Barnosky: The reason that's relevant now is because we're entering into a phase where we have got the same two things acting in a much more scaled up way.
Narrator: For years there has been a very polarized debate between those who favor humans as the main cause of the extinctions versus those who blame climate change. In a recent review, Barnosky concluded it was both.
Barnosky: I think what's significant about this study is it shows that really in some places, we can pretty clearly document that it was climate change. In other places, we can pretty clearly document that climate probably wasn't so important – that it was human population pressure.
Narrator: For Science Today, I'm Larissa Branin.