The Sixth Extinction: An Unnatural History

along the road, we saw a rainbow assortment of species: golden tanagers
the color of buttercups, blue-gray tanagers the color of cornflowers, and
blue-necked tanagers, which are a flash of dazzling turquoise. We also saw
a silver-beaked tanager with a bright red belly and a flock of Andean cock-
of-the-rocks, known for their flamboyant scarlet feathers. Male cock-of-
the-rocks have a disk-shaped crest on the top of their heads and a raspy
call that makes them sound demented.
At various points in earth history, the sorts of creatures now
restricted to the tropics had much broader ranges. During the mid-
Cretaceous, for example, which lasted from about 120 to 90 million years
ago, breadfruit trees flourished as far north as the Gulf of Alaska. In the
early Eocene, about 50 million years ago, palms grew in the Antarctic, and
crocodiles paddled in the shallow seas around England. There’s no reason
to suppose, in the abstract, that a warmer world would be any less diverse
than a colder one; on the contrary, several possible explanations for the
“latitudinal diversity gradient” suggest that, over the long term, a
warmer world would be more varied. In the short term, though, which is
to say, on any timescale that’s relevant to humans, things look very
different.
Virtually every species that’s around today can be said to be cold-
adapted. Golden tanagers and cock-of-the-rocks, not to mention bluejays
and cardinals and barn swallows, all made it through the last ice age.
Either they or their very close relatives also made it through the ice age
before that, and the one before that, and so on going back two and a half
million years. 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.