by Nate Silver
The third claim—that water vapor will also increase along with gases like CO2, thereby enhancing the greenhouse effect—is modestly bolder. Water vapor, not CO2, is the largest contributor to the greenhouse effect.21 If there were an increase in CO2 alone, there would still be some warming, but not as much as has been observed to date or as much as scientists predict going forward. But a basic thermodynamic principle known as the Clausius–Clapeyron relation, which was proposed and proved in the nineteenth century, holds that the atmosphere can retain more water vapor at warmer temperatures. Thus, as CO2 and other long-lived greenhouse gases increase in concentration and warm the atmosphere, the amount of water vapor will increase as well, multiplying the effects of CO2 and enhancing warming.
This Isn’t Rocket Science
Scientists require a high burden of proof before they are willing to conclude that a hypothesis is incontrovertible. The greenhouse hypothesis has met this standard, which is why the original IPCC report singled it out from among hundreds of findings as the only thing that scientists were absolutely certain about. The science behind the greenhouse effect was simple enough to have been widely understood by the mid- to late nineteenth century, when the lightbulb and the telephone and the automobile were being invented—and not the atomic bomb or the iPhone or the Space Shuttle. The greenhouse effect isn’t rocket science.
Indeed, predictions that industrial activity would eventually trigger global warming were made long before the IPCC—as early as 189722 by the Swedish chemist Svante Arrhenius, and at many other times23 before the warming signal produced by the greenhouse signal had become clear enough to be distinguished from natural causes.
It now seems almost quaint to refer to the greenhouse effect. In the mid-1980s, the term greenhouse effect was about five times more common in English-language books24 than the phrase global warming. But usage of greenhouse effect peaked in the early 1990s and has been in steady decline since. It is now used only about one-sixth as often as the term global warming, and one-tenth as often as the broader term climate change.25
This change has largely been initiated by climate scientists26 as they seek to expand the predictive implications of the theory. However, the pullback from speaking about the causes of the change—the greenhouse effect—yields predictably misinformed beliefs about it.*
In January 2012, for instance, the Wall Street Journal published an editorial27 entitled “No Need to Panic About Global Warming,” which was signed by a set of sixteen scientists and advocates who might be considered global warming skeptics. Accompanying the editorial was a video produced by the Wall Street Journal that was captioned with the following phrase:
A large number of scientists don’t believe that carbon dioxide is causing global warming.
In fact, very few scientists doubt this—there is essentially no debate that greenhouse gases cause global warming. Among the “believers” in the theory was the physics professor William Happer of Princeton, who cosigned the editorial and who was interviewed for the video. “Most people like me believe that industrial emissions will cause warming,” Happer said about two minutes into the video. Happer takes issue with some of the predictions of global warming’s effects, but not with its cause.
I do not mean to suggest that you should just blindly accept a theory in the face of contradictory evidence. A theory is tested by means of its predictions, and the predictions made by climate scientists have gotten some things right and some things wrong. Temperature data is quite noisy. A warming trend might validate the greenhouse hypothesis or it might be caused by cyclical factors. A cessation in warming could undermine the theory or it might represent a case where the noise in the data had obscured the signal.
But even if you believe, as Bayesian reasoning would have it, that almost all scientific hypotheses should be thought of probabilistically, we should have a greater degree of confidence in a hypothesis backed up by strong and clear causal relationships. Newly discovered evidence that seems to militate against the theory should nevertheless lower our estimate of its likelihood, but it should be weighed in the context of the other things we know (or think we do) about the planet and its climate.
Healthy skepticism needs to proceed from this basis. It needs to weigh the strength of new evidence against the overall strength of the theory, rather than rummaging through fact and theory alike for argumentative and ideological convenience, as is the cynical practice when debates become partisan and politicized.
Three Types of Climate Skepticism
It is hard to imagine a worse time and place to hold a global climate conference than Copenhagen in December, as the United Nations did in in 2009. During the winter solstice there, the days are short and dark—perhaps four hours of decent sunlight—and the temperatures are cold, with the wind whipping off the Øresund, the narrow strait that separates Denmark from Sweden.
Worse yet, the beer is expensive: the high taxes on alcohol and pretty much everything else in Denmark help to pay for a green-technology infrastructure that rivals almost anywhere in the world. Denmark consumes no more energy today than it did in the late 1960s,28 in part because it is environmentally friendly and in part because of its low population growth. (By contrast, the United States’ energy consumption has roughly doubled over the same period.29) The implicit message seemed to be that an energy-efficient future would be cold, dark, and expensive.
It is little wonder, then, that the mood at Copenhagen’s Bella Center ranged far beyond skepticism and toward outright cynicism. I had gone to the conference, somewhat naively, seeking a rigorous scientific debate about global warming. What I found instead was politics, and the differences seemed irreconcilable.
Delegates from Tuvalu, a tiny, low-lying Pacific island nation that would be among the most vulnerable to rising sea levels, roamed the halls, loudly protesting what they thought to be woefully inadequate targets for greenhouse-gas reduction. Meanwhile, the large nations that account for the vast majority of greenhouse-gas emissions were nowhere near agreement.
