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 track for a sixth mass extinction. The world’s species already seem to be 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 could push many 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 rapidly (though scientists are still debating these estimates, with some lower, some far higher).

So what happens if the extinction rate does speed up? That’s one of the questions that New Yorker science writer Elizabeth Kolbert explores in her 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: Let’s start by walking through the history of science here. Back in the 18th century, no one even knew that there were any extinct species. How did we get from there all the way to realizing that there had been five of these mass extinction events in Earth’s history?

Elizabeth Kolbert: There is an interesting history there. Up until the early 1800s, the concept of extinction didn’t really exist. Even early in the 19th century, you had Thomas Jefferson hoping that when he sent Lewis and Clark to the Northwest, that they would find mastodons roaming around. Mastodon bones had been unearthed — there was a very famous one unearthed in New York and displayed in Philadelphia — and people thought they must still exist somewhere.

But right around that time, a French naturalist named Georges Cuvier came to the realization that look, if these animals were out there, we would have seen them by now. They are not there. And that made sense of a lot of things. There were these bones that were very, very hard to explain. And more and more of them as Europeans colonized the New World, they were getting these bones shipped to them. It made sense of these weird nautical creatures that had been found that no one ever found.

S0 extinction actually predated the concept of evolution by about half a century — people knew that things went extinct, even though they didn’t really understand how species came into being. But there was still some debate. Cuvier thought that when extinctions happened, it must be because the Earth changed quickly and catastrophically. Why else would an animal that was perfectly suited to life on this planet go extinct? His theory became known as “catastrophism.” And Charles Lyell and Charles Darwin came along and said, that’s ridiculous, the Earth changes slowly, we’ve never seen a catastrophe, that’s because they don’t exist.

That paradigm persisted until the 1980s and 1990s. That was when Walter Alvarez and his father Luis Alvarez came up with the theory that an asteroid impact had done in the dinosaurs. And that idea was actually resisted for the same reasons — the dominant view was that the Earth does not change quickly. But then it was proven.

And so now the prevailing view of change on planet Earth, as one paleontologist put it, is that the history of life consists of long periods of boredom interrupted occasionally by panic. It usually changes slowly, but sometimes it changes fast, and when it does, it’s very hard for organisms to keep up.

BP: 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?

EK: 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 [the end-Ordovician], 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: Now, at some point scientists realized that present-day extinction rates seem to be elevated — that species may now disappearing faster than the normal “background” rate. (Though precise estimates are tricky because measuring that background rate turns out to be very difficult.) How did they realize this?

EK: I think a point that’s important to make is that, normally, you shouldn’t be able to see anything go extinct in the course of a human lifetime. The normal background rate of extinction is very slow, and speciation and extinction should more or less equal out. But that’s clearly not what is happening right now. Any naturalist out in the field has watched something go extinct or come perilously close. Even children can name things that have gone extinct.

So as soon as this concept of background vs. mass extinction came into being in the 1980s, people realized that what we’re seeing today is not just background extinction. Now, whether you make the jump to say that a major mass extinction is going on or just an elevated extinction rate, that’s up for debate. But if you are looking at this in a rigorous way, you can see that something unusual is going on.

BP: One thing your book explores is that no single factor will drive current and future extinctions. There’s hunting and poaching. There’s deforestation. There are invasive species. 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.

BP: Are there lessons we can learn from past extinctions that provide clues for what the current changes hold?

EK: A lot of people are trying to tease out what survived previous extinctions and ask what are the characteristics of those that survived. It’s called the selectivity of extinction events. Why did some groups survive and others didn’t? It turns out to be, 65 million years after the fact, very, very difficult.

But speaking very broadly, the species that tend to survive mass extinction events often tend to be very widely distributed, or groups that have a lot of species. I’m not sure whom that’s going to help today, but that seems to be the pattern.

BP: In your book you talk about this quasi-experiment in Brazil dating back to the 1970s, where ranchers had down swaths of rain forest at random and scientists could study the effects on species. What did we learn about deforestation and extinction from that?

