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, CO 2 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.
“For plants to tolerate warmer temperatures there’s all sorts of things
that they could do,” Silman told me. “They could manufacture special
proteins. They could change their metabolism, things like that. But
thermal tolerance can be costly. And we haven’t seen temperatures like
those that are predicted in millions of years. So the question is: have
plants and animals retained over this huge amount of time—whole
radiations of mammals have come and gone in this period—have they
retained these potentially costly characteristics? If they have, then we
may get a pleasant surprise.” But what if they haven’t? What if they’ve
lost these costly characteristics because for so many millions of years
they provided no advantage?
“If evolution works the way it usually does,” Silman said, “then the
extinction scenario—we don’t call it extinction, we talk about it as ‘biotic
attrition,’ a nice euphemism—well, it starts to look apocalyptic.”