“What Should Carbon Cost?” by Gilbert E. Metcalf, is unsuccessful in answering the question of how to calculate the most appropriate carbon tax rate. The uncertainties are too great. And probable impacts of climate change are beyond the scale of usual economic analyses. The use of integrated assessment models (IAMs) to calculate climate damage is like employing Newtonian physics to analyze phenomena far outside its range of applicability.

Metcalf states that “the richer future generations are compared to us, the less we should feel compelled to incur costs now to make them better off. That leans toward a high discount rate.” But it is quite possible that future generations will be poorer than us as a result of the climate change that is already baked into current atmospheric carbon dioxide—and because perpetual economic growth is not sustainable. If so, ethical and economic considerations suggest a negative discount rate.

Further, Metcalf mentions “low-probability, high-damage” catastrophes such as runaway heating caused by thawing permafrost. There is much uncertainty about such events. But “uncertainty” is different from “low probability.” Their probability may be high with the surprising six-degree-Celsius increase in economist William D. Nordhaus's cited analysis—three times the limit recommended by climate scientists.

DICK WALTON Billings, Mont.

METCALF REPLIES: Economist Frank Ramsey's rule states that the richer future generations are, the higher the discount rate. Walton is correct that the poorer they are, the lower that rate should be. But 2,000 years of history suggest our best estimate is of rising economic well-being going forward. As for permafrost melting, the probability may be high if we fail to reduce emissions, but the ultimate damages after accounting for unforeseen feedbacks still involve considerable uncertainty. The larger point, however, is that the possibility of catastrophes complicates measuring the social cost of carbon.


In “Tales of the Dying Brain,” Christof Koch describes near-death and out-of-body experiences in humans. He does not mention nonhuman animals, however.

Many animal species, vertebrate and invertebrate, exhibit a temporary deathlike state called thanatosis, or tonic immobility, when confronted or physically touched by predators. Physically, vertebrates exhibit reduced respiratory rate, bradycardia and hypotension. Recovery takes at least minutes after the threat is removed.

Does tonic immobility relate to human near-death experiences?

BERND ESCHE via e-mail

Koch's article reminded me of a comment by a Baptist minister who had attended many bedside deaths. He related the story of one individual who, on his deathbed, saw glimpses of heaven and called out, “They are all there,” naming deceased members of his family. When the minister was asked about this near-death experience, his response surprised and pleased me. He said he believed that such an experience of heaven was whatever the dying person believed heaven would be.

JEAN HOWARD via e-mail

KOCH REPLIES: In response to Esche: the possibility of animals having near-death experiences (albeit shorn of their cultural context) during thanatosis is fascinating. Death feigning may indeed trigger such a state. The methodological challenge would be to train animals to subsequently report something about their internal condition—say, by pressing levers or some other simple motor behavior—in a way accessible to an external, trained observer.

Howard reports a wise comment by a Baptist minister. To me, this astute observation—that people from different faiths and cultures experience their own idiosyncratic heaven and hell—argues against the hypothesis that near-death experiences reveal a single, universal truth about the hereafter. Instead everyone seems to be granted a very different vision of an afterlife that is formed by their own expectations and upbringing.


Ben Santer does a very good overview of our nation's current president in “Failure to Lead” [Forum]. He does leave out one very important occurrence, however: Donald Trump's trade adviser circulated a memo, dated January 29, 2020, that warned the West Wing that the coronavirus was coming and would be a pandemic. Of course, history is rife with moments in which disasters would have been lessened or stopped if leaders had listened to their subordinates.

RALPH KUNDTZ, Jr. Akron, Ohio


In “The Vaccine Quest,” Charles Schmidt describes the new DNA and RNA techniques being pursued for a speedy vaccine for the novel coronavirus SARS-CoV-2. The article left me with a nagging question about possible side effects: With these methods, once your cells make virus antigens, do they not display them on their surface—akin to waving a red cape at a bull? Could a vaccine thus cause your antibodies to savage your own cells?

I cannot help but think of the 1966 sci-fi movie Fantastic Voyage, in which a group is shrunk and placed into a scientist's body. In the film, a miniaturized Raquel Welch injures some of the scientist's cells, causing multiple antibodies to attack her.

ROBERT A. LEE Brewster, N.Y.

SCHMIDT REPLIES: The answer to Lee's perceptive question is that the number of human cells expressing SARS-CoV-2's spike protein is very small, and their location is limited to sites close to where a given vaccine is administered. Those cells could potentially be killed by the immune reaction to the spike protein. But they are so few in number that there would be no adverse effects if they were eliminated.


“Landing on the Right Foot,” by Leslie Nemo [Advances], discusses problems caused by two different types of “feet” used in measurements: the international foot and the U.S. survey foot. It cites an engineer's account of a building that was constructed near a landing strip and had to lose its top floor at the last minute to avoid obscuring planes' glide path because of that discrepancy.

But the article says the ratio between the two types is only 0.999998. With that calculation, the difference would only come to a small fraction of an inch for a mile-high building.

ALBERT D. MASON via e-mail

THE EDITORS REPLY: A handful of letters requested more information about how such a small difference in the length of the foot could lead to a building needing to be a floor shorter. A structure's location is designated in coordinates, which are typically rendered in meters and then converted to feet—and these values can be in the millions. At such high numbers, the otherwise minuscule differences between U.S. survey and international feet add up. In this case, surveyors and designers from different companies used different versions of the foot, resulting in the building being built two feet farther south than intended. This result meant the top of the building intersected with planes' east-west flight paths to a nearby airport. Had everything gone to plan, the flight path would have sat just south of the building.