| Proc Natl Acad Sci U S A | |
|---|---|
| Ceballos, Gerardo; Ehrlich, Paul R | Moreover, we have unleashed a mass extinction event, the sixth in roughly 540 million years, wherein many current life forms could be annihilated or at least committed to extinction by the end of this century |
Stearns, Beverly Peterson; Stearns, S.
7.