President Obama had arrived at the conference empty-handed, having burned much of his political capital on his health-care bill and his stimulus package. Countries like China, India, and Brazil, which are more vulnerable than the United States to climate change impacts because of their geography but are reluctant to adopt commitments that might impair their economic growth, weren’t quite sure where to stand. Russia, with its cold climate and its abundance of fossil-fuel resources, was a wild card. Canada, also cold and energy-abundant, was another, unlikely to push for any deal that the United States lacked the willpower to enact.30 There was some semblance of a coalition among some of the wealthier nations in Europe, along with Australia, Japan, and many of the world’s poorer countries in Africa and the Pacific.31 But global warming is a problem wherein even if the politics are local, the science is not. CO2 quickly circulates around the planet: emissions from a diesel truck in Qingdao will eventually affect the climate in Quito. Emissions-reductions targets therefore require near-unanimity, and not mere coalition-building, in order to be enacted successfully. That agreement seemed years if not decades away.
I was able to speak with a few scientists at the conference. One of them was Richard Rood, a soft-spoken North Carolinian who once led teams of scientists at NASA and who now teaches a course on climate policy to students at the University of Michigan.
“At NASA, I finally realized that the definition of rocket science is using relatively simple psychics to solve complex problems,” Rood told me. “The science part is relatively easy. The other parts—how do you develop policy, how do you respond in terms of public health—these are all relatively difficult problems because they don’t have as well defined a cause-and-effect mechanism.”
As I was speaking with Rood, we were periodically interrupted by announcements from the Bella Center’s loudspeaker. “No consensus was found. Therefore I suspend this agenda item,” said a French-sounding woman, mustering her best English. But Rood articul
ated the three types of skepticism that are pervasive in the debate about the future of climate.
One type of skepticism flows from self-interest. In 2011 alone, the fossil fuel industry spent about $300 million on lobbying activities (roughly double what they’d spent just five years earlier).32, * Some climate scientists I later spoke with for this chapter used conspiratorial language to describe their activities. But there is no reason to allege a conspiracy when an explanation based on rational self-interest will suffice: these companies have a financial incentive to preserve their position in the status quo, and they are within their First Amendment rights to defend it. What they say should not be mistaken for an attempt to make accurate predictions, however.
A second type of skepticism falls into the category of contrarianism. In any contentious debate, some people will find it advantageous to align themselves with the crowd, while a smaller number will come to see themselves as persecuted outsiders. This may especially hold in a field like climate science, where the data is noisy and the predictions are hard to experience in a visceral way. And it may be especially common in the United States, which is admirably independent-minded. “If you look at climate, if you look at ozone, if you look at cigarette smoking, there is always a community of people who are skeptical of the science-driven results,” Rood told me.
Most importantly, there is scientific skepticism. “You’ll find that some in the scientific community have valid concerns about one aspect of the science or the other,” Rood said. “At some level, if you really want to move forward, we need to respect some of their points of view.”
A Forecaster’s Critique of Global Warming Forecasts
In climate science, this healthy skepticism is generally directed at the reliability of computer models used to forecast the climate’s course. Scott Armstrong, a professor at the Wharton School at the University of Pennsylvania, is such a skeptic. He is also among the small group of people who have devoted their lives to studying forecasting. His book, Principles of Forecasting, should be considered canonical to anybody who is seriously interested in the field. I met with Armstrong in his office at Huntsman Hall in Philadelphia. He is seventy-four years old but has a healthy goatee and looks perhaps fifteen years younger.
In 2007, Armstrong challenged Al Gore to a bet. Armstrong posited that what he calls his “no-change” forecast—global temperatures would remain at their 2007 levels—would beat the IPCC’s forecast, which predicted continued warming. Gore never accepted the bet, but Armstrong proceeded to publish the results without him. The bet was to be resolved monthly—whichever forecast was closer to the actual temperatures for that month won the round. Through January 2012, Armstrong’s no-change forecast had prevailed over the IPCC’s forecast of slow-but-steady warming in twenty-nine months out of forty-seven.33
FIGURE 12-2: ARMSTRONG–GORE BET
Armstrong told me that he does not doubt the science behind the greenhouse effect per se. “I mean there has been a little bit of warming,” he told me. “But nobody is arguing about that over the last 150 years.”
But Armstrong does have some grievances with the majority view on global warming.* In 2007, at about the same time he proposed his bet to Gore, Armstrong and his colleague Kesten Green subjected global warming forecasts to what they called an “audit.”34 The idea was to see how well global warming forecasts, especially those produced by the IPCC, abided by his forecasting principles.
The Armstrong and Green paper claimed to find the IPCC forecasts wanting; it suggested that they had failed to abide by seventy-two of eighty-nine forecasting principles. Eighty-nine forecasting principles35 is probably too many.36 Nevertheless, most of Armstrong’s principles are good rules of thumb for forecasters, and when applied to global warming forecasts they can be simplified into what is essentially a three-pronged critique.
