A skeptic's guide to climate change
Ever since the rise of concern about climate change during the 1980s, 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 840°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 10.8 square feet of its surface. This is a significant amount. You could run a 60-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 instruments 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 1960s and 1970s 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 1960s and
1970s 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 1. 1 and 1.40 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 1980 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 climate 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.
This is not the end of the story, however. While we can be fairly certain that more greenhouse gases in the air will push the atmosphere to further warming, big uncertainties remain about how the planet will respond. An assessment of the sensitivity of global temperatures to outside forcing —whether to changes in sunlight or the addition of greenhouse gases—mostly revolves around disentangling the main feedbacks: the things changed by an altered climate that influence the climate in turn. Positive feedbacks reinforce and amplify the change, and run the risk of producing a runaway change—the climatic equivalent of a squawk on a sound system. Negative feedbacks work in the other direction, moderating or even neutralizing change.
The current climate models concur with Arrhenius that the planet will amplify the warming. But skeptics believe that nature has strong stabilizing forces that will act as negative feedbacks and head off climate change. They don't by any means agree on how this will work. Some say a warmer world will be a cloudier world, providing us with more shade from the sun. Others, like the respected Massachusetts Institute of Technology meteorologist Richard Lindzen, have argued that the higher reaches of the troposphere might actually become drier, reducing the greenhouse effect of water vapor. Many of these arguments reflect legitimate uncertainty among climate scientists, though some of the negative feedbacks proposed by the skeptics, such as cloud processes, could equally turn into major positive feedbacks and make the IPCC projections too small.
Where does this leave us? Actually, with a surprising degree of scientific consensus about the basic science of global warming. When the science historian Naomi Oreskes, of the University of California in San Diego, reviewed almost a thousand peer-reviewed papers on climate change published between 1993 and 2003, she found the mainstream consensus to be real and near universal. "Politicians, economists, journalists and others may have the impression of confusion, disagreement or discord among climate scientists, but that impression is incorrect," she concluded. The disagreements were mainly about detail. The consensus, stretching from Tyndall through Arrhenius to the IPCC, lived on.
For hard-line skeptics, of course, any scientific consensus must, by definition, be wrong. As far as they are concerned, the thousands of scientists behind the IPCC models have either been seduced by their own doom-laden narrative or are engaged in a gigantic conspiracy. For them, the greater the consensus, the worse the conspiracy. The maverick climatologist Pat Michaels, of the University of Virginia in Charlottesville, says we are faced with what the philosopher of science Thomas Kuhn called a "paradigm problem." Michaels, who is also the state meteorologist for Virginia, one of the United States' largest coal producers, and a consultant to numerous fossil fuel companies, says: "Most scientists spend their lives working to shore up the reigning world view—the dominant paradigm—and those who disagree are always much fewer in number." The drive to conformity, he says, is accentuated by peer review, which ensures that only papers in support of the paradigm appear in the research literature, and by public funding of research into the prevailing "paradigm of doom."
Even if you accept this cynical view of how science is done, it doesn't mean that the orthodoxy is always wrong. The fact that scientists universally agree that the world is round does not make it flat. Many of the same claims that are now made against the global warming "paradigm" were once made about the "AIDS industry" by people who disputed that HIV caused AIDS. Some governments took their side for a long time, and their citizens are now living with the consequences. Where are those skeptics now? Some of them can be heard making the case against climate change.
But all that said, I do think the skeptics are important to the arguments about climate science. The desire for consensus is always likely to lead the mainstream scientific community to don blinkers. This has not only blotted out the arguments of skeptics but also sidelined results from the handful of "rogue" climate models that keep turning up tipping points that could tumble the world into much worse shape than what is currently predicted by the mainstream. One scientist told me in the corridors of a conference in early 2005: "By ignoring these outliers, IPCC has failed for ten years to investigate the possible effects of more extreme climate change."
So, despite their sometimes cynical motives, the skeptics have served a purpose in picking away at the IPCC orthodoxy. As in politics, every good government needs a good opposition. And though their arguments have often been opportunistic and personal, the skeptics have spotted the stifling impact of consensus-building. They are, if nothing else, helping to keep the good guys honest. The pity is that they have not done a better job, by engaging in more real science and less empty rhetoric. And in their enthusiasm to debunk climate change, they have failed to grasp one alarming possibility: that the IPCC could be underestimating, not overestimating, the threat that the world faces.
Lidia Rosa Paz was at a loss. She caught my arm and pointed despairingly into the raging river. Out there, about 50 yards into the water, was the spot where, until days before, she had lived. On the night of October 28, 1998, her shantytown of Pedro Dias, in the town of Choluteca, in Honduras, had been washed away, taking more than a hundred people to their deaths. Lidia had survived, but every one of her possessions was gone. "What will I do now?" she asked. I didn't have an answer.
