Kerry Emanuel wasn’t the first scientist ever to postulate that hurricanes might intensify in a warmer world. He was merely the first to perform the detailed theoretical work necessary to flesh out that speculation. The concept itself, however, had emerged on several prior occasions, including one very unexpected one just after the turn of the nineteenth century. These episodes—which are based upon the author’s research but should not be taken to represent a complete historical survey—suggest that there may have always been something fairly intuitive about the relationship between hurricanes and heat.
No one today remembers Alfred Russel Wallace as a scientist who offered any particular insight into hurricanes. If he is remembered at all, it’s usually as one of history’s classic second fiddles—the man who mailed Charles Darwin an essay describing the very concept of evolution by natural selection that Darwin had been holding close to his chest for well over a decade, fearful of publishing because of the reaction he knew it would provoke.
But if Wallace nearly beat Darwin, he was many decades ahead of his time in speculating, albeit only briefly, that changes to the Earth’s climate might strengthen the planet’s most powerful and destructive breed of cyclonic storms. The thought appeared in 1903’s Man’s Place in the Universe, a book in which Wallace expounded a theory of how the Earth had been perfectly designed to accommodate human life, and which included a remarkable paragraph about hurricanes:
It is outside the zone of the equable trade-winds, and in a region a few degrees on each side of the tropics, that destructive hurricanes and typhoons prevail. These are really enormous whirlwinds due to the intensely heated atmosphere over the arid regions already mentioned, causing an inrush of cool air from various directions, thus setting up a rotatory motion which increases in rapidity till equilibrium is restored. The hurricanes of the West Indies and Mauritius, and the typhoons of the Eastern seas, are thus caused. Some of these storms are so violent that no human structures can resist them, while the largest and most vigorous trees are torn to pieces or overturned by them. But if our atmosphere were much denser than it is, its increased weight would give it still greater destructive force; and if to this were added a somewhat greater amount of sun-heat—which might be due either to our greater proximity to the sun or to the sun’s greater size or greater heat-intensity, these tempests might be so increased in violence and frequency as to render considerable portions of the earth uninhabitable.
From a modern scientific perspective, much of Wallace’s logic here seems foreign. Today scientists view hurricanes as natural heat engines that draw their energy from warm ocean water and moist air. So contrary to Wallace’s assumptions, an “inrush of cool air” would weaken a hurricane, as would greater atmospheric “density” or mass (which would reduce wind speed by increasing the pressure, which is simply the weight of air molecules from above). Still, it’s striking that Wallace thought an increase in heat from the sun, bearing down on the Earth, might strengthen these deadly storms. Perhaps he knew of the theories, not uncommon at the time, that invoked variations in solar intensity to explain changes in weather as well as longer-term climate cycles. It’s tantalizing to imagine Wallace going one step farther and surmising that the planet might warm, and hurricanes strengthen, not because of direct changes in solar intensity but due to an increased atmospheric capacity to “trap” heat that originated from the sun as it radiated back upward from the Earth.
In a 1954 essay on hurricanes, the Swedish meteorologist Tor Bergeron, who had been a key member of the Bergen School and is perhaps best known today for his discovery of so-called occluded fronts, echoed Wallace by speculating about possible changes in hurricane intensity. Then he took an added cognitive step. As Bergeron wrote:
Another problem, of much more far-reaching consequences, presents itself. What kind of secular changes may have existed in the frequency and intensity of the hurricane vortices on our Earth? And what changes may be expected in the future? We know nothing about these things, but I hope [to] have shown that even quite a small change in the different factors controlling the life history of a hurricane may produce, or may have produced, great changes in the paths of hurricanes and in their frequency and intensity. A minor alteration in the surface temperature of the sun, in the general composition of the earth’s atmosphere, or in the rotation of the earth, might be able to change considerably the energy balance and the balance of forces within such a delicate mechanism as the tropical hurricane. During certain geological epochs, hurricanes may have been just as frequent as the cyclones of our latitudes, or they may have occurred all over the oceans and within all coastal regions, and they may have been even more violent than nowadays. During other periods they may have been lacking altogether.
Not only does this passage predict avenues of hurricane research being pursued today, such as paleotempestology (see Chapter 14, “Hurricane Climatology”). Bergeron echoes Wallace’s speculation about the relationship between solar temperatures and hurricane strength but then goes farther, explicitly mentioning the effect of possible changes in the “general composition of the earth’s atmosphere,” as might occur due to increasing concentrations of greenhouse gases like carbon dioxide. Bergeron’s discussion of the “energy balance” within hurricanes implies a thermodynamic understanding of these monster storms, a perspective that inevitably lends itself to the notion that hurricane intensity might increase with more heat added to the system.
Apparently Bergeron also lectured on the hurricane-climate linkage in the 1960s. Florida State University meteorologist T. N. Krishnamurti recalls a Bergeron seminar given at the University of California-Los Angeles in the 1960s that discussed global warming and its potential intensifying effect upon hurricanes. Krishnamurti, who was teaching at UCLA at the time, distinctly remembers Bergeron describing the storms as heat engines and discussing how they might grow stronger. “Back then it was surprising to hear,” Krishnamurti says. Perhaps Bergeron’s thoughts in this area sprang from familiarity with the work of a mentor, Svante Arrhenius, a fellow Swede and one of the first scientists involved in early discoveries about the greenhouse effect. (Arrhenius recommended Bergeron to his friend Vilhelm Bjerknes, who in turn brought the young scientist to Norway to join the Bergen School.)
Finally, in addition to Wallace and Bergeron, the scientists who originally came up with the heat-engine theory of hurricanes also mooted the implications of their work for hurricane intensification. In an interview, Joanne Simpson (formerly Malkus) recalled that back in the days when they published their influential work on hurricane structure and energetics, she and Herbert Riehl had “played around” with the idea that the strength of the hurricane heat engine they had described might be altered by a change in the climate. “We thought, well, if the ocean gets a lot warmer, would we have more hurricanes, or more intense hurricanes?” says Simpson. But, she continues, “This was just talk, we didn’t write anything up about it, although we showed clearly that the local heat source is necessary for the storm to deepen.”
Given their timing, such speculations about a hurricane-climate linkage by Bergeron, Malkus, and Riehl shouldn’t seem entirely surprising. By the 1950s and 1960s, two central pieces of scientific theory could sustain such thinking. First, hurricanes had been described as heat engines; and second, there were reasons to suspect that humans might be involved in a heating of the world’s oceans. In tandem with an active Atlantic hurricane era and Herbert Riehl’s introduction of the heat-engine theory, global warming itself was reemerging as a subject of inquiry and, for the first time, of at least limited public concern.
Nevertheless, the idea of hurricane intensification caused by climate change was not destined to be built upon at the time, for a mul titude of reasons. Soon CISK would emerge on the scene in the world of hurricane science, deemphasizing the role of the oceans. As the mid-century cooling period began, meanwhile, global warming slipped from the political and scientific radar. So did hurricanes themselves, as the frequency of major storms declined significantly in the Atlantic basin. Neither subject would become a matter of much significant public attention again until the late 1980s, when Emanuel’s hurricane theory linked them permanently, even as global warming became a topic of political discussion—and very intense Atlantic hurricanes began to reappear.