The background rate of extinction that Ceballos’s team used was one that says two vertebrate species will go extinct per 10,000 species, per 100 years. Based upon this estimate, nine vertebrate species should have disappeared since 1900. In reality, 477 known species have gone extinct since then. This is a conservative estimate in and of itself, since to confidently declare a species extinct takes a significant amount of time and effort to be certain the species is actually gone. Ceballos exhorts that the evidence produced by this study is “incontrovertible” and that our global human society is without a doubt the cause. Climate change, overpopulation, income inequality, land degradation, and overexploitation of animal species have all contributed to the accelerating rates of extinction.