To the editors:

Who would have thought someone could write a popular book about the physics of climate? I certainly did not. When I created a course at the University of California, Berkeley, to cover climate change through the lens of math and physics, I thought no more than a handful of students would sign up. Fast-forwarding to 2021, that course had nearly a thousand students enrolled and Lawrence Krauss’s book The Physics of Climate Change was near the top of Amazon’s rankings in the categories of both climate change and physics. It turns out that a fair number of people want an accessible introduction to climate change that is more quantitative than the hand-waving fare that tends to fill newspapers and television broadcasts.

If the reader is skeptical of the broad acceptance of climate science espoused in both Krauss’s book and Robert Socolow’s review, I will deal with the elephant in the room right away: Krauss and Socolow are right.

Socolow writes that The Physics of Climate Change should be valued for its approach, in which it communicates to physicists that “climate science is being done well and its major conclusions are correct.” I agree. He also writes that “Krauss displays a fine grasp of the early history of climate science.” I agree. Indeed, that part was a pleasure to read, and it was consistent with my understanding of scientific developments in the 1800s. Socolow writes that “Krauss’s explanation of the connection between atmospheric CO₂ buildup and surface warming is as good as any I have seen.” Again, I agree.

Of course, I would hardly be seen as a good academic if I did not declare some quibbles with my colleague’s review—and I do have some—but I will save them for the end. What is more important to emphasize is our shared view that Krauss’s book is an accurate portrayal of the science. Therefore, I will first lay out some of the big ideas of global warming portrayed in The Physics of Climate Change about which I suspect Socolow and I are in substantial agreement.

At the heart of the book is a tour through the basic facts of global warming as seen through the lens of physics. Early on, Krauss explains how greenhouse gases, including carbon dioxide, affect the flows of energy through the atmosphere. This brings him to a point that I reveal to my students with great fanfare: the Earth’s surface receives twice as much energy in invisible light from the atmosphere as it receives in sunlight. This fact is shockingly counterintuitive for most people. After all, why do we not notice the atmosphere’s warming rays, and, if nuclear fusion powers the Sun, what powers the atmosphere? It takes time to answer these questions, but the time is well-spent. Far from being a curiosity, the back-and-forth flows of this invisible light are essential for the habitability of the planet.

Indeed, this point was appreciated nearly two centuries ago by one of the greatest mathematicians and physicists of all time: Joseph Fourier. It is from here that Krauss embarks on a survey of climate-science history, one that I join Socolow in lauding. As Krauss notes, it was in Fourier’s 1824 publication that the greenhouse effect was first described, although not by that name. Referring to the invisible infrared light as “dark heat,” Fourier wrote, correctly, that

the temperature [of the Earth] can be augmented by the interposition of the atmosphere because heat in the state of light finds less resistance in penetrating the air than in re-passing into the air when converted into dark heat.¹

Climate science is not a new field, but one that will soon be celebrating its 200th anniversary.

The original objectives of climate science were to explain the habitable temperature of the planet and to make sense of the periodic glaciation of North America and Europe that was seen in the geologic record. It is only in the past sixty years or so that climate science has been applied in a concerted way to studying manmade global warming. But even that, as a topic, is not entirely new. In 1896, Svante Arrhenius, a Nobel Laureate and pioneering physical chemist, calculated how much the Earth would warm due to the continued burning of coal, oil, and gas.² Referring to carbon dioxide as “carbonic acid,” Arrhenius concluded in 1908 that “the slight percentage of carbonic acid in the atmosphere may by the advances of industry be changed to a noticeable degree in the course of a few centuries.”³ What he did not foresee was the exponential rise in humanity’s rate of fossil-fuel burning. In 1988, NASA’s James Hansen testified to Congress that manmade global warming had been detected. Instead of centuries, it had taken just eighty years for the change to become noticeable.

One of the ways in which Krauss’s book shines is in providing the reader with context from Earth’s past. “Perspective is everything,” he writes. “The same data viewed in different contexts can appear to present a vastly different picture.” I could not agree more. Some commentators, especially those antagonistic to climate-change mitigation, tend to narrow the perspective, focusing on trivial matters, much as a magician distracts his audience with a flourish of his hand. In that narrowing of perspective, authentic data can then be used to portray global warming as either fictitious or lacking in seriousness. In contrast, Krauss widens the perspective. For example, the book contains no fewer than nine plots of the historical concentration of atmospheric carbon dioxide. Why so many? The reason is that each of those—varying in timespan from sixty years to nearly a million—provides a different way to look at our current predicament.

