Leonard: Background
For my first novel, The Inheritors: A Climate Fable, I invoked a theoretical type of extreme tropical cyclone, dubbed by Dr. Kerry Emanuel (who has made it his life’s work to study extreme tropical weather) a “hypercane.” This type of storm has never been observed, but the equations that govern tropical cyclone intensity via thermodynamic processes predict that it would form under certain conditions.
Until recently, it was believed that those conditions could only occur from an asteroid or comet strike or the eruption of an underwater volcano. And, to be sure, hypercanes of the most extreme possible intensity could only occur under such conditions (in which case, particularly with a bolide strike, we’d have a lot of problems to worry about in addition to the storm!). But less extreme conditions could produce “less extreme” hypercanes, and these “lesser” conditions are entirely plausible under anthropogenic climate change.
Without going into too much granular detail on the subject, a tropical cyclone has a maximum potential intensity that it can reach. The storm’s actual intensity will usually be further limited by factors such as wind shear, the entrainment of dry air, and sea surface upwelling, but the potential intensity that it can reach is governed by the temperature at the tropopause (this is the limit of the troposphere, the slice of the earth’s atmosphere where most weather occurs), the surface air humidity, and the sea surface temperatures.
For the same surface air humidity level and tropopause temperature, hotter seas mean higher potential intensities.
For the same sea surface temperature and surface air humidity level, colder tropopause temperatures mean higher potential intensities.
The tropical storm or hurricane essentially acts as a thermodynamic heat engine. It draws hot, moist air from the near-surface, vents some of it upwards through the “chimney” of the eyewall, dissipates some in rainfall and near-surface evaporation, and dissipates some through radiation as this air descends again to the surface. A tropical cyclone seeks thermodynamic equilibrium, and in those that we have always seen, they achieve it through these processes. When a storm is taking in more heat from the ocean than it is currently dissipating, it intensifies. When it is dissipating more heat than it takes in, it’s weakening. When it is dissipating exactly as much as it takes in from the surface, the storm is in thermodynamic equilibrium. This is why its maximum potential intensity is controlled by sea surface temperature and tropopause temperature.
At least, that’s how it’s supposed to work, and how it always has worked within recorded human history. Hypercanes are a different theoretical beast. In the hypercane case, the storm cannot reach thermodynamic equilibrium through the typical processes, because the temperature difference between sea surface temperature and tropopause temperature is too great. It takes in too much heat too fast to vent it out through the eye or dissipate it through surface fluxes. In this case, the equations predict that the storm would experience runaway intensification. The factor that would eventually end up limiting its intensity in this case would be its own internal friction from the high winds it obtained. This is a much slower and less efficient process than the usual tropical cyclone processes, so the storm would reach a far greater intensity than anything we’ve ever observed, and it would maintain it for a long time.
A hypercane would be defined by its internal heat-dissipation processes and whether it was “supercritical” (the technical term for “no equilibrium through fluxes and venting”), rather than having any specific wind speed or surface pressure. It is not “Category Six” or anything of the sort. Indeed, there’s a good case that it should not really even merit the term “hurricane” (or “typhoon” or the equivalent), since it would be a different kind of tropical cyclone, and should not be considered on the Saffir-Simpson Hurricane Wind Scale, if one ever were to form. But on planet Earth, the “weakest” hypercane would probably have central pressures of 700 millibars and winds over 400 mph, based on a solution of the thermodynamic equations for tropical cyclones.
Dr. Emanuel’s early research papers on the topic, “The Maximum Intensity of Hurricanes” (1988) and “Hypercanes: A possible link in global extinction scenarios” (1995), for which he had co-authors, focused on the most extreme scenario: In the case of the 1995 paper, a hypercane formed from 50°C sea water. This, fortunately, cannot happen under even the worst projections of climate change. (It could happen with a bad enough asteroid strike, but again, we’d have pretty serious problems of many kinds in that case!) The hypercane that Dr. Emanuel et al. modeled from these conditions reached almost 200 mb and nearly Mach 1 winds, as Edward Kirby says in the story.
