The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

For most of the Pleistocene temperatures were significantly lower than they are now—such is the rhythm of the orbital cycle that glacial periods tend to last much longer than interglacials—and so an evolutionary premium was placed on being able to deal with wintry conditions. Meanwhile, for two and a half million years, there’s been no advantage in being able to deal with extra heat, since temperatures never got much warmer than they are right now. In the ups and downs of the Pleistocene, we are at the crest of an up. To find carbon dioxide levels (and therefore, ultimately, global temperatures) higher than today’s requires going back a long way, perhaps as far as the mid-Miocene, fifteen million years ago. It’s quite possible that by the end of this century, CO2 levels could reach a level not seen since the Antarctic palms of the Eocene, some fifty million years ago. Whether species still possess the features that allowed their ancestors to thrive in that ancient, warmer world is, at this point, impossible to say.