Through a tour of deep time, Krauss points out that carbon-dioxide concentrations are higher now than anytime over the past 800,000 years. Considering that humans evolved into existence only around 300,000 years ago, this is a staggering fact: no prior generations of humans have ever inhaled outside air with so high a concentration of carbon dioxide. The last time atmospheric carbon-dioxide concentrations were this high was likely three or four million years ago during the time of our non-human ancestor Australopithecus.

The book also treats the reader to the late nineteenth-century calculation by Arvid Högbom that explains how it is possible that humans could transform the atmosphere to this degree. Högbom calculated that the carbon dioxide in the entire preindustrial atmosphere, if reduced to coal, would cover the Earth with a layer of coal only one millimeter thick. Visualized in this way, it is easier to understand how humans have succeeded in burning nearly an equal amount of carbon from fossil fuels extracted from deposits that are localized, but vastly exceeding one millimeter in thickness. Even more worrying, the amount of fossil carbon that humans have the capacity to burn, mainly in the form of coal—which can be converted to gas and oil through industrial processes—is ten times the amount of carbon that was originally in the atmosphere as carbon dioxide. Not only do we have the capacity to alter the atmosphere’s carbon-dioxide content, but we have the capacity to change it dramatically.

To broaden the perspective further, Krauss, in his own analogy, plays the role of the Ghost of Christmas Future. The furthest into the future that most forecasts of climate extend is the year 2100. To his credit, Krauss moves the discussion well beyond 2100 and into the deep future. The reader is presented with the growing scientific understanding—informed by theory, modeling, and the geologic record—of the effective permanence of global warming. Even if humans suddenly stopped burning fossil fuels, the temperature change accrued up to that point would remain more or less at that value for thousands of years.

Whether intentional or not, Socolow seems to imply that this level of persistence is just a guess when he refers to it as an “assumption.” Certainly, scientists do not know exactly how much the Earth’s temperature will change after the cessation of emissions—in the centuries following, it could warm some or it could cool some—but the most likely outcome is that the temperature stays roughly constant in those subsequent centuries. To check this understanding, scientists look to the last major emission of carbon into the atmosphere, which occurred some 55 million years ago. As its name implies, the Paleocene–Eocene Thermal Maximum (PETM) was a hot period in Earth’s past, which was triggered by a release of carbon dioxide several times what humans have emitted so far, but still well within our capacity to emit. In that PETM event, the temperature rose and remained elevated for about 100,000 years, during which animals evolved smaller bodies in response to the sustained heat. This helps to confirm our understanding of the carbon cycle: we know with high confidence that the Earth will not return to preindustrial temperatures by itself within the lifetime of anyone reading this article, or of their grandchildren, or of their grandchildren’s grandchildren.

Even though elevated temperatures from global warming will persist for thousands of years, the Earth will not remain static. Far from it, the great ice sheets of Greenland and Antarctica will respond on their millennial timescales, shrinking their extent and filling up the oceans with, most likely, many meters of sea-level rise. As Krauss notes, Greenland was probably a small fraction of its current size 400,000 years ago even though temperatures then were comparable to what they are today, and a melting of Greenland alone can raise the sea level by seven meters. Indeed, a rough estimate is that the Earth’s glaciers and ice sheets respond in a way that generate a committed sea-level rise—that is, the future increase in sea level once the ice has equilibrated to the current climate—of several meters for every degree Celsius of global warming. Mean global temperatures are already at one degree Celsius above their preindustrial levels and are climbing fast. As Socolow notes, even “a sea-level rise of one meter will be highly problematic for the world’s coastal cities.” So while some commentators dismiss sea-level rise as having little impact over the coming decades, the broader perspective shows just how permanently and dramatically we are transforming the Earth.

Of course, there is much more to be said about the impacts of global warming, but only so much can be fit into a short book. The PETM is a good point of reference for the big-picture implications of extreme global warming, but it goes unmentioned. Also unmentioned is a potentially powerful long-term feedback mechanism: the melting of massive deposits of flammable ice, methane clathrate, that sit at the bottom of the ocean, which, as they release methane, could greatly amplify the human effect on climate. And the book leaves unaddressed many of the most concerning potential consequences of global warming, including mass extinction and the introduction of heat index values that are incompatible with human physiology.

