Climate alarmists have a problem. Over the past couple decades, the Intergovernmental Panel on Climate Change (IPCC) has substantially reduced its projections of warming. It had projected global warming by the end of the century to be 3 to 5°C but “best estimates now put us on track for business-as-usual warming” to be only 2 to 3°C (p. 218). Many critics say that these revised estimates are still far too high (see, for example, Nicola Scafetta, The Holy Grail of Climate Change: Quantifying Climate Sensitivity, in E. Calvin Beisner and David R. Legates, editors, Climate and Energy: The Case for Realism, Regnery, 2024), but either way these new lower estimates cause a problem. R. Jisung Park puts it this way: “It is the risk that, once it becomes clear that the path of warming we are currently on is not so obviously catastrophic, climate change ceases to be a problem worthy of scarce mental bandwidth and political capital” (p. 219)—and, one might add, scarce research funds.

This is not to say that Park isn’t concerned about climate change. On the contrary, he thinks that it is an extremely important issue, but rather than approaching it as an existential risk, he aims to reframe it as a “slow burn.” “The central premise of this book is that the subtle setbacks of a changing climate may comprise some of its most important challenges: imperceptibly elevated health risks spread across billions of people; pennies off the dollar of corporate profitability, the immobilizing erosion of coastal and agricultural livelihoods, young people learning less, old people remembering less, many of us arguing more” (p. 3). Climate change “is a very big deal even if it does not turn out to be the end of the world” (p. 234).

Park, an assistant professor at the University of Pennsylvania where he holds appointments in the School of Social Policy and Practice and the Wharton School of Business, aims to examine how climate problems—especially higher temperatures—already damage lives and economies, to project these patterns into the future, and to consider informed, data-driven mitigation and adaptation policies. He draws on his own research and hundreds of other recent studies. The result is an excellent, approachable book, which will inform almost any reader. While painstakingly trying to distance himself from doomsaying alarmism, however, he concludes that the true social cost of carbon (and therefore the appropriate tax) is around $190 per ton—a figure that is far above the usual estimate, such as Nobel-laureate William Nordhaus’s latest estimate around $62 per ton. A $190 Pigouvian tax would increase the cost of gasoline by another $1.70 or so per gallon. It would make coal uneconomical adding about $320 to the current price that’s around $120 per ton. It would also stifle natural gas, adding a tax around $10 per 1,000-cubic-feet to the current price, which is currently about $3.10. (See Institute for Energy Research, 2022.) This tax would drive most fossil fuels off the menu, but cheap fossil fuels have been merely a “caffeine boost” (p. 10) and “sugar high” (p. 19), as Park sees it. (For a well-informed dissenting view on the essential value of fossil fuels, see Timothy Terrell, The Role of Energy in Human Thriving, in E. Calvin Beisner and David R. Legates, editors, Climate and Energy: The Case for Realism, Regnery, 2024.)

Do the hidden costs of warming, the marginal external costs of greenhouse gas emissions, really add up this high? Let’s walk through Park’s key findings.

Climate change is projected to cause more large storms. Storms can destroy physical capital, but their impact on human capital should not be ignored and has now been estimated. Park is coauthor of an important article (Isaac M Opper, R Jisung Park, Lucas Husted, The Effect of Natural Disasters on Human Capital in the United States, Nature Human Behaviour 7 [9] [2023]: 1442–53) that links estimates of property damage from natural disasters—hurricanes, floods, tornadoes, wildfires—in the U.S. between 1990 and 2018 to educational outcomes in the localities they affect. He estimates that the largest of these—something like Hurricane Katrina—are “highly disruptive to learning” and have reduced average test scores by about 0.02 standard deviations and college enrollment by 0.04 standard deviations. Those figures strike me as surprisingly small and not all that “disruptive.” Linking this estimate to other studies cited by Park on how education affects incomes suggests that a major climate-related natural disaster may reduce the life-time earnings of affected students by about $1000 to $2000. The Bureau of Labor Statistics estimates that the life-time earnings of a high school graduate are about $1.3 million ($2.3 million for those with a bachelor's degree). That’s a pretty small percentage. The COVID lockdowns had a substantially larger impact, reducing test scores around .1 (math) to .15 (reading) standard deviations in OECD countries.

But storms aren’t the big issue, it’s heat that mostly causes the “slow burn” as Park sees it. Another of his interesting papers (Hot Temperature and High Stakes Performance, Journal of Human Resources,57, no. 2 [2022]: 400–34) examines the impact of hot weather on Regents Exam scores in New York City. Hot temperature reduces performance by up to 13 percent of a standard deviation. But the key is that many New York City schools aren’t air conditioned. Park notes that “teachers’ unions have petitioned for classroom air-conditioning on multiple occasions” (p. 75), as if taxpayers are simply stingy and won’t spend funds on vulnerable children. New York State spends almost $25,000 per pupil per year, the highest figure in the nation. That’s about two and a half times as much as is spent in my state, North Carolina. Yet all the schools have air conditioning around here. (Spending is even higher in New York City, about $36,000 per pupil—the highest among the one hundred largest school systems in the nation!)

