what was going on? Are hurricanes becoming more destructive as global warming kicks in? Is there worse to come? The answer matters not just to the people in the firing line around the Gulf of Mexico and the Caribbean, or across the tropics in the Indian Ocean and the Pacific: if there's more severe disruption to oil production in the Gulf, or supertyphoons hit economic powerhouses like Shanghai or Tokyo at full force, we'll all feel the impact.

Until 2005, most of the world's leading hurricane experts were sanguine. The upsurge in the number of hurricanes in the North Atlantic in the previous decade had been just part of a normal cycle. Hurricanes had been strong before, from the 19405 through the 196os. Climate models suggested that even a doubling of carbon dioxide levels in the atmosphere would increase hurricane intensity by only Io percent or so. But that year the consensus was shattered. A flurry of papers claimed that hurricanes had grown more intense during the past thirty-year surge in global temperatures. Not more frequent, but more intense, with stronger winds, longer durations, more unrelenting rains, and even less predictable tracks. The trend was apparent in all the world's oceans, they said. From New Orleans to Tokyo, nobody was immune.

One of the authors, Kerry Emanuel, of the Massachusetts Institute of Technology, said: "My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential and-taking into account an increasing coastal population-a substantial increase in hurricane-related losses in the 21st century." Coming just weeks after the destruction of New Orleans, that sounded like a clear message to Corky Perret and the people of the Gulf Coast. No point in rebuilding, because the next superhurricane could be just around the corner. But the claims produced a schism among the high priests of hurricane forecasting. Many, like the veteran forecaster William Gray, of Colorado State University, said that they saw no upward trend and no human fingerprint. They accused the authors of the latest papers of bias and worse. So who was right?

Hurricanes are an established part of the climate system. There have always been hurricanes. They start off as clusters of thunderstorms that form as warm, humid air rises from the surface of the tropical ocean. As the air rises, the water vapor condenses, releasing energy that heats the air and makes it rise even higher. If enough storm clouds gather in close proximity, they can form what Emanuel calls a "pillar" of humid air, extending from the ocean surface for several miles into the troposphere. The low pressure at the base of the pillar sucks in more air, which picks up energy in the form of water vapor as it flows inward, and releases it as it rises. This lowers the pressure still further.

Meanwhile, the rotation of Earth, acting on the inward-flowing air, makes the pillar spin. If conditions are favorable, a tropical storm can rapidly turn into a hurricane as wind speeds pick up. Its power is staggering: Chris Landsea, of the National Oceanic and Atmospheric Administration, in Miami, has calculated that an average hurricane can release in a day as much energy as a million Hiroshima bombs. Luckily for all concerned, only a tiny fraction of this energy is converted into winds.

Worldwide there are about eighty-five tropical cyclones each year, of which about sixty reach hurricane force. That figure has been fairly stable for as long as people have been counting hurricanes. But the distribution of the hurricanes varies a great deal from year to year. In 2005, for example, the Atlantic was battered but the Pacific was relatively peaceful. On the face of it, global warming is likely to make things worse. The initial pillar of humid air forms only when the temperature of the sea surface exceeds 78°F. As the world's oceans warm, ever-larger areas of ocean exceed the threshold. There has been an average ocean warming in the tropics of 0.5 degrees already.

What is more, every degree above the threshold seems to encourage stronger hurricanes. When Katrina went from a category i to a category 5 hurricane back in August 2005, the surface of the Gulf of Mexico was around 86"F, which, so far as anyone knows, was a record. Whether or not climate change can be blamed for the record sea temperatures (and most would guess that it can), those temperatures certainly helped Katrina strengthen as it slipped across the Gulf from Florida toward the Louisiana coast.

This simple link between sea surface temperatures and hurricane formation and strength has encouraged the view that a warmer world will inevitably lead to more hurricanes, stronger hurricanes, and the formation of hurricanes in places formerly outside their range. But the world is not that simple, says William Gray. What actually drives the updrafts that create the storm clouds, he says, is not the absolute temperature at the sea's surface but the difference in temperature at the top of the storm. Climate models suggest that global warming will raise air temperatures aloft. So, if he is right, while the current sea surface temperatures necessary to create hurricanes may be 78°F or more, it could in future rise to 82° or more. In the final analysis, Gray argues, the hurricane-generating potential of the tropics may remain largely unchanged.

There are other limitations on hurricane formation. However hot the oceans get, air cannot rise everywhere. It has to fall in some places, too, whatever the ocean temperature. And many incipient hurricanes are defused by horizontal winds that lop off their tops. Climate models suggest that global warming will increase wind speeds at levels where they would disrupt hurricanes. Other disruptions include dust, which often blows across the Atlantic during dry years in the Sahara.

But some trends will make big storms more likely. Most tropical storms fizzle out because they lose contact with their fuel-the heat of warm ocean waters. This happens most obviously when a hurricane passes over land, but it also happens at sea. As the storm grows, its waves stir up the ocean, mixing the warm surface water with the generally cooler water beneath. The surface water cools, and that can be the end. In practice, a hurricane can grow only if the warmth extends for tens of yards or more below the surface. But with every year that passes, warm water is penetrating ever deeper into the world's oceans. That is clearly tied to global warming. And it is setting up ideal conditions for more violent thunderstorms. Katrina is again an object lesson here. It continued to strengthen as it headed toward New Orleans, because it moved over water in the Gulf of Mexico that was very warm, not just at the surface but to a depth of more than 300 feet.

The past decade in the North Atlantic has seen a string of records broken. The period from 1995 to 1998 experienced more Atlantic hurricanes than had ever before occurred in such a short time-a record broken only in 2004 and 2005. The 1998 season was the first in a loo-year record when, on September 25, four hurricanes were on weather charts of the North At lantic at one time. And not long afterward came Hurricane Mitch, the most destructive storm in the Western Hemisphere for 200 years. Feeding on exceptionally warm waters in the Caribbean, it ripped through Central America in the final days of October 1998, its torrential rains bringing havoc to Honduras and Nicaragua and killing some io,ooo people in landslides and floods.