The Biotest perch exposed to warm nuclear effluent could adapt only up to a point, and then they hit a physiological ceiling. At a certain threshold — 4.6 degrees Celsius above their Biotest high temperatures — the perch were more not likely than their unadapted Baltic-dwelling neighbors to survive a temporary heat shock. The perch had adapted, but they were living on the edge of survivability, and this only became clear by hitting them even harder.

Humans will also have their own ceiling. Sherwood and Huber note that the second law of thermodynamics “does not allow an object to lose heat to an environment whose TW” — that’s the wet bulb temperature again — “exceeds the object’s temperature, no matter how wet or well ventilated.” In other words, once a certain threshold is reached, we can no longer cool ourselves by convection or evaporation. Our environment becomes, in effect, a steam room; sweating, our great adaptive tool, is no longer effective. Those savanna areas where evolutionary changes were said to have taken place — places like Kenya’s Turkana Basin¹⁸ — were hot, but not necessarily the wet-bulb hellscapes that some climate change models are projecting.

Studies have identified the “critical thermal maximum” as 35 degrees Celsius, which is the same as human skin temperature. (Our core temperature is slightly higher, at 37 degrees.) As Jeremy Pal and Elfatih Eltahir write in Nature Climate Change, “35 degrees Celsius is the threshold value of Tw beyond which any exposure for more than six hours would probably be intolerable even for the fittest of humans, resulting in hyperthermia,” or heatstroke. Vanos points out that the survivability metric is “just based off of pure physics, [and has] never really been tested.”

Sometime this century, however, it may be. In recent meteorological history, wet-bulb temperature has rarely maxed out at more than 31 degrees Celsius. And good thing: One South African mine study found that “90 percent of all heatstroke cases occurred at wet-bulb temperatures of 30.0 degrees Celsius.”¹⁹ But Pal and Eltahir project that the deadly 35-degree threshold will increasingly be reached by the end of the century, in regions like the Persian Gulf and the densely inhabited regions around the Indus and Ganges river basins.²⁰ They are talking about temporary heat waves, and, presumably, people living in those temperatures will, if they can, make behavioral adaptations: staying out of the sun, retreating into air conditioning, burrowing underground like desert tortoises.

But not everyone will be able to burrow and withdraw. And as temperatures continue to climb and indoor air-conditioning systems are strained, some adaptive responses will begin to kick in. In the very long-term, could we adapt to the new climate normal the way our savanna-dwelling forebears once did? Could our organs change, like the European perch’s did? Could our sweat become more better suited for aridity? Would we become taller and longer limbed so as to dissipate heat more reliably? Could our metabolisms undergo a permanent shift?

Humans are master adapters. The “variability selection” hypothesis, promoted by Rick Potts, director of the Smithsonian Institution’s Human Origins Program, suggests that we did not evolve to meet the needs of one environment but to the exigencies of novel and changing environments. “Key hominid adaptations, in fact, emerged during times of heightened variability,”²¹ Potts writes. We are adaptive machines, living in a remarkable variety of conditions across the planet. Witness those living at elevated environments, with much less oxygen than the rest of us require, presumably thanks to genetic variations passed on from ancestors.²² Similarly, archaeologist Patrick Roberts and anthropologist Brian Stewart argued this month in Nature Human Behavior that humans have carved out a unique ecological niche, which they call “the generalist specialist,” defined by an ecological plasticity.²³ “Not only did [our species] occupy and utilize a diversity of environments, but it also specialized in its adaptation to some of these environmental extremes.”

But it’s hard to pinpoint specific climate-related adaptations.

Arslan Zaidi, a researcher in genetics in the Department of Anthropology at Penn State, has tried to find some. He is the co-author of a paper²⁴ investigating the idea that variations in human nostril shape may be the result of adaptations to climate — the thought being that people in warmer climates have wider nostrils, which helps as a sort of internal air conditioning for the human body. The paper finds that nares width is only “weakly correlated” with temperature, and that other factors, like sexual selection, might have played equally important roles.

This speaks to the difficulty in trying to anticipate a human physiological response to rising temperatures. “Evolution,” Zaidi says, “is not as deterministic as one might think.” Randomness enters the picture, via genetic drift and mutation, so it “impossible to predict with confidence what’s going to happen in the future.” Natural selection, he adds, is “a weak evolutionary force in humans, especially when it comes to traits that are not lethal or absolutely essential for our survival at an early age.” Heat is already a threat, but we don’t go out of our way to subject our bodies to it.

Evolution also takes a long time. “There is a wait for beneficial mutations to arise — if they ever will — then reach a high enough frequency in the population to be ‘noticed’ by selection,” Zaidi says.“We don’t know how fast adaptation will or can occur,” says epidemiologist Woodward. “But it seems unlikely that biological change will be sufficient. The rate of change is the critical factor, and warming on this occasion is occurring orders of magnitude faster than ever before, as far as we can tell.”

It’s not clear whether climate change will become the latest episode in an evolutionary history full of successful adaptations, or whether the changes this time around will simply be too rapid. But we are warming up our very own Biotest Lake, and the clock is ticking.

Update: An earlier version of this story misstated the date that the Swedish plant went online. It was 1980. The story also misidentified Jennifer Vanos’ affiliation. She is at UCSD.