Since we’re apparently not going to do much about global warming for at least a few years, we might as well examine past events for some insight into what our own immediate future may hold. There is no shortage of climate-linked catastrophes to go by. Mass extinctions, regardless if they coincide with giant volcanoes or space rock impacts, always involve massive changes in global climate. But many climatologists zero in on one geologically recent event, called the Paleocene-Eocene Thermal Maximum or PETM, as the best analogue to the loaded smokestacks the planet is breathing through now.
The PETM starts about 55.5 million years ago, just 10 million years after the more well-known K-T event that signaled the end of non-avian dinosaurs and about every other animal over 50 pounds in weight on land or by sea. The latter part of the Paleocene was a strange time, both in terms of biota and climate. Before the changes all started, the temperature was comparable to today, everywhere except the polar regions. Mammals had emerged from their hidey-holes and evolved into more diverse forms. Some clear precursors of modern grazers and predators are seen in the fossil record, but very few blades of anything like modern grass yet exists to support later, more familiar ice-age megafauna. Raptors were long gone, but a new family of giant, carnivorous flightless birds had evolved to fill a most similar, terrifying niche. Early primates diversified and prospered, some living comfortably beyond the Arctic Circle. Even the geography was getting recognizable, as tectonic forces slowly guided the major continents toward their current destinations and shapes.
But all those plants and animals are in trouble: the temperature is set to spike and the climate about to change, quickly and dramatically. It will come in two rapid-fire pulses, one of which seems to have happened almost overnight by the patient standards of geology. When it’s all over, the average global temperature will be 5 to 8 °C (9 to 14 °F) warmer. Which is about where we may be headed in the next century or two.
The Paleocene is a fascinating epoch that saw the evolutions of some of the most vicious—and some of the most adorable—mammals. One of the most interesting was the plesiadapiformes, a sort of early-model lemur, and the forerunners of ambulocetids, a sort of mammalian crocodile that would eventually evolve into all the whales and dolphins on Earth today.
But what really characterizes the PETM came into view just a few decades ago, as paleontology and geochemistry conspired to tease out the details of what was up until then a poorly-defined extinction event. Some of the initial clues are a sharp drop in some microorganisms, even deep water foraminifera, and large blooms in others, including various kinds of nasty algae. The world warmed up—a lot. The resulting acidification almost certainly devastated reefs and appears to have reached into the mud lying at the bottom of the deepest trenches.
Generally speaking, when you see huge shifts in climate like this driven by natural causes (unless someone wants to get into some kind of uncharted alien intervention or the rise and fall of intelligent cynodonts burning fossil fuels), it means more than one factor may have ganged up. There are plenty of contenders, like a volcanic field in Canada dating to about the right time; or changes in earth’s orbit that, in the absence of ice caps, may have tipped the climate over; or the clathrate gun hypothesis; or events as dramatic as the great carbon comet impact, and even as subtle as the Isthmus of Panama pinching off the Atlantic and Pacific, and on and on. But the evidence or magnitude (or both) of all these is lacking in some way. We really don’t know.
What is clear is a bunch of plants and animals died, en masse, on land and by sea. Everything from muck-sucking marine worms to some of the latest furry inventions of evolution disappeared. Whole taxons up to the family level were wiped out. Other vertebrate lines, particularly mammals, evolved furiously, radiating into new eco-niches, presumably under intense selective pressure. And it happened fast by natural standards, in two waves. The first may have lasted only a few millennia, and the second perhaps 20,000 years or less.
By the time it was over a new epoch had dawned, the Eocene. It’s an epoch that would end up stretching out for so long it might need to be broken into several distinct sub-epochs. And the world soon grew tired of the heat, and settled down to a more equitable climate.
The PETM can tell us a lot about how high and how fast the temperature might change, and it gives us hints about the biological fallout. The time bore enough similarities to today, both in terms of climate and biota, that it’s about the best we have to go by when guessing what lies in wait for us—and soon.
But here’s the thing that will be different this time: there seems to have been no permanent ice at either pole during the Paleocene. So sea levels didn’t change much, even where temperatures soared. Perhaps a score of thick mountain glaciers melted, the thermal expansion of water might have kicked in, and raised sea levels a few feet. But that appears to be the size of it. This time, sea levels will eventually rise at least dozens of feet and maybe 100 or more, and fairly quickly. The effect that would have on current human civilization, where about one-half the world’s 8 billion people live in poverty near coastlines or large rivers or bays, is chilling to contemplate.