It turns out that air conditioning can be a pretty effective “silver bullet” to beat the heat. Park references a study estimating that in weeks with six or seven days above 90 degrees, productivity in U.S. auto plants fell noticeably (about 8 percent), but the study covers the period from 1994 to 2005, and air conditioning use has spread a lot since then. Despite this putative effect, almost all the auto plants built in the U.S. in the past two decades have been in places that often see temperatures rise above ninety degrees throughout the summer—especially in the South. Industry-level studies such as this one, don’t seem to add up to much in countries like the United States, as shown by a broader study by Ishan Nath. Nath finds that “the effect of heat on worker productivity varies significantly with average national income. For the least adapted firms in poor countries with moderate climates, a day with a maximum temperature of 40° C (104° F) reduces annual output by 0.4 percent, which comes out to roughly the equivalent of erasing a full working day. In middle income countries, the impacts of a similarly hot day were roughly half as large, and in rich countries the effect was negligible on average” (p. 199, emphasis added). These numbers suggest that very hot temperatures can have substantial negative productivity effects, but that air conditioning can erase them.

This brings us to the most important negative effect of higher (or rising) temperatures—their effect on health and longevity. Park argues very effectively that official statistics on heat-related and cold-related mortality are deeply flawed. Heat increases mortality from things like heart conditions and cancer, but they will be listed as the cause of death, not the heat itself. He cites a study by Olivier Deschenes and Michael Greenstone, which uses 72 million death certificates from 1968 to 2002 and finds that an additional day with average temperatures in the 90° F range increases annual mortality by 0.1 percent. That’s very high, but it is not clear that this tells us anything about the relationship today or in the future. At the beginning of this period only 25 percent of American households had air conditioning, by the middle it was 61 percent. Today it is about 88 percent. Perhaps more useful is an important recent study “Valuing the Global Mortality Consequences of Climate Change Accounting for Adaptation Costs and Benefits,” which was published in the Quarterly Journal of Economics. As Park explains, the authors uncover a U-shaped relationship where extreme cold and hot temperatures increase mortality rates, especially for the elderly, and use these estimates to project the rise in global mortality rates by the end of the century, accounting for adaptations and income growth that are likely to occur. They estimate that climate change will increase global mortality rates by 73 deaths per 100,000 per year. If correct, this is very significant and the authors estimate the marginal external cost of an additional ton of CO2 today at $36.60. However, the estimate relies on an extreme and unlikely scenario, the IPCC’s RCP 8.5 scenario, which includes an eight-fold increase in coal consumption. The authors estimate that under the more plausible RCP 4.5 scenario, the cost of carbon is about half this amount—and it is even smaller (only $6.70) when using a more realistic discount rate—3 percent instead of 2 percent—to the RCP 4.5 scenario. The estimated damages of carbon emissions would be even smaller under scenarios that recognize global population is likely to peak within a few decades.

Yet, Park holds firm to the $190 per ton damage estimate—even though the high-end estimates of damages that he discusses don’t seem to add up to anything so high—and points out that $190 multiplied by global emissions adds up to about 6 percent of global GDP. Interestingly if the same number were multiplied by recent historical emissions and compared to recent historical global GDP, it appears that the problem is shrinking relatively. $190 times emissions in 2000 equaled 14.5 percent of its global GDP, for example.

To summarize, I am extremely skeptical of Park’s high-end estimate of the marginal external cost of greenhouse gases and many of his attempts to extrapolate from the past to the future. However, this book deserves to be widely read because it puts the focus where it should be—not on unlikely catastrophes from carbon emissions but on the quotidian effects of climate change. It is a sweeping survey of recent empirical literature on the subject, coherently pulling together findings on the relationship between climate and a wide range of outcomes—education, productivity, and mortality, as noted above, but also things like disease, mental health, aggression, crime, and workplace injuries—and always with an eye toward different impacts among different socio-economic groups both within the U.S. and around the globe.

Park closes with policy considerations, recommending both hefty carbon taxes and lots of adaptation—a veritable “adaptation rush” (p. 243). Bravo to the second of these, which has generally been taboo among those concerned about climate change. However, one must be wary here too because when this involves the government it will inevitably entail substantial rent seeking, such as the proposed plan to protect the New York City area with “urban sea defenses” (p. 242) costing taxpayers (mostly those outside the area) $119 billion!

All in all, the most ubiquitous and surefire adaptation to high temperatures, both today and into the future, is likely to be good old fashioned air conditioning. In China, 60 percent of households now have air conditioning. Park shows that people in poorer countries are especially hard hit by hot temperatures, but I suspect that it won’t be long until air conditioning becomes widespread in developing countries, as their incomes rise and AC prices fall. Park argues that “climate change looks to be a force that could reverse a significant fraction of the gains in the livelihoods made among the world’s bottom billion” (p. 207). I would be surprised, not primarily because he overestimates how much warming will occur, but because he doesn’t seem to realize how much the bottom billion have gained in recent years. The World Bank estimates that GDP per capita (2015 dollars) in the least developed countries (LDCs) has risen about 67 percent since 2000. That’s a tremendous achievement that is unlikely to be reversed by any warming of temperatures—although adopting policies to make energy less affordable (like banning fossil fuels) could put a nasty dent in it.