With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (4 page)

On his death, Keeling's bosses at Scripps were kind enough to call the Keeling curve "the single most important environmental data set taken in the loth century." Nobody disagreed. One writer called him the man who "measured the breathing of the world."

Thanks to Keeling's curve, the ideas of Arrhenius and Callendar were rescued from the dustbin of scientific history. It seemed he was right that people could tamper with the planetary thermostat. Climatologists, many of whom had predicted in the 196os that natural cycles were on the verge of plunging the world into a new ice age, began instead to warn of immi nent man-made global warming. As late as the early 197os, U.S. government officials had been asking their scientists how to stop the Arctic sea ice from becoming so thick that nuclear submarines could not break through. But by the end of the decade, President Jimmy Carter's Global 2000 Report on the environment had identified global warming as an urgent new issue, and the National Academy of Sciences had begun the first modern study of the problem.

A vast amount of research has been conducted since. For the past decade and a half, the IPCC has produced regular thousand-page updates just to review the field and pronounce on the scientific consensus. But in some ways, mainstream thinking on how climate will alter as carbon dioxide levels rise has not advanced much in the century since Arrhenius. Thanks to Keeling, we know that those levels are rising; but little else has changed.

Only in the past five years, as researchers have learned more about the way our planet works, have some come to the conclusion that changes probably won't be as smooth or as gradual as those imagined by Arrhenius-or as the scenarios of gradual change drawn up by the IPCC still suggest. We are in all probability already embarked on a roller-coaster ride of lurching and sometimes brutal change. What that ride might feel like is the central theme of this book.

2

TURNING UP THE HEAT

A skeptic's guide to climate change

Ever since the rise of concern about climate change during the i98os, the scientists involved have been dogged by a small band of hostile critics. Every time they believe they have seen them off, the skeptics come right back. And in some quarters, their voices remain influential. One leading British newspaper in 2004 called climate change a "global fraud" based on "left-wing, anti-American, anti-West ideology." And the best-selling author Michael Crichton, in his much-publicized novel State of Fear, portrayed global warming as an evil plot perpetrated by environmental extremists.

Many climate scientists dismiss the skeptics with a wave of the hand and return to their computer models. Most skeptics, they note, fall into one of three categories: political scientists, journalists, and economists with little knowledge of climate science; retired experts who are aggrieved to find their old teachings disturbed; and salaried scientists with overbearing bosses to serve, such as oil companies or the governments in hock to them. If the skeptics are to be believed, the evidence for global warming and even the basic physics of the greenhouse effect are full of holes. The apparent scientific consensus exists only, they say, because it is enforced by a scientific establishment riding the gravy train, aided and abetted by politicians keen to play the politics of fear. Much of this may sound hysterical. But could the skeptics be on to something?

First, the basic physics. As we have seen, much of this goes back almost two centuries. Fourier and Tyndall both knew that the atmosphere stays warm because a certain amount of the short-wave radiation reaching Earth from the sun is absorbed by the planet's surface and radiated at longer infrared wavelengths. Like any radiator, this warms the surrounding air. They knew, too, that this heat is trapped by gases-such as water vapor, carbon dioxide, and methane-that have a "greenhouse effect," without which the planet would be frozen, like Mars. But you can have too much of a good thing. Our other planetary neighbor, Venus, has an atmosphere choked with greenhouse gases and is broiling at around 84o"F as a result. And that is a worry. For, thanks to Keeling's curve, there can be no doubt now that human activity on planet Earth is raising carbon dioxide in the atmosphere to roughly a third above pre-industrial levels.

The effect this has on the planet's radiation balance is now measurable. In 2001, Helen Brindley, an atmospheric physicist at Imperial College London, examined satellite data over almost three decades to plot changes in the amount of infrared radiation escaping from the atmosphere into space. Because what does not escape must remain, heating Earth, this is effectively a measure of how much heat is being trapped by greenhouse gases-the greenhouse effect. In the part of the infrared spectrum trapped by carbon dioxide-wavelengths between 13 and 19 micrometers-she found that less and less radiation is escaping. The results for the other greenhouse gases were similar.

