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What is mass extinction?

Author of "The Sixth Extinction," Elizabeth Kolbert, talks to Ezra Klein about the next mass extinction. What follows is a transcript of their conversation.

Elizabeth Kolbert: A mass extinction is defined as a moment in time — geologically speaking, a short moment in time — when the diversity of life on Earth plummets. One very well-known paleontologist has described the whole history of life as long periods of boredom, interrupted occasionally by panic. And these are the moments of panic.

Your chances as a species of going extinct at any given moment are very, very low, except during moments of mass extinction, when they skyrocket. In the case of the five major mass extinctions of the last half billion years, roughly three-quarters of all species have been eliminated at those moments. Then, after that, diversity starts to tick up again, but it takes somewhere usually between five and 10 million years for that recovery process to take place.

Ezra Klein: And human beings might be causing another mass extinction.

Elizabeth Kolbert: Yes. We're really inured to extinction. We see stories all day. There was just a story, I think it was today, about how lemurs, 90% of all lemur species on Madagascar are endangered. Many are critically endangered, down to like 18 individuals. So we're just like, "Oh, another extinction," as if that's something that happens all the time.

What history shows us is that if you, Ezra Klein, should not see even one species going extinct in the course of your lifetime. But you're a young man and species have already gone extinct in your lifetime. So something really unusual is going on. You should not be able to see a single species of mammal go extinct in your lifetime.

Ezra Klein: What level of die off do you need for something to be a mass extinction, or even a minor mass extinction?

You should not be able to see a single species of mammal go extinct in your lifetime.

Elizabeth Kolbert: Well, the minor mass extinction is a little bit fuzzy, but the major ones, the five — they're sometimes called the big five — the cutoff, if you will, is roughly 3/4 of all species on the planet disappear in a relatively short amount of time.

Ezra Klein: Conventional wisdom is that the mass extinction that led to the death of the dinosaurs was an asteroid hitting the earth. But what other kinds of events, and recognizing that not everything is fully proven, have typically been behind these kinds of extinction events?

Elizabeth Kolbert: It's sort of an unfinished story, as you say. The impact hypothesis — that an asteroid impact was what ended the cretaceous and did in the dinosaurs — was confirmed, only in the 90s. So then people went back, and they sort of thought, well, we're going to find an asteroid impact at all of these junctures. That would make sense. That would be very elegant, and we're going to find that. They really looked hard, and they couldn't find any evidence of that.

There's sort of a consensus that the first one, which happened 440 million years ago, when most of life was confined to the water, was caused by this snap sort of glaciation. The world suddenly got very cold. So that's the working hypothesis there. The most severe mass extinction of all time was about 250 million years ago at the end of the Permian period. It seems pretty clear that was caused by some kind of massive outpouring of carbon dioxide, which caused really serious global warming and acidified the oceans, changed the chemistry of the oceans very radically. One of the really sobering things to think about is that is what we're doing, massively pouring carbon dioxide into the air and into the water. So increasingly, people are drawing parallels between what we're doing and the worst mass extinction.

Ezra Klein: So what do we think happened to cause that outpouring of carbon dioxide? Obviously at that period we didn't have a mob of SUVs roaming the world. So how did all the carbon dioxide get released?

Elizabeth Kolbert: Well, right. You've put your finger on a big scientific mystery, to be honest. It was so much carbon dioxide that it's been very difficult for people to even think of what the source might be. The sort of lead candidate is a burst of volcanism — these volcanic events that happen and create these huge, what are called igneous provinces. A lot of Siberia is covered with this ancient lava from this event. That's sort of the lead candidate. But people who have tried to date that, those dates have not exactly lined up, so we're not sure.

Ezra Klein: One of the really terrifying parts of your almost nonstop-terrifying book, is that the quantity of carbon dioxide that we are emitting at the moment, every day, every year, every month, every year, is not just similar too, but potentially faster than the carbon dioxide emission that led to that extinction.

Elizabeth Kolbert: Yes, yes, yes. Scientists have come up with amazing methods of following these cold cases, a 250-million-year-old murder mystery, teasing out from things like shells how much carbon was poured into the atmosphere at that point in time, and have concluded that the rate that we are pouring CO2 into the air is certainly comparable, and perhaps greater than what was occurring then.

Ezra Klein: This I thought was one of the really fascinating conceptual things in the book that you really emphasize is it’s the rate of change. So the issue is not that the world has not changed before, it's just that it does so in very slow geologic time frames. But that in these mass extinction events, there’s a rate of change that organisms simply can't adapt to. The way you say it in the book is that the rules of life change more rapidly than life can actually keep up. I thought that was fascinating because our intuition around global warming, with our tiny blink of a geologic-eye life spans, is that it is happening slowly. It's not that much warmer it was than a week ago. But in terms of the earth, and in terms of how rapidly species evolve, this is shocking, dramatic, fast-forward, training-montage-style change.

