Michael Oppenheimer. Photo: Princeton

This week, to accompany our cover story on worst-case climate scenarios, we’re publishing a series of extended interviews with climatologists on the subject — most of them from the “godfather generation” of scientists who first raised the alarm about global warming several decades ago.

Now a professor at Princeton, Michael Oppenheimer was the longtime chief scientist of the Environmental Defense Fund’s Climate and Air program, and was an early force in the U.N. efforts on climate change that resulted, ultimately, in the Kyoto protocol. He has also been a long-standing participant in the U.N.’s IPCC climate-reporting project, including serving as the lead author of its 2007 Fourth Assessment. But he has also, over the years, spoken out about the limitations of the IPCC process and the reports it produces.





How do you think about the IPCC process and their estimates? I know you’ve been critical in the past.

I’ve been involved in the IPCC since the first assessments, and while I have criticisms of the way certain aspects of the climate system have been assessed by the IPCC and the way the judgments were reported, I certainly think the whole process has done what it was supposed to — to give government a basis for proceeding to develop policy rather than spending their time arguing about the science. All the governments accept the science. So IPCC has done a good job, but it could do a better job.

My own involvement in the question you’ve asked goes back 30 years —more than that, actually. The key question: What are the risks that we really ought to worry about, what are the risks that ought to frame policy including outcomes that might be unlikely but, were they to occur, would have massive impact. These are the kinds of things that I’ve long thought that policy makers ought to be paying attention to, they are the kinds of things that the intelligence community and the Defense Department has long focused on.

They are not focused on the average of what might happen, they worry about the surprises — what Donald Rumsfield notably tried to distinguish when he talked about the “known unknowns” and the “unknown unknowns.” We have both categories with climate. There are things we can identify as potentially happening but can’t really quantify very well — those are the known unknowns. And then are the things that we are constantly challenged by, that happened and which we didn’t really predict or even identify as a possible outcome — the unknown unknowns. In the latter category you might put the speed at which ice has been lost in Greenland and Antarctica, by processes that were not anticipated and barely thought about at all in the early IPCC reports. The classic surprise in all of this wasn’t in climate, it was in the ozone — the appearance of the ozone hole.

That’s really the best example of an unknown unknown showing up.

What are the frontiers of understanding right now as you see them? What are the things we should be trying to study more, or things that haven’t yet been integrated into our understanding?

For a long time — going back at least to 1983, the ice sheets and the potential for rapid loss of ice was thought about, and attempts were made to quantify the likelihood. In fact there was enough attention early on it that I shouldn’t even have called it an unknown unknown — that should be in the category of known unknown.

For decades, scientists have tried to model the ice sheets better, and to observe them better. The field has grown over time, and we’ve started to accumulate a lot of data and started seeing these bizarre things happening in the mid-1990s. The scientific consensus had landed on the idea that ice sheets couldn’t move fast, they could only move glacially, to make a little joke. It turned out that things were happening rapidly in both ice sheets. The old picture was that it was going to be millennia. And if the West Antarctic ice sheet were to disgorge its ice sheet, which would result in something like four meters, if it did that over the course of a century that would be huge. That idea was totally disfavored, until we started to see, over the last 20 years or so, we started to see that ice sheets were beginning to move fast in various parts, and some very surprising things were happening, like the collapse of a gigantic part of an ice shelf in the course of a few weeks in 2002.

That was important not just because it showed how fast ice can disintegrate, but also because the ice behind that ice shelf began discharging faster into the ocean, causes a faster sea-level effect. Other things like that began to happen quite a lot. But it remains, to a large extent, an unknown unknown. There is no well-accepted way of incorporating ice melt into climate models. That, to me, remains, of the things we have reason to believe have a fairly good chance of happening over time as the world warms, that remains the most risky. Because we know if we warm the world enough, it’s going to happen.

And the reason we know that the last time the world was two degrees warmer, sea levels were between six and nine meters higher. And that could only happen if parts of Antarctica and the Greenland ice sheet had melted. So we know it’s going to happen with only a relatively modest warming, we just don’t know how fast, and that’s critically important.