These findings alone should be enough to establish for even the most diehard skeptic that man-made greenhouse gas emissions are making the atmosphere warmer. Climate models developed by the U.S. government's space agency, NASA, estimate that Earth is now absorbing nearly one watt more than it releases per io.8 square feet of its surface. This is a significant amount. You could run a 6o-watt light bulb off the excess energy supplied to the area of the planet that a modest house occupies.

More contentious is whether we can actually feel the heat. Direct planet-wide temperature records go back 150 years. They suggest that nineteen of the twenty warmest years have occurred since 1980, and that the five warmest years have all been since 1998. Could the thermometers be misleading us? That has to be a possibility. The records, after all, are not a formal planetary monitoring system; they are just a collection of all the data that happen to be available.

Two important criticisms are made. One is that satellite sensors and in struments carried into the atmosphere aboard weather balloons do not back up the surface thermometers. The instrument data suggest that if air close to the surface is warming, that warming is not spreading through the bottom 6 miles of the atmosphere, known as the troposphere, in the way that climate scientists predict. If true, this is very worrying, says Steve Sherwood, a meteorologist at Yale University and author of a study of the problem: "It would spell trouble for our whole understanding of the atmosphere."

Not surprisingly, skeptics have given great play to the suggestion that satellites "prove" the surface thermometers to be at fault. Not so fast, says Sherwood. The satellite data are untrustworthy, because they measure the temperature in the air column beneath a satellite and cannot easily distinguish between the troposphere, which is expected to be warming, and the stratosphere, which should be cooling as less heat escapes the lower atmosphere. Further, satellites do not provide direct measurements in the way that thermometers do. Temperatures have to be interpreted from other data, which creates errors. The scientists running the instruments accept that the results "drift." Every week, says Sherwood, they recalibrate their satellite measurements according to data from weather balloons. In effect, therefore, the long-term average data from satellites are creatures of the balloon data.

So how good is the balloon data? Here Sherwood found a surprisingly obvious flaw-obvious, at any rate, to anyone who has left an ordinary thermometer out in the sun. The sun's ultraviolet rays shining on the bulb force the temperature reading continuously upward so that it no longer measures the air temperature. The true air temperature can be captured only in the shade, unmolested by the sun's direct rays. Thermometers on weather balloons, it turns out, are no different. They are "basically cheap thermometers easily read by an electric circuit," says Sherwood. They, too, show spurious readings when in the sun.

Meteorologists have recently fixed the problem by shielding the thermometers attached to weather balloons inside a white plastic housing. But this was rarely done thirty years ago. Sherwood concludes that "back in the T96os and 197os especially, the sun shining on the instruments was making readings too high." And that, he says, is the most likely explanation for why balloon measurements do not reveal a warming trend.

Two further observations back up this interpretation. First, spurious readings should not be a problem when the sun goes down, so 196os and 197os readings at night should be reliable. And sure enough, nighttime balloon data over the past thirty years show a warming trend. Second, the data from both balloons and satellites show a strong cooling in the stratosphere-which is likely only if more heat is truly being trapped beneath it, in the troposphere.

Another serious criticism of the surface-temperature trends is that measurements by surface thermometers have been biased by the growth of cities. The concrete and tarmac of cities retain more heat than rural areas, especially at night. The argument is that over the decades, more and more temperature-measuring sites have become urban, so the temperature trends reflect the urbanization of thermometers rather than real warming. The "urban heat island," as researchers call it, is undoubtedly real. Cities do hang on to heat. But is it skewing the global data?

