The sixth mass extinction

There have been five mass extinction events in the history of the earth, the first at the end of the Orodovician (445mya), the second at the end of the Devonian (375mya), the third at the end of the Permian (254mya), the fourth at the end of the Triassic (204mya), and the fifth at the end of the Cretaceous (71mya). It's been commonplace among conservation biologists to refer to current human-caused extinctions as the sixth mass extinction.¹

It's been clear for a long time that current rates of extinction are much higher than they have been at any time in the recent geological past.²There are many caveats associated with estimates of current rates of extinction, and there are lots of uncertainties, but there's little doubt that extinction rates are much higher now than they were before the rise of modern humans. What hasn't been as clear is whether the scale of the extinction really is as great as that of the mass extinctions of the past -- until now.

Writing in this week's Nature, Anthony Barnosky and colleagues grapple with the data that are available on contemporary extinctions and use a variety of techniques to compare current rates of extinction with the 75% extinction threshold used to define mass extinctions in the fossil record. As you can see, the estimated extinctiion intensities are already verging on that threshold.³


After reviewing all of the evidence this is what the authors conclude:

Even taking into account the difficulties of comparing the fossil and modern records, and applying conservative comparative methods that favour minimizing the differences between fossil and modern extinction metrics, there are clear indications that losing species now in the 'critically endangered' category would propel the world to a state of mass extinction that has previously been seen only five times in about 540 million years. Additional losses of species in the 'endangered' and 'vulnerable' categories could accomplish the sixth mass extinction in just a few centuries. It may be of particular concern that this extinction trajectory would play out under conditions that resemble the 'perfect storm' that coincided with past mass extinctions: multiple, atypical high-intensity ecological stressors, including rapid, unusual climate change and highly elevated atmospheric CO₂. The huge difference between where we are now, and where we could easily be within a few generations, reveals the urgency of relieving the pressures that are pushing today's species towards extinction.

Yes, Virginia. We are living in the sixth mass extinction in the history of life. And we are responsible for it.