global warming circa 56 million years ago -- 1/15/16

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Today's selection -- from The Horse by Wendy Williams. Climate change has varied widely through time. The temperature increase during the Paleocene-Eocene Thermal Maximum (PETM) of 56 million years ago, in which global temperatures rose at least 5°C for 200,000 years, may have helped lead to the rise of mammals:

"[During the PETM] it was hot. ... In fact, it was as though there was a sudden explosion of heat, as remarkable in its own way as the fall of the asteroid had been 10 million years earlier. Curiously, this explosion of heat also marks the appearance of Polecat Bench, [Wyoming's] horses and primates. This was a time when temperatures in some places shot up by 6 or 8 degrees Celsius in a very short time period, lingered at those heights, then, almost as sud­denly, dropped back down. The cause of this heat spike remains elusive, but it may have been due to large bursts of methane that bubbled up from the deep ocean.

"On temperature charts that track the rise and fall of heat through­out our planet's history, the heat spike looks to me like the outline of the Eiffel Tower. The anomaly is officially called the Paleocene-Eocene Thermal Maximum, PETM for short, but I prefer to think of it as the Eiffel Tower of Heat, with its sharp lines of ascent and descent that mimic so closely the graceful lines of the Parisian landmark. It's a weird event.

"And it's doubly weird that both horses and primates may owe their existence, in part, to its existence: the spike marks the beginning of the Eocene, when not just horses and primates, but most modern mammal groups finally came into their own. Many of our major mammal groups trace their first appearances to this puzzling heat spike. It's as though the whole world had become a giant petri dish brought to a boil by a colossal Bunsen burner. And voilà! A world that was drowning in post­asteroid misery suddenly experienced a global spring. ...

"It's hard for us to imagine in our twenty-first-century world, where we accept the cruel reality of sweltering summers and freezing winters, but for a good deal of Earth's history, including most of the Eocene, the planet enjoyed fairly uniform temperatures. For example, during the Eocene, the world north of the Arctic Circle was so warm that crocodiles flourished there. There were no ice caps, of course, and so much freshwater flowed into the Arctic Ocean that a layer of fresh­water sat like a lens over the salt water. The freshwater Azolla fern was plentiful. Forests of redwoods and walnut trees grew there. Pale­ontologists have found the remains of giant ants usually associated with the tropics."