There have been five mass extinctions in Earth’s history. Now we’re facing a sixth.

By Brad Plumer
11 February 2014

(Washington Post) – There have been five mass extinction events in Earth’s history. In the worst one, 250 million years ago, 96 percent of marine species and 70 percent of land species died off. It took millions of years to recover.

Nowadays, many scientists are predicting that we’re on pace for a sixth mass extinction. The world’s species are already vanishing at an unnaturally rapid rate. And humans are altering the Earth’s landscape in far-reaching ways: We’ve hunted animals like the great auk to extinction. We’ve cleared away broad swaths of rain forest. We’ve transported species from their natural habitats to new continents. We’ve pumped billions of tons of carbon-dioxide into the atmosphere and oceans, transforming the climate.

Those changes are pushing more species to the brink. A 2007 report from the Intergovernmental Panel on Climate Change suggested that 20 to 30 percent of plant and animal species faced an increased risk of extinction this century if the planet keeps warming (though scientists are still debating these exact numbers, with some going far higher).

So what happens if the extinction rate speeds up? That’s one of the questions that New Yorker science writer Elizabeth Kolbert explores in her excellent new book, The Sixth Extinction, an in-depth look at the science of extinction and the ways we’re altering life on the planet. We spoke by phone this week about the topic. […]

Brad Plumer: Nowadays, scientists are aware of five mass extinction events in the past, starting with the End-Ordovician Extinction 450 million years ago and up to the End-Cretaceous Extinction that killed off the dinosaurs 66 million years ago (see chart). Is there a lot we still don’t know about what caused these events?

Elizabeth Kolbert: Yes, absolutely, although it depends. So I think with the dinosaurs, [the asteroid theory] is quite widely accepted at this point. There was a big paper in Science on this subject last year, although there are still a couple of holdouts.

The worst mass extinction of all time came about 250 million years ago [the Permian-Triassic extinction event]. There’s a pretty good consensus there that this was caused by a huge volcanic event that went on for a long time and released a lot of carbon-dioxide into the atmosphere. That is pretty ominous considering that we are releasing a lot of CO2 into the atmosphere and people increasingly are drawing parallels between the two events.

The very first extinction event, seems to have been caused by some kind of sudden cold snap, but no one’s exactly sure how that happened. But then, with the other two, the causes of those are pretty murky and people have tried to come up with a unified theory for these extinctions, but that hasn’t worked at all. The causes seem to be pretty disparate. […]

BP: One thing your book explores is that there’s no one factor causing modern-day extinctions. There’s hunting. There’s deforestation. There are changes in land use. There’s climate change and the acidification of the oceans. Which of these stands out as most significant?

EK: To me, what really stood out … And I always say, look, I’m not a scientist, I’m relying on what scientists tell me. And I think many scientists would say that what we’re doing to the chemistry of the oceans could end up being the most significant. One-third of the carbon-dioxide that we pump into the air ends up in the oceans almost right away, and when CO2 dissolves in water, it forms an acid, that’s just an unfortunate fact.

The chemistry of the oceans tends to be very stable, and to overwhelm those forces is really hard. But we are managing to do it. When people try to reconstruct the history of the ocean, the best estimate is that what we’re doing to the oceans or have the potential to do is a magnitude of change that hasn’t been seen in 300 million years. And changes of ocean chemistry are associated with some of the worst extinction crises in history. [more]

There have been five mass extinctions in Earth’s history. Now we’re facing a sixth.