Hers was one story from a night when floods and landslides ripped apart the small Central American country's geography, leaving more than 10,000 Hondurans dead and 2 million homeless. It was the night that Hurricane Mitch, the most vicious hurricane to hit the Americas in 200 years, came calling, and dumped a year's rain in just a few hours. Choluteca is in southern Honduras, on the Pacific coast, far from the normal track of Caribbean hurricanes. When the radio issued storm warnings that night, neither Lidia nor any of her neighbors took much notice. "Hurricanes never come here," she told me. Or at least they never had.
I was in Honduras a couple of weeks after the hurricane had struck. The devastation was appalling. Huge floods had rushed down rivers and into the capital, Tegucigalpa, in the mountainous heart of the country, ripping away whole communities. A thousand people lost their lives beneath a single slide that landed on the suburb of Miramesi. Another stopped just short of the American embassy in the capital. Rivers changed their paths right across the country, obliterating towns. And flash floods on steep hillsides buried whole communities under mud. Sixty percent of the country's bridges were destroyed, along with a quarter of its schools and half its agricultural productivity, including nearly all its banana plantations. The first visitors to the southern town of Mordica reported, "All you can see is the top of the church." Ministers said the country's economic development had been put back twenty years.
For tens of millions of people across the world, the violence of Mitch is an omen. Many climatologists believe that Mitch, a ferocious hurricane made worse by the warm seas that allowed it to absorb huge amounts of water from the ocean, was a product of global warming—and a sign of things to come for the hundreds of millions of inhabitants of flood-prone river valleys and coastal plains across the world; for those living on deforested hillsides prone to landslips; and for many millions more who do not yet know that they are vulnerable in a new era of hyperweather. People like Lidia before Mitch hit.
Those who do not believe that global warming is a real and dangerous threat should visit places like Choluteca and talk to people like Lidia. It may not convince them that climate change is making superhurricanes and megafloods. But it will show them the forces of nature untamed and the human havoc caused when weather breaks its normal shackles. For hundreds of millions of people, these issues are no longer a matter for computer modeling or debate in the corridors of Congress or future forecasts.
They are about real lives and deaths. The question is not: Can we prove that events like Mitch are caused by climate change? It is: Can we afford to take the chance that they are?
The year 1998 was the warmest of the twentieth century, perhaps of the millennium. It was also a year of exceptionally wild weather, and few doubt that the two were connected. That year, besides the storms, the rainforests got no rain. Forest fires of unprecedented ferocity ripped through the tinder-dry jungles of Borneo and Brazil, Peru and Tanzania, Florida and Sardinia. New Guinea had the worst drought in a century; thousands starved to death. East Africa saw the worst floods in half a century—during the dry season. Uganda was cut off for several days, and much of the desert north of the region flooded. Mongol tribesmen froze to death as Tibet had its worst snows in fifty years. Mudslides washed houses off the cliffs of the desert state of California. In Peru, a million were made homeless by floods along a coastline that often has no rain for years at a time. The water level in the Panama Canal was so low that large ships couldn't make it through. Ice storms disabled power lines throughout New England and Quebec, leaving thousands without power or electric light for weeks. The coffee crop failed in Indonesia, cotton died in Uganda, and fish catches collapsed in the Pacific off Peru. Unprecedented warm seas caused billions of the tiny algae that give coral their color to quit reefs across the Indian and Pacific Oceans, leaving behind the pale skeletons of dead coral.
All a coincidence? Not according to the IPCC. Some of the damage was caused by an intense outbreak of a natural climate cycle in the Pacific known as El Nino. Every few years, this causes a reversal of winds and ocean currents across the equatorial Pacific, for a few months taking rains to drought regions and droughts to normally wet areas. But as we shall see in Chapter 30, there is growing evidence that El Ninos are becoming stronger and more frequent under the influence of global warming. This is probably part of a pattern identified by the IPCC, in which, all around the world, the weather is becoming more extreme and more unpredictable as the world warms. And 1998, the warmest year yet, was the epitome of the trend.
The heat is intensifying the hydrological cycle. Globally, average annual rainfall increased by up to 10 percent during the twentieth century, because warming has increased evaporation. Locally, the trends are even stronger. The floods that inundated Mozambique in 2000 occurred because maximum daily rainfall there had risen by 50 percent. In the eastern U.S., the proportion of rain falling in heavy downpours has increased by a quarter. In Britain, winter rain falls in intense downpours twice as often as it did in the 1960s. There are similar patterns in Australia, South Africa, Japan, and Scandinavia. Even the Asian monsoon has become more intense but less predictable. At the same time, dry areas in continental interiors have become drier, causing deserts to spread. The year 1998 was the first in a run of years of intense drought that stretched from the American West through the Mediterranean to Central Asia.
At the time of this writing, no other year has been as hot as 1998—and no other year so climatically violent. Unless, that is, you were caught in one of the record number of tropical storms in the North Atlantic in 2005. But if you want to know what the first stage of climate change is shaping up to be like, look no further than 1998.
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