How likely is it that the world will warm not just by 2 degrees Fahrenheit by the end of this century, but by 11 degrees? What would happen to the planet? And, to avoid a much hotter world, what should we do and when should we do it? In Climate Shock, Gernot Wagner, the lead senior economist at the Environmental Defense Fund, and Martin L. Weitzman, a professor of economics at Harvard University, address those questions.

They claim that if the concentration of carbon dioxide in the air rises from its current level of 400 parts per million (ppm) to 700 ppm, the probability that the world will warm by more than 11 degrees is 11 percent. If that were to happen, they claim, we would experience serious rises in ocean levels and more and more-violent storms, to name two major consequences. To avoid that, they propose a Pigovian tax of at least $40 per ton of carbon dioxide emitted and, they say, “We must act now.” If we don’t substantially reduce our carbon usage soon, at some point we will find ourselves using “geoengineering” to reduce the earth’s temperature by a few degrees. And they fear that geoengineering could get out of control and have unintended consequences. Better, they say, to impose a stiff carbon tax now.

How convincing is their case? Not very. They could be right, but they don’t tell us nearly enough to justify their most important claim: an 11 percent probability of a much warmer climate. And, while they often profess relative certainty in the body of the book, they tend to relegate some of the most important doubts and controversies to the footnotes. That’s a problem because few people read footnotes. Also, the authors judge competing policy responses to global warming asymmetrically. Specifically, they advocate a stiff carbon tax throughout, always claiming that it’s the obviously right thing to do, without ever considering whether such a tax might have unintended consequences. But when they consider geoengineering solutions—technological methods to alter the climate that they admit would cost a small fraction of the carbon tax—they raise the specter of unspecified unintended consequences and even construct a scenario in which a mysterious foreign government could engage in unchecked geoengineering.

Why a tax? / Consider their claim, early in the book, that “the latest research suggests that climate change will lead both to more and bigger storms” (italics in original). But turn to the footnotes and you see that they are less certain about those storms than they seem in the text. First, they point to a 2005 study’s finding that “hurricanes had intensified over the preceding three decades.” Then they write, “The ensuing scientific debate seems to have settled with the conclusion that climate change does indeed lead to more intense hurricanes but that their frequency may not change (or may even go down slightly).” Notice three things: First, they admit that there’s a debate among respected climate scientists, something you don’t get a hint of unless you read the footnote. Second, Wagner and Weitzman reference only the sources on their side, the main one being Massachusetts Institute of Technology meteorologist Kerry A. Emanuel. Third, one of the consensus views of the scientific debate, that the number of hurricanes will not increase, directly contradicts their conclusion in the body of the book. Finally, in that same footnote, they write, “That scientific debate isn’t settled, yet the physical signs are sadly clear.” How could the physical signs of something that hasn’t happened yet be clear, sadly or otherwise?

In their discussion of geoengineering, they point out that the 1991 eruption of the Mount Pinatubo volcano, with its spewing of sulfur into the atmosphere, reduced global temperatures by about 0.5 degrees Celsius (0.9 degrees F) the following year. We could achieve Mount Pinatubo-like results, and in a controlled way, by pumping comparable amounts of sulfur dioxide into the air: “About 20 million tons of sulfur dioxide managed to wipe out the global warming effects of 585 billion tons of carbon dioxide in the atmosphere” (italics in original). The authors also point out that such solutions would cost somewhere between $1 billion and $10 billion dollars per year. “Those,” they write, “are the engineering costs of getting temperatures back down to preindustrial levels.” That cost, they point out, amounts to just pennies per ton, versus a hefty $40 carbon tax per ton. So why not go the sulfur route?

Yet Wagner and Weitzman argue against that idea. Even if doing so would reduce temperatures by the same amount everywhere, they argue that “it would still be hard to agree on the ‘right’ temperature.” That’s true, but that’s also true of any policy to deal with global warming, including their proposed carbon tax. If they are right about the effects of a carbon tax, the higher the tax, the lower the earth’s temperature will be. Why would it be easier to agree on the temperature one wants to achieve with a carbon tax than on the temperature one wants to achieve with geoengineering? They also argue that to use a geoengineering solution, “we would need strong, global institutions and well-formed governance processes.” But that’s also true of a globally agreed-upon carbon tax.

They reveal the depth of their opposition to geoengineering when they write that “[we] hope we are wrong about the seemingly unstoppable drive toward geoengineering.” Geoengineering technology, they write, “is too cheap and too readily available.” That’s a strange criticism coming from economists. We economists tend to think that, all other things equal, solutions that are cheap and readily available can’t be too cheap or available. Their fear, they write, is that a solution would be “geoengineered in some ‘rogue’ fashion.” But as noted above, any government solution to climate change has problems. A carbon tax could be implemented in a “rogue” fashion also, used to go after politically unpopular industries and firms and to exempt those with political pull.

