Lessons From a Supervolcano

For starters, knowing what the massive upheaval of Indonesia's Toba supervolcano did to the planet's climate (it might have cooled global temperatures enough to kill vegetation for years on end and perhaps hasten an ice age) offers sobering insight into what pumping billions of tons of chemicals into the atmosphere as we're now doing could result in. In this interview, Shindell shares his thoughts on Toba's impact then and now.

Planet cooler

NOVA: I want to ask you about the lessons Toba might have for us today, but first a few questions about Toba itself. You've modeled how the world's climate likely reacted to Toba's eruption 75,000 years ago. What did you find? What happened?

Drew Shindell: Well, we found it was such a powerful eruption that it really could send the planet's temperatures dropping quite rapidly and keep them there for quite awhile. This isn't a long time period relative to ice ages, but you can make the planet cold for many years. And when people are trying to survive based on growing crops, [such cooling] each year for many years—on the order of 10 years or so—that can have pretty damaging consequences.

Some experts have said that Toba might have even triggered an ice age. Can supereruptions do that?

Well, I don't think a supereruption could really trigger an ice age. But I think that if we were on the verge of falling into one anyway, a supereruption could certainly provide that little push to get us rolling down the hill into an ice age. It could bring one on a little bit earlier.

Do you think that happened with Toba?

It's hard to say. There are a lot of signs that it did make the Earth quite a bit colder, and that cooling period extended for a long time. Part of the uncertainty that remains is that [the climate] seems to have warmed [up again] about 1,000 years afterwards before really settling into the actual depths of the ice age. So it's unclear whether Toba really did push us in and then something else happened 1,000 years later, or whether the effects of Toba were to make it cooler but it was really a temporary thing, and then we went into the ice age that we would have gone into anyway.

What causes the climate in such a case where an ice age develops to slow its runaway cooling, reverse, and ultimately return the planet to a warmer period?

Well, climate cycles are really driven by changes in the alignment between the Earth and the sun, and these things change extremely slowly. There are a few different cycles having to do with the shape of the Earth's orbit and the tilt of its axis, but the cycles range from about 20,000 to 100,000 years [in length].

"Clearly crops will fail for year after year after year, and the whole planet would probably plunge into a terrible famine."

As the amount of energy reaching the Earth changes at different times of year, different parts of the Earth will be exposed to a different amount of sunlight, depending on the orbital configuration. How much energy reaches the northern latitudes, where you have a lot of land area and can develop ice sheets, is really the key factor.

So you go into an ice age if you have a reduction in the amount of sunlight reaching these northern high latitudes around 60 degrees that's enough to allow snow to persist during the summer. If the snow stays throughout the summer, it piles up year after year after year and you develop an ice sheet, which is a strong amplifier of the cooling of the country there in the first place by reflecting a lot of sunlight.

Then you just plunge into an ice age. And you really have to wait until the temperatures warm up enough—again from the change in the astronomical alignment—to start melting that ice and bring you out of an ice age.

Extinction machine

Experts say it's only a matter of time before another eruption of Toba's magnitude occurs. If such a supereruption were to occur today, how would its effects on the climate differ from those 75,000 years ago, given our globally warming world?

Well, there would be a number of differences. There are a couple of main factors, in my opinion, that would each go in somewhat opposite directions. One is we have a lot of modern technology that we didn't have back then, 75,000 years ago, of course. If the climate is degraded and there's less energy from the sun reaching the surface so that it gets darker and crops can't grow, we have some ways to get around that. We have things like our own ability to generate power. In some ways this would make it a little bit easier for us to deal with the climate problems induced by a supereruption.

But in most other respects, I would expect that it would be even more devastating than it was back then, largely because there are just so many people on the planet now. If six billion people need to be fed, you really can't tolerate a vast disruption in our ability to grow crops. If so much ash and soot are thrown up into the air that sunlight can't reach the surface, clearly crops will fail for year after year after year, and the whole planet would probably plunge into a terrible famine. That's what we think happened after the eruption of Toba. And if you look at the genetic studies, it seems there was a huge wave of extinctions right around then. Probably that was a result of the same thing—the blocking of the sun by the eruption.

And Toba was not even the biggest supereruption that's been recorded. If a really huge supereruption were to occur today, could the climate potentially change so drastically as to threaten us as a species, or threaten life on Earth?

Well, Toba was extremely large, and it did wipe out a lot of species, it seems, 75,000 years ago. But several of the other mass extinction periods, genetic bottlenecks if you will, in Earth's history are related to the timing of very, very massive volcanism. One of the most famous is, of course, the extinction of the dinosaurs. There was also apparently a large asteroid impact, but it is clear that there was a huge episode of volcanism right around the exact same time. And there are other periods when mass extinctions across different boundaries occurred, and in a time where again these are linked to volcanism.

So in the past we have a lot of evidence that genetic bottlenecks, when large fractions of the species alive on Earth at the time were wiped out, are really correlated with volcanism. I would say that if another supereruption were to occur, it would definitely have the potential to wipe out vast numbers of species living on the Earth. And, of course, there will be; it's just a question of when. We really don't have any capability to predict these things. Fortunately, they seem fairly rare.

Climate shifter

What lessons might Toba have for us today then?

Toba offers us a few lessons that I think are interesting in a time of global warming. What we're seeing now is that as we put a lot of greenhouse gases into the atmosphere, the climate is slowly changing. But what something like Toba tells us, or may be telling us, is that the Earth's climate has sensitivities where if you push it too far, it might not come back. The cooling after Toba, as I've said, seems to have lasted about a thousand years.