Elizabeth Kolbert: Absolutely. That is really the heart of the book. Eventually, our whole human history will be compressed in geological time down to this very thin layer. Like we look back on the past and see this very thin layer that represents, say, the end of the Cretaceous period. Change is occurring so fast that it will look not dissimilar to an asteroid impact, because the difference between a year, a hundred years, a thousand years is erased in geological time.

Just to give one example that I think people can relate to, or understand, the Arctic ice cap, which has been there for millions of years, is probably going to be gone in your lifetime. You're going to be able to go to the North Pole in the summertime and swim around, paddle around, if you were in the Polar Bear Club, or whatever. That is a huge change on the surface of the earth, and it will occur in the lifetime of a lot of people who are alive today.

Ezra Klein: I want to ask the same question here from 2 different directions. First direction is, I think, in some ways the sort of selfish human direction. People sometimes ask "Who cares?" Who cares if there are fewer kinds of snails or fewer kinds of bats? What are we getting as a race out of having all these different kinds of animals in the tropics?

Elizabeth Kolbert: I get asked that question all the time. I sort of have two answers for it. One is, we're talking about life on this planet. It took tens of millions of years to evolve to this point, and it's unraveling very, very quickly. If you don't care about that, I'm just not sure what you would care about.

But if you still don't care, I would say that at these moments of mass extinction, it seems that the rules of the survival game change. Once very dominant groups, for example, the dinosaurs — the dinosaurs were not doing anything wrong, there was nothing wrong with the dinosaurs — and they were gone, 100 percent of them were gone. We don't know exactly why, why they were particularly vulnerable, but they're gone. When you're changing the rules of the survival game, as we are, then you don't know where that game is going to end up.

Ezra Klein: That gets to the other direction I wanted to ask that question from. I kept imagining, reading your book, this book being read by someone 5,000 years from now. It is such a monstrous thing to imagine that we know that we are killing off a tremendous quantity of life on this planet yet, because it’s inconvenient to think about or difficult to change, we just don't worry about it. I think to somebody with a larger time frame than our own to imagine that there was a species existed here, briefly, and they didn't really care that they were wiping everything else out — I don't think we would be judged kindly by that society.

Elizabeth Kolbert: No. I don't think we would. Obviously, I'm going to argue, having written on this subject, that we shouldn't be judged kindly. One of the scientists that I went out with to the Great Barrier Reef, a guy named Ken Caldeira, who's at Stanford, made the point to me when we were out on the reef, "Look, if the Romans, whom we feel a pretty significant connection to, had burned through all the fossil fuels the way we are burning through them now, this reef would not exist. It would already be gone."

At the rate that we are burning through fossil fuels, there are very, very robust predictions that coral reefs will start to disappear around the middle of this century. There will be no more coral reefs by the end of this century. We are not that far from the Romans. And yet, had they done what we did, the world would be a totally different place.

Ezra Klein: Is this kind of die off a warning system for us, too? There's a lot we actually can't survive: earthquakes, fires, tsunamis. In the face of really tremendous natural disaster or change, our capacity to endure is not always what we'd hoped. Back in 2006, when you wrote your last book, the global goal was to keep warming under 2 degrees Celsius. It now looks very clear that we're going to blow by that. We may go up to 4 degrees Celsius. It's plausible that we could go higher over the next couple hundred years. That's a swing comparable in size to the swing that produced the Ice Age. It isn't obvious to me on the kind of rate of change we're doing that, particularly if things don't go exactly as we hope on the innovation curve in the next hundred years, that we're going to have such an easy time handling it.

Elizabeth Kolbert: There are two levels at which you have to consider the future of human beings and humanity. One is on the level of the species. Species can survive if a few individuals survive. If a few dinosaurs had survived, we might well still have dinosaurs.

But then we have human society, which is much more vulnerable, and much more interconnected than random pockets of people. Many people who are looking at this issue would say, okay, what's really under very serious risk here, what we're really, really putting at risk, is not necessarily our own survival as a species. Because we are very clever, and there are a lot of us, and we live all over the planet. Most species do not. But our society depends on stability. If you start really destabilizing the natural systems on which we depend, destabilizing the climate, we just don't know what's going to happen with that.

Ezra Klein: The book has this fascinating dimension where it's this incredibly beautiful travelogue of the apocalypse for a lot of species. It reminded me of that book that was big a couple years ago, "100 Places to See Before You Die." This is like "A Dozen Places You Have to See Before We Destroy Them." In those travels, what was the most hopeful thing you saw, or that you heard, along the way?

Elizabeth Kolbert: That's a really good question. I went to the Amazon — it's not actually in the book — but I went out into the forest with the people who are actually trying to incentivize people not to chop down the rain forest, not to illegally log. We talked to some of these people who used to make a living, and it wasn't a good living, to be honest, illegally logging. For a relatively small amount of money, they had been converted to people protecting the forest. That was really a great experience, and a moving experience.

It was a very small scale thing. It would have to be ratcheted up by a million-fold, or whatever. But it was a hopeful thing that people who had seen themselves as having an investment in cutting down the forest could turn within a relatively short amount of time to seeing themselves as having an investment in not cutting down the forest. I think that there are ways to shift our thinking.

This text has been lightly edited for clarity.