First, Armstrong and Green contend that agreement among forecasters is not related to accuracy—and may reflect bias as much as anything else. “You don’t vote,” Armstrong told me. “That’s not the way science progresses.”
Next, they say the complexity of the global warming problem makes forecasting a fool’s errand. “There’s been no case in history where we’ve had a complex thing with lots of variables and lots of uncertainty, where people have been able to make econometric models or any complex models work,” Armstrong told me. “The more complex you make the model the worse the forecast gets.”
Finally, Armstrong and Green write that the forecasts do not adequately account for the uncertainty intrinsic to the global warming problem. In other words, they are potentially overconfident.
Complexity, uncertainty, and the value (or lack thereof) of consensus views are core themes of this book. Each claim deserves a full hearing.
All the Climate Scientists Agree on Some of the Findings
There is an unhealthy obsession with the term consensus as it is applied to global warming. Some who dissent from what they see as the consensus view are proud to acknowledge it and label themselves as heretics.37 Others, however, have sought strength in numbers, sometimes resorting to dubious techniques like circulating online petitions in an effort to demonstrate how much doubt there is about the theory.* Meanwhile, whenever any climate scientist publicly disagrees with any finding about global warming, they may claim that this demonstrates a lack of consensus about the theory.
Many of these debates turn on a misunderstanding of the term. In formal usage, consensus is not synonymous with unanimity—nor with having achieved a simple majority. Instead, consensus connotes broad agreement after a process of deliberation, during which time most members of a group coalesce around a particular idea or alternative. (Such as in: “We reached a consensus to get Chinese food for lunch, but Horatio decided to get pizza instead.”)
A consensus-driven process, in fact, often represents an alternative to voting. Sometimes when a political party is trying to pick a presidential nominee, one candidate will perform so strongly in early-voting states like Iowa and New Hampshire that all the others drop out. Even though the candidate is far from having clinched the nomination mathematically, there may be no need for the other states to hold a meaningful vote if the candidate has demonstrated that he is acceptable to most key coalitions within the party. Such a candidate can be described as having won the nomination by consensus.
Science, at least ideally, is exactly this sort of deliberative process. Articles are published and conferences are held. Hypotheses are tested, findings are argued over; some survive the scrutiny better than others.
The IPCC is potentially a very good example of a consensus process. Their reports take years to produce and every finding is subject to a thorough—if somewhat byzantine and bureaucratic—review process. “By convention, every review remark has to be addressed,” Rood told me. “If your drunk cousin wants to make a remark, it will be addressed.”
The extent to which a process like the IPCC’s can be expected to produce better predictions is more debatable, however. There is almost certainly some value in the idea that different members of a group can learn from one another’s expertise. But this introduces the possibility of groupthink and herding. Some members of a group may be more influential because of their charisma or status and not necessarily because they have the better idea. Empirical studies of consensus-driven predictions have found mixed results, in contrast to a process wherein individual members of a group submit independent forecasts and those are averaged or aggregated together, which can almost always be counted on to improve predictive accuracy.38
The IPCC process may reduce the independence of climate forecasters. Although there are nominally about twenty different climate models used in the IPCC’s forecast, they make many of the same assumptions and use some of the same computer code; the degree of overlap is significant enough that they represent the equivalent of just five or six independent models.39 And however many models there are, the IPCC settles on just one forecast that is endorsed by the entire group.
> Climate Scientists Are Skeptical About Computer Models
“It’s critical to have a diversity of models,” I was told by Kerry Emanuel, an MIT meteorologist who is one of the world’s foremost theorists about hurricanes. “You do not want to put all your eggs in one basket.”
One of the reasons this is so critical, Emanuel told me, is that in addition to the different assumptions these models employ, they also contain different bugs. “That’s something nobody likes to talk about,” he said. “Different models have different coding errors. You cannot assume that a model with millions and millions of lines of code, literally millions of instructions, that there isn’t a mistake in there.”
If you’re used to thinking about the global warming debate as series of arguments between “skeptics” and “believers,” you might presume that this argument emanates from a scientist on the skeptical side of the aisle. In fact, although Emanuel has described himself as conservative and Republican40—which is brave enough at MIT—he would probably not think of himself as a global warming skeptic. Instead, he is a member in good standing of the scientific establishment, having been elected to the National Academy of Sciences. His 2006 book41 presented a basically “consensus” (and extremely thoughtful and well-written) view on climate science.
Emanuel’s concerns are actually quite common among the scientific community: climate scientists are in much broader agreement about some parts of the debate than others. A survey of climate scientists conducted in 200842 found that almost all (94 percent) were agreed that climate change is occurring now, and 84 percent were persuaded that it was the result of human activity. But there was much less agreement about the accuracy of climate computer models. The scientists held mixed views about the ability of these models to predict global temperatures, and generally skeptical ones about their capacity to model other potential effects of climate change. Just 19 percent, for instance, thought they did a good job of modeling what sea-rise levels will look like fifty years hence.