Wagner and Weitzman do address other geoengineering solutions, such as carbon capture and storage and ocean fertilization. The latter refers to dumping “iron or other nutrients into surface waters to make them more fertile grounds for natural carbon dioxide uptake.” An advantage of this is that it would encourage the growth of fish. But, they argue, most of these approaches “run head-on into the free-rider problem” because, they assert, these efforts would be so much more expensive than shooting sulfur “into the stratosphere to create an artificial sun shield.” But in fact, the iron solution is actually quite cheap. Besides, although there is a free-rider problem, that is hardly a show stopper; their preferred option of a stiff carbon tax also has large free-rider problems. What happens, for example, if China and India rely on the United States to impose a tax and they don’t impose one?

Wagner and Weitzman write, “Loss of human lives, ecosystems, or food aren’t [sic] compensated so readily by increased consumer electronics.” They seem to be saying that global warming will reduce food output, but one of the main effects of increasing carbon dioxide, as climate scientist Craig Idso has pointed out, is greater plant growth, which means more food, not less. One would have expected two economists to examine the benefits of global warming as well as the costs. Only in a footnote do they tell us that some of the effects of carbon “may also decrease” the estimated social cost of carbon. Unfortunately, they don’t tell us which. Do they have agriculture in mind? We don’t know. This is another example of their hiding doubts in the footnotes.

11 percent alarmism / Wagner and Weitzman’s strongest and most important claim is that there is an 11 percent chance the earth will warm by 11 degrees or more. How do they achieve this level of specificity? By using as inputs some fairly sketchy probability estimates made by other climate scientists and the Intergovernmental Panel on Climate Change. If you have total trust in those estimates, you can trust the authors’ 11 percent scenario. But there is good reason to not trust the inputs. Judith Curry, a climate scientist at the Georgia Institute of Technology, writes:

More than a dozen other observation-based studies have found climate sensitivity values lower than those determined using global climate models, including recent papers published in Environmetrics [sic] (2012), Nature Geoscience (2013), and Earth Systems Dynamics (2014). These new climate sensitivity estimates add to the growing evidence that climate models are running “too hot.” Moreover, the estimates in these empirical studies are being borne out by the much-discussed “pause” or “hiatus” in global warming—the period since 1998 during which global average surface temperatures have not significantly increased.

Assume for a moment that you were convinced by the authors’ claim that there is an 11 percent chance of an 11 degree (or more) increase in world temperatures, the results of which would be very bad. What would follow? As noted, they want a $40 per ton carbon tax immediately. They write: “We once had decades to turn the climate ship around. Not anymore.”

Is that because the high temperatures are just around the corner? No. Recall that in their view the 11 percent probability happens if we get to 700 ppm of carbon dioxide in the atmosphere. Unless we change our behavior, they argue, this would happen by 2100. But they seem to waffle substantially on this estimate. They get to 700 ppm by 2100 by counting “other greenhouse gases.” But they give us enough information to calculate that we will hit 700 ppm in 2165 because of an increase of 2 ppm of carbon dioxide per year and a current level of 400 ppm. Finally, they write that we won’t know the precise temperature that results from increased greenhouse gas concentrations until “hundreds of years into the future.”

But whether the amount of time we wait until knowing is 85 years or 200 years, why the urgency? The closest they come to an answer is an analogy with a “civilization-as-we-know-it-altering” asteroid that has a 5 percent chance of hitting Earth in 100 years. They write, “We wouldn’t say that we should be able to solve the problem in at most a decade, so we can sit back and relax for another 90 years.” True. But there are more than a few decades between now and 90 years from now. Or, back to their estimate that we could hit very high temperatures by 2100, there are more than a few decades between now and 2090. Moreover, no one I know on either side of this debate advocates that we do nothing. We all want to know more and develop various technologies—and all of that is happening. Even if we decide that a carbon tax is the right path, we couldn’t wait 10 or even 20 years until we know more?

Schneider strategy / The authors quote the late climatologist Steve Schneider, who lamented at a 2010 conference that he and fellow climatologists felt obligated to attend and discuss solutions because they had not been listened to decades earlier. If Schneider’s name sounds familiar, you may remember him for writing the following:

On the one hand, as scientists we are ethically bound to the scientific method, in effect promising to tell the truth, the whole truth, and nothing but—which means that we must include all the doubts, the caveats, the ifs, ands, and buts. On the other hand, we are not just scientists but human beings as well. And like most people we’d like to see the world a better place, which in this context translates into our working to reduce the risk of potentially disastrous climatic change. To do that we need to get some broad-based support, to capture the public’s imagination. That, of course, entails getting loads of media coverage. So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have. This “double ethical bind” we frequently find ourselves in cannot be solved by any formula. Each of us has to decide what the right balance is between being effective and being honest. I hope that means being both.

At book’s end, the authors cast aside all doubts, writing: “We ought to do everything in our power to prevent further climatic changes. It’s not a question of if we should set a price on carbon but how high it should be.” But wait. What happened to the geoengineering solution that could be done for pennies on the dollar or, more accurately, pennies on the $40? Is this ending the authors’ version of the Schneider strategy?