What kind of warming is likely? I know that there are so many factors that go into that, including how we adapt; what is the likelihood we stay below two degrees?

I think the likelihood that we stay below two degrees even with diligent efforts was relatively small — maybe 20 percent, in my view — before the Trump withdrawal from Paris. And the likelihood is now increased markedly that we’re not going to make the two degrees. Personally I would rate the likelihood as under 10 percent. So I think we need to be prepared for a world where we are going to have eventually a large sea-level rise, for a world in which we have extended episodes of unbearable excess heat, for a world where eventually crop yields will decline significantly in parts of the world and cause food-security problems that go over the edge at least periodically, and that means more starvation and malnutrition. Where natural ecosystems like coral reefs, some of them, are going to be doomed. We’re probably already bought into a world we’re not going to like very much — and the likelihood of other surprising outcomes is increasing markedly. One of the ones that’s already been pointed to that is of special concern is a shutdown or slowing of the ocean conveyor belt.

Can you walk me through what that would mean, if that slowed down significantly?

It’s called the MOC, and it performs several functions — it basically transfers heat, humidity, and salinity from the southern latitudes to the high latitudes. That’s one factor in keeping parts of Europe warmer than they would otherwise be, another factor is that a bunch of Europe has a semi-maritime climate. Projections do show that the extreme parts of NW Europe, under simulations that shut-down the circulation, do show a colder climate, rather than a warmer climate. Other parts of Scandinavia could get much colder. Depending on how fast that happens, it could be very damaging. Changes can occur over the course of a decade, or less in some cases. But we don’t know how fast those switches can happen. In a warm period like we’re in now, or even in a “super warm” period like it looks like we’re gonna be in in the future. That’s one of the big known unknowns. And, potentially, it could be quite threatening, because if it happens as rapidly as it did in the distant past it would be difficult for societies to adapt.

You were talking about things we need to prepare for. You mentioned significant sea-level rise and unbearable heat …

Yeah. Let me talk about the unbearable heat a little bit. It’s an emerging area. It focuses on what’s called the wet-bulb temperature — a combination of heat and humidity. Humans simply cannot survive outdoors above a certain wet-bulb temperature doing any sort of even normal levels of work, like construction jobs, farmer jobs, children playing. Normal levels of effort can be life-threatening in a situation where the wet-bulb temperature gets too high. And there are now starting to be projections showing that beginning with brief periods, and then expanding to many days at a time, potentially — when you get beyond the end of the century — eventually getting to most of the summer, in some areas, you’re looking at wet-bulb temperatures that are just too high for humans to carry on normal outdoor activity. That would be shocking for more areas.

It wouldn’t happen overnight. We would have some chance to adjust. But it would make the world a totally different place in those areas than it is now. During the warm season, you’d have to live in a bubble. And some people might argue that it’s in currently air-conditioned environments.

Some places, in very hot parts of the world, many people do live in air-conditioned environments. But I don’t like to live that way. It’s clearly destructive to ecosystems, and in addition in much of the developing world people can’t afford air-conditioning. In the future — 50 years from now, say — many people still won’t be able to afford it. That’s true in parts of the United States as well. So we’re talking about the development of a life-threatening situation.

That’s the most extreme case, but even with the garden-variety heat increases that middle-of-the-road climate projections show, you’re going to get big increases, even in the immediate projections. If you look at what’s projected for even 35 years from now, in the mid-latitudes, for business-as-usual scenarios, we see that the number of days that used to be the 10 percent hottest — the 90-degree days in NYC in 1950s — by 2040 or 2050 they become 30 percent of the days. So the days that used to be the 10 percent hottest now occur 30 percent of the time. The former 10 percent is now recorded 15–17 percent of the time. The number of days that now occur 5 percent of the time are gonna occur maybe 15 or 20 percent of the time by the time you get to 2040 or 2050. These are huge changes, and they’re not in some weird future world, they’re in your lifetime. Think about the children. Think about the changes in the world of our children. It’s humongous. That isn’t if something really weird happens that we don’t expect. That’s just garden-variety climate change that we can project.