That is not what happens in this novel. It is meant to be hard science fiction about climate change, and climate change is not going to do that to us.
However, the tropical cyclone intensity equations indicate that, at surface air of 75 percent relative humidity, a borderline hypercane could form with merely 35°C water and -70° upper atmosphere temperature.
In my modeling of this storm with the WRF-ARW model (the flagship weather model for research, updated regularly), I was unable to reproduce that. That doesn’t mean the equations are incorrect. They are not. What it means is that the implementation of the WRF model imposed certain compromises and restrictions on how the storm was simulated.
But I was determined not to write anything that could not be explicitly backed up with a simulation to support it, so I used slightly more extreme initial conditions in order to get a hypercane from WRF. Those conditions were 37°C seawater (human body temperature) and -85°C tropopause temperature. These conditions, and an appropriate vertical air profile all the way up, caused the flagship weather model to generate a storm that sure looks like it’s a hypercane, and which the equations predict should be a hypercane.
Sea surface temperatures in the Gulf of Mexico have routinely reached 34°C in recent years. It happens every year. And upper-atmospheric temperatures over Florida, where The Inheritors: A Climate Fable takes place, have reached at least -78°C, with surface air only 3° warmer in a real atmospheric sounding than the sounding that I used for my WRF storm simulation. Admittedly, to get air that cold at the top, you cannot have an ongoing heat wave. But that is precisely what happens in the novel: A record heat wave turns the water into hot soup, and then the heat wave breaks.
We’ve never managed to draw the “inside straight from hell,” as Edward Kirby puts it, all at once before. But there’s no reason we couldn’t in the future.
The Inheritors: A Climate Fable does does not go immediately to the most outrageous, extreme type of hypercane conceivable. It is grounded in what is actually possible with climate change alone, according to the thermodynamic equations—and confirmed possible by simulation.
Explore Hurricane/Hypercane Leonard, the real protagonist of The Inheritors, and if you have the computing resources to do so, you can even simulate it yourself.
Citations:
Emanuel, K. A., 1988: The maximum intensity of hurricanes, Journal of the Atmospheric Sciences, Vol. 45, No. 7, pp. 1143-1155. Article Link
Emanuel, K. A., K. Speer, R. Rotunno, R. Srivastava, and M. Molina, 1995: Hypercanes: A possible link in global extinction scenarios, Journal of Geophysical Research, Vol. 100, No. D7, pp. 13755-13765. Article Link
Li, S., Jaroszynski, S., Pearse, S., Orf, L., Clyne, J., 2019: VAPOR: A Visualization Package Tailored to Analyze Simulation Data in Earth System Science. Atmosphere. 10(9):488. https://doi.org/10.3390/atmos10090488
Lin, J., and K. Emanuel, 2024: Why the lower stratosphere cools when the troposphere warms. Proceedings of the National Academy of Sciences, Vol. 121, e2319228121, https://doi.org/10.1073/pnas.2319228121. Article Link
Skamarock, W. C., J. B. Klemp, J. Dudhia, D. O. Gill, Z. Liu, J. Berner, W. Wang, J. G. Powers, M. G. Duda, D. M. Barker, and X.-Y. Huang, 2019: A Description of the Advanced Research WRF Version 4. NCAR Tech. Note NCAR/TN-556+STR, 145 pp. doi:10.5065/1dfh-6p97
Visualization & Analysis Systems Technologies, Visualization and Analysis Platform for Ocean, Atmosphere, and Solar Researchers (VAPOR version 3.8.0) [Software]. Boulder, CO: UCAR/NCAR — Computational and Information System Lab, 2023. doi:10.5281/zenodo.7779648
(Please leave Dr. Emanuel alone unless you have a legitimate question for him about his research. He does not know me, and I very much doubt he wants to answer questions about somebody’s work of fiction. I’m citing his work just as a scientist would cite it in a paper. The writer of that paper is responsible for it, not the cited authors.)