“…About 56 million years ago Earth became very hot again—as hot as it ever has been. This was the so-called Paleocene-Eocene Thermal Maximum, the PETM. Are we headed for that instead?
“This is the same as the Great Dying. Scientists no longer think that methane played a major role in the PETM. But there is a different lesson. The PETM is notable for the rapid warmup—it happened not across millions of years but in as short as 10,000 or 20,000 years. This is very rapid from a geological standpoint, although, again, it’s 100 times slower than today. The warming spike happened on top of an already warm planet; it took the planet to temperatures higher than anything that’s documented in the geological record.
“The PETM reached levels of heat that would be dangerous for human beings, and we are already encountering wet-bulb temperatures [an estimation of the effect of temperature and humidity] that are deadly in some parts of the world. The PETM would have been a world where large parts of the planet were too hot for humans. So people say, “Oh, look, life adapted.” There was a massive miniaturization of some species. Horses shrunk 30 percent in order to adapt [smaller bodies, with a higher ratio of surface area to volume, have less trouble shedding heat]. The reality is that when you see something so dramatic as horses shrinking by 30 percent, that means there would have been very large amounts of maladaptive species; there would be a massive loss of life along the way. The idea is that human beings can just adapt, but those selective pressures don’t favor anyone…”
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