The Biological Dynamics of Forest Fragments Project in Brazil (Smithsonian Tropical Research Institute)

EK: Right, the Biological Dynamics of Forest Fragments Project. It’s in the Amazon rain forest north of the city of Manaus. What happened there was that this area was already being converted into ranches, so in collaboration with some American scientists, they deforested it in an interesting way. They left these square patches surrounded by ranch. You can see it from the air, it’s quite striking. And this group of scientists surveyed the habitat in this forest before everything was cut down and then monitored it for 35 years.

And what you find are variations on this theme of loss. First most of the primate species don’t survive in these smaller patches or even in the bigger patches of forest. Then you lose a lot of your bird species. In some cases species leave, and in some cases, when you maroon them in small patches of habitat, their populations shrink, and very small populations are just more vulnerable to chance.

So when people talk about the dangers of habitat fragmentation, on the one hand, a big animal that needs a large range can’t survive in a small patch. But it’s also smaller animals that don’t need that much space become vulnerable to the dynamics of small populations.

BP: Did that Brazil project yield any lessons for protecting rain forest habitats?

EK: Don’t deforest! For one. But also in the 1980s there was this battle about protecting forests, and whether it was better to do it in lots of little patches or in one big patch. And this project has resolved that. You need big areas if you want to preserve biodiversity, for the reasons I just mentioned.

BP: You discuss global warming in your book. And the big concern here seems to be that a lot of species are adapted to particular climate ranges, and if those heat up, some species may not be able to move or relocate fast enough to more suitable climates. How much do we really know about these dynamics?

EK: What people are finding, what the scientists that I was out in the Peruvian cloud forest with are finding, is that things move at very different rates. People have calculated how fast species would have to move to keep up with rising temperatures, whether it’s moving up a mountain or moving to higher latitudes.

And some organisms can keep up with that fantastically high pace — for example, in Peru, there was this one genus of tree called Schefflera, which is sometimes used as a house plant, and that genus is moving really fast up the mountain. But some of the other plants weren’t moving at all, and others were moving but not nearly fast enough.

So the lesson is that all those pretty complicated relationships, which in the tropics have been been pretty stable for a long time, are going to break up. And we just don’t know what the fall-out from that is going to be.

BP: So you end up with pretty wide estimates for how many species could go extinct if the planet heats up this much. Some studies suggest that 20 percent to 30 percent of species are at risk of extinction if the planet warms 2°C. Other scientists think those estimates are flawed.

EK: There’s still a lot we don’t know here. You often hear that what we’re doing is a planetary experiment, but we only have one planet, and we can only run this experiment once. So some of these modeling efforts get pretty complicated. Just because a species lives in a certain climate under a certain set of conditions, could it live under different conditions? Or is this just where it’s maximally competitive? What happens if some of your competitors are disadvantaged? We just don’t know. Life turns out to be incredibly complicated.

BP: That brings me to another question. Most of the people in your book who study these trends tend to think they’re horrible news. Did you come across researchers who had a more optimistic view?

EK: I guess one point to make: Even in moments of extremes, certain organisms do thrive. They’re sometimes called “disaster taxa,” and they do very well. After the End Permian extinction, which was the worst mass extinction of all time, there was an animal called Lystrosaurus, was a pig-sized animal that just did phenomenally well. It was the biggest animal on the planet, you find fossils everywhere. And the question of why did it do so well? We just don’t know.

But some things will thrive. Some things will thrive in an acidified ocean because all of their competitors will drop out. So some things will do well, and undoubtedly there will be surprises. But I have not met anyone who hasn’t said, we’re going to be vastly simplifying the web of life. A lot of things are going to drop out. It’s hard to make predictions of what they are.

BP: There’s another angle in your book that tends to get less attention. The spread of people across continents has transported all sorts of species to new habitats — and sometimes that’s had catastrophic results, like when the brown tree snake was introduced into Guam and wiped out the native birds. Is this sort of exchange speeding up, or are there efforts to slow it down?