"Is there a chance that we'll push things into a new state where they won't simply return to where they were before? I think yes."

To the best extent that we can understand it, even if you throw a lot of particles and ash and chemicals up into the atmosphere from a supereruption, they hang around for awhile—in the case of a very large eruption like Pinatubo in the Philippines in 1991, about three years or so; in the case of a supereruption, maybe a decade or so. But they're going to fall out just by their weight. It's not really so much a function of how much is up there, but just how long it takes for them to settle down out of the atmosphere. So the fact that the cooling seems to have lasted for a thousand years really implies that there are thresholds that can be crossed. There are ways that you can push the Earth's climate, and it doesn't simply bounce back when you stop pushing.

Those are important lessons when we try to consider what we should do about global warming at the present. Is there a chance that we'll push things into a new state where they won't simply return to where they were before? I think yes, there is a pretty high probability of that happening.

I think many people still tend to think of the climate as so huge and robust that it takes thousands of years to change radically. But that just isn't true, is it?

No, it apparently really isn't true. And actually most scientists had the same point of view for a long time. It took us a very long time to obtain evidence, and awhile afterwards to really be convinced by the evidence, that in the past climate has varied quite rapidly, sometimes with large changes taking place over times as short as a decade or so.

We have evidence from ice cores, which have a record of what happened to past climate, from both Greenland and Antarctica. As techniques have improved to extract information from these, we can get higher and higher temporal resolution and really see that sometimes changes can happen extremely rapidly.

One of the most interesting things going on now is the dramatic increase in the melt area and the rate at which ice is melting in Greenland. One of the things that always comforted us a little bit was there was so much inertia in the ice sheets that we thought we really didn't have to worry about them all that much. It takes millions and millions of years to build up an ice sheet, and presumably it would take an extremely long time as well for them to break down.

But as we get more evidence about what's happened in the past, and as we see what's going on now, we're beginning to doubt that that's really true, meaning how quickly they can break down. It does take millions of years for snow to pile up to a depth of a couple of miles as in Greenland, but it seems that snow and ice sheets can fall apart much more quickly, which makes sense when you think about it. If something starts to melt and breaks up into pieces and they all slide out into the ocean, it's just gone really quickly. And we see that's what's happened in the past.

We have new measurements that seem to indicate that just during the last five years or so the rate at which water has been coming off of Greenland has approximately doubled. It says that these things are really sensitive, and, in fact, the time over which they can respond might be much, much less than we thought.

Cautionary tale

Al Gore's new documentary, An Inconvenient Truth, shows quite dramatic images of the Greenland ice sheet melting. As a climate specialist, what's your take on the film? Have you seen it?

I haven't actually seen the film, but I've seen Al Gore's presentation, and I've been told that it's basically the same material. I thought that the material in the presentation was very well done. It was pretty carefully chosen so as to convey, in my opinion, a pretty accurate picture of what the scientific community thinks. It's not the most extreme viewpoint on either side but kind of the general consensus of what's been going on.

There are, as you said, dramatic pictures of what's been going on in the Arctic—the sea ice retreat and the melting over Greenland. These are really what's happening. This is what the data shows. It's not really anybody's opinion; this is just what the new measurements are showing. Even five years ago, we wouldn't have believed a lot of this. But now that's what the satellites are seeing, aircraft are seeing, people on the ground. It's all quite consistent and rather alarming.

"What the past tells us is that bad things can sometimes happen."

What would you see needing to happen, climaticalogically speaking, for world leaders to realize what most scientists now realize and to take action? What would convince them, do you think?

I'm not sure there's an easy answer to that question. It seems to me that world leaders in many countries have been fairly receptive to at least trying to take initial steps to deal with the problem. That hasn't been so much the case here in the United States.

But I think maybe one of the best analogies is the case of smoking. It was a difficult problem to deal with because the hazards induced by smoking were way off in the future, so people didn't really want to quit smoking. They didn't really feel that it was that imperative to do it right away, and there wasn't really that leadership from above. It eventually came about, partially through the courts, but partially through general popular opinion changing.

I think that's part of what might happen from something like Gore's film. As more and more people see this, and as more and more people realize what the problem is, even though it's far away in the future, what has to happen is the American public has to put pressure on the leadership, has to say "This is not going to be a problem within your elected term of office" (which is generally two to six years in our government) "but it is a problem for you because we, the people who are electing you, are saying it's a problem." That has to be the transition, and I think we're starting to see more of that.

You see things as a climatologist on scales that a geologist does—tens of thousands of years. Yet things, as you've said, can also happen very quickly. How do you get the point across to people that things can happen quickly, that, say, we could jump to a whole new steady state of climate?

Well, I like to give examples from the past. There are periods where we have seen things shift pretty abruptly. I mentioned before that we have evidence from the ice cores, and what we see is there have been times when something dramatic happened and climate seemed to really flip within even a decade or so. So there's really a history of it.

I mean, if you just see something happen in a climate model, sure there's lots of uncertainties in the model, and how do you really know if you should trust what this model says? Even if every model in the world said "In 10 years something dramatic is going to happen," how would you really know whether to believe that when the models are mostly used for looking at what's happened in the recent past, which hasn't been as dramatic?

But when you see that the records from the past really show that this has happened before, then we know that what we humans are doing is kind of an unplanned, uncontrolled experiment where we are putting a lot of stuff in the atmosphere and pushing at the Earth's climate system in a funny way. And when you start doing that, what the past tells us is that bad things can sometimes happen.