Global warming touches so many aspects of the Earth system and the life it sustains that climate science is naturally interdisciplinary. Perhaps this is what Socolow had in mind when he wrote that the field of climate science “urgently needs to recruit mid-career scientists from neighboring disciplines.” But I do not agree that there is a need, in Socolow’s words, for a “broad structural change required to advance climate science.” Such a statement suggests that climate science as a field is broken, perhaps in need of fixing by some saviors from the physics department. It is not. Physicists are welcomed—even encouraged—to join the study of Earth’s climate, but climate science already has a great many physicists working within its ranks, many of whom entered climate science during or after their PhD in physics. I am one of them. Physicists have made, are making, and will continue to make important contributions to the field.

Socolow also argues that a higher priority should be placed on the scientific effort to understand “the real threat posed by climate change” so that society can make optimal decisions. Even though I am a scientist engaged in that effort, I think this is an unrealistic view of policy making. Any political decision to mitigate emissions hinges not just on scientific facts, but also on the public’s values and their understanding of the facts. Regarding values, I have no doubt that the vast majority of Americans want to bequeath to their children a planet that is just as livable as the one they have enjoyed. Sadly, however, the public is far from understanding even the basic facts of climate change: according to a 2021 Gallup poll, two thirds of Republicans do not even know we have warmed the planet.⁴ Therefore, while the ongoing science is important, I would place a far higher priority on telling the public what we already know. Krauss’s book is a valuable contribution to that effort.

David Romps

Robert Socolow replies:

I am replying briefly here to David Romps’s letter that addresses my review of Lawrence Krauss’s book The Physics of Climate Change. I commented at length in this journal in reply to a previous letter by Nadir Jeevanjee, which also responded to my review of Krauss. Both Romps and Jeevanjee reinforce the assessment, shared by Krauss and me, that climate science has been an invaluable tool in identifying climate change as a serious threat to human well-being. The four of us also agree, I infer, that over the next few decades climate science has the potential to deliver a much-refined understanding of how quickly specific quantitative features of climate change will change.

Romps objects to one sentence in my review, and this offers me an opportunity to separate two issues that, unfortunately, I combined in that sentence. I wrote: “As part of the broad structural change required to advance climate science, the field urgently needs to recruit mid-career scientists from neighboring disciplines.” Romps does not see the need for broad structural change, which he reads to mean fixing a “broken” climate science. Moreover, he understands me to be urging that physicists be allowed to fix it, indeed to be its “saviors.”

My overpacked sentence calls for two only loosely related changes: a restructuring of climate science and a greater contribution to climate science from neighboring disciplines. By “restructuring” I do not mean fixing something that is broken but rather giving greater priority to new tasks. The task I most have in mind is delineating the high tail of the distribution of outcomes, where adverse positive feedbacks are particularly damaging. Right now climate science cannot report how quickly bad outcomes will arrive, such as the first meter of sea-level rise or unlivable combinations of temperature and humidity in specific tropical regions. The underlying reason is the elusiveness of several critical feedback loops. A risk-oriented global climate change program would, first of all, be a larger one, and would, second of all, aim major multi-year campaigns at each of these feedback loops.

Others can construct a more informed list of such campaigns than I can, but such a list would probably include campaigns to elucidate leakage rates from permafrost and methane clathrates, self-reinforcing ice sheet contraction, and the response of low-lying reflective clouds to warming. Each is an area of climate science pursued with dedication but far less aggressively than the scientists themselves would wish. My conclusion from many conversations with climate scientists, especially younger ones, is that they agree that sorting out the worst potential risks from climate change deserves greater priority within climate science. They wish there were less inertia inhibiting such a shift.

As for greater contributions from neighboring disciplines, I am writing here as a recruiter, not as someone looking for saviors. I have spent decades encouraging, with occasional success, my colleagues in physics and in aerospace engineering to seek ways to contribute to climate science. My expectation is that, as a side benefit, influential science and engineering leaders who join climate science will be able to attract additional funding so as to enable faster and deeper learning about the Earth.