This seems unlikely. The largest areas of warming have been recorded over the oceans, and the greatest magnitude of warming is mainly in polar regions, distant from big centers of population. The skeptics should finally have been silenced by a neat piece of research in 2004 by David Parker, of the Hadley Centre for Climate Prediction, part of Britain's Met Office in Exeter. He figured that the urban heat island effect should be most intense when there is no wind to disperse the urban heat. So he divided the historical temperature data into two sets: one of temperatures taken in calm weather, and the other of temperatures taken in windy weather. He found no difference. So, while nobody denies that the urban heat island effect exists, it is not sufficient to upset the reliability of global trends in thermometer readings.

There are other disputes, which we might call "second order," because they are about circumstantial evidence of climate change. Is it true, for instance, that temperatures at the end of the twentieth century were really hotter than at any other time in the past millennium? That is the claim made by U.S. researcher Michael Mann. He produced a controversial graph dubbed the "hockey stick," which used data from tree rings and other "proxy" sources to show that the millennium comprised 950 years of stable temperatures and a sudden upturn at the end. The arguments, which we will look at in more detail later, continue as to whether Mann's data are correct. And in the end, we may simply never know enough about past temperatures to be sure. But however the dispute goes, it doesn't change the basic science of the greenhouse effect. And in any event, it should be no part of the case for future climate change that past climate did not vary. It rather obviously did. As this book will argue, there is no comfort in past variability. Quite the contrary.

Similarly, there is room for uncertainty about the cause of the rise in temperature over the past 150 years, which is, depending on how you draw your average for recent years, put at a global average of between i. i and 1.4 F. The warming itself is real enough, but that doesn't necessarily mean that humans are to blame. It could be natural.

One argument is that more radiation reaching us from the sun can account for most of the warming of the past 150 years. This case was made best by the Danish scientists Knud Lassen and Eigil Friis-Christensen in 1991. They found a correlation between sunspot activity, which historically reflects the energy output of the sun, and temperature changes on Earth from 1850 onward. Time-based statistical correlations are notoriously tricky, because they can happen by chance; but the Danes' correlation looked convincing, and prominent skeptics took up the case. However, newer data have convinced Lassen that solar activity cannot explain more recent climate change. Declining sunspot activity since i98o should have reduced temperatures on Earth. Instead, they have been rising faster than ever.

Overall, this particular dispute has been good for science, and the skeptics can claim a tie. Climate scientists who once put all global warming since 1850 down to the greenhouse effect now concede that up to 40 percent was probably due to the sun. Solar changes may have been the main cause of the substantial global warming in the first half of the twentieth century, for instance. But there is no way the sun's activity can explain the dramatic warming since 1970.

Both sides play one last trick. Web sites run by skeptics regularly publish temperature graphs from particular places that show no warming, suggesting that the whole idea of global warming is a myth. But climate scientists are almost as guilty when they indiscriminately attribute every local warming to global trends, whereas well-understood local cli mate cycles may be the more likely cause. The case for setting up local climate "watchtowers" in parts of the planet known to be sensitive to climate change, such as the Arctic, remains strong. But they will never provide unambiguous proof of global change, because global warming has not canceled out natural variations in local climate systems. What is so remarkable about recent trends is not local events but the global reach of warming. Virtually no region of the planet is spared. This is in contrast to natural oscillations that mostly just redistribute heat. The greenhouse effect is putting more energy into the entire climate system. Occasionally that causes cooling and other weird weather, but mostly it causes strong warming.

To summarize the current state of affairs: the global trends are real. No known natural effect can explain the global warming seen over the past thirty years. In fact, natural changes like solar cycles would have caused a marginal global cooling. Only some very convoluted logic can avoid the conclusion that the human hand is evident in climate change. Indeed, to think anything else would be to flout one of the central tenets of science. The fourteenth-century English philosopher William Ockham coined the principle of Ockham's razor when he argued that, if the evidence supported them, the simplest and least convoluted explanations for events were the best. Changes in greenhouse gases are the simple, least convoluted explanation for climate change. And those changes are predominantly man-made.