Minnesota fisheries supervisor Brad Parsons, holds a 12 kg bighead carp, at the state’s Department of Natural Resources headquarters in St. Paul, Minn. on April 20, 2011. The 86 cm carp, caught two days earlier in the St. Croix River near Prescott, Wis., is among several invasive Asian carp species that could cause serious damage to the Great Lakes region’s aquatic ecosystems. (Richard Marshall/The St. Paul Pioneer Press/Associated Press)

EK: There are certain moments of time where you see a huge exchange of species. After Columbus arrived in the New World, there was this huge exchange. And as global travel becomes very rapid, that speeds up exchanges. Organisms that couldn’t survive on the Mayflower could survive in a modern supertanker or plane and get transported from one continent to another. So we’ve ratcheted things up a notch.

So we don’t do as much purposeful moving of species as we used to — where we’ve decided we’d like to have this bird in a new place. We’ve done a lot to prevent that. You’re not supposed to just take a bird from South America and release it in Australia. But the unconscious transport of species, I think there’s no doubt that is increasing very dramatically as the sheer amount of cargo increases.

And it can still have devastating effects. Look at the Asian carp, working their way toward the Great Lakes. There’s the Asian longhorn beetle, a pretty recent invader causing tremendous damage to forests in this country. There’s the emerald ash borer, quite a recent one, which has led to all these signs in the Northeast telling people not to move firewood, to avoid moving these invaders around.

There are zebra mussels, which recently moved into Massachusetts, where they weren’t, taking over lakes there. The disease that’s killing off bats in the Northeast and in the D.C. area, that’s an invasive pathogen that was brought in, it’s a fungus. We can just name one thing after another. And I’m sure if we have this conversation a year from now there will be new ones that we know about.

BP: Now what about attempts to save species from extinction? What are some of the more interesting efforts you encountered?

EK: A lot of them involve zoos or conservation organizations. So there are these really fascinating and pretty ugly animals called hellbenders, they’re these big salamanders that could feature in a horror movie. They are very endangered, and what people are trying to do is raise them to a certain size at the Bronx Zoo, and then repopulate streams in upstate New York.

Also at the Bronx Zoo there’s this amazing project with this endangered bird from an island in the Pacific [the maleo]. It lays these enormous eggs that have to be incubated in volcanic soil. They bury the egg and the egg is warmed by volcanic activity in the area, which is just amazing. So the zoo is trying to make an incubator that mimics these volcanic soils. Then they trick the birds, by taking away their eggs so that they lay another. And there are hundreds and hundreds of these efforts.

BP: Don’t these sorts of efforts tend to favor “charismatic” animals over things like tiny organisms in the ocean that could affect entire food webs?

EK: Yes. We only see what we see. And we don’t know where the link is that may turn out to be absolutely crucial, because we’re not participating in the food web at that level of specificity. The really scary thing is when scientists find organisms at the bottom of the food chain that can’t survive under conditions that we predict will occur in next century or so. That has happened. Then you can potentially get these big knock-on effects on the food chain. If you talk to marine scientists, that’s exactly what they’re worried about.

And you might be able to raise that pteropod in a tank, but it really doesn’t matter. Because we’re talking about things that exist on a massive scale. Too numerous to count. That’s what keeps the food chain going.

BP: As a final question, what’s the big thing you took away after reporting and writing this book?

EK: Here’s the big thing I took away, and it’s a very sobering thought: Many of our best qualities as humans —our creativity, our cleverness, our cooperation, the fact that we can work in these huge societies, and pass knowledge on from generation to generation — those things can turn out to be damaging. It’s not just that we go out and poach things, although that’s a problem. We’ve very smart and inventive and we can change the planet by doing things that have no evil intent. For example, going on vacation and bringing a bat fungus from Europe to the United States completely unintentionally. So it’s not always clear how you would separate out what we do just by being human from what we do that has all of these unfortunate side effects.

Interview has been lightly edited for length and clarity.