Are there things that seem to you like unlikely possibilities that you don’t think we’ve paid enough attention to — or should be thinking of as more likely than conventional wisdom holds?Conventional wisdom doesn’t have a good reputation anymore. In the climate business there isn’t very much conventional wisdom, simply because a lot of times a lot of the predictions have been … Some of the predictions have been smack-on. Some have been a little too high. But maybe the more interesting ones have been too low. And so ice sheets and sea-level rise, those features and the Arctic, the way the icy parts of the world have behaved, have changed more rapidly than expected. And that has massive implications.

The only things we have delivered wisdom about are the relatively simple and straightforward parts of the problem where there’s been a consensus for a long time. Like “Is the Earth warming?” and “Is the Earth’s warming caused by the buildup of green house gases?” and “How much will we have to cut emissions in order to stabilize and eventually return the climate to something that resembles what it was in the recent past?”

But for these sort of edgy parts of the problem there is no delivered wisdom, so everybody has a very open mind about these questions — the periphery of the problem, whether it’s things that we’re not sure about or we don’t have good ways to model them but where they could have very high impact outcomes. You talk about, for instance, the feedback in the system if this generation of the permafrost ecosystem leads to a lot of methane being released into the atmosphere — methane being a powerful greenhouse gas that would further amplify the warming. Somewhat more unlikely but also less known is what happens if the so-called methane hydrates that are on the continental shelf at the sea bed or even in deeper waters get liberated as warming penetrates. How much methane will that put into the atmosphere? That whole feedback system is not well understood. That lack of understanding has disturbed and energized a lot of scientists, so there’s now increasing focus on it.

It’s been a little bit hard for me to get a handle on that material, because it seems like a few years ago there was more direct anxiety about it as a short-term risk. There seems to be diminishing fear about that generally speaking.

What you should do is take a look at the IPCC report. They did about as good a job as they could do. And in that report the IPCC has, I think in chapter six, a substantial section. And that was the first time I think they really tried to incorporate some actual numbers. If you read it you’ll get the feeling that there is a lot that’s unknown about it. What they caution about it is just because we see some bubbles — there’s some bubbling up in Siberia — doesn’t mean that there’s about to be a catastrophe. Because methane has probably leaked from these things for a long time. The question is, how much will and can it accelerate. And we just don’t know. So that’s an important area where progress needs to be made.

Are there any other things in that category that you want to be sure to mention? Other things that we’re unsure about but should be worrying a bit more about?

The next category of things I worry about aren’t physical systems.

These are ecosystems and social systems. When you talk about coral reefs disintegrating, that is an ecosystem disintegrating. You’re not just talking about something that’s pretty. You’re talking about something that’s not only highly biodiverse, but which economies that a lot of otherwise quite poor nations depend on. The coral reefs go to tourism, so it’s fishing boats, the fishing. It’s a lot to worry about.

One of the ones that’s been talked about lately, I think a little bit too loosely sometimes, was the connection between climate change, human migration, and conflict. It’s been sort of a leap: The climate in the Middle East has been unusually dry in the last few decades, that generated problems with food security, and that’s the reason we have a mess right now. It’s obviously a lot more complicated than that. A lot of work has been done recently using entirely new methods on the relationship between both climate change and human migration, and separately the relationship between climate change and human conflict. And there’s a very strong case now that for all types of conflict, running from interpersonal one-on-one violence up to a large-scale war, that there’s a relationship there, and that climate change tends to generate conflict and the hotter the world gets now, the more likely it is that there’ll be conflict at all scales. That research is new. It’s somewhat controversial. But it needs to be paid very careful attention to because we live in a world that doesn’t need anymore conflict.

Is there a researcher focused on that you can point me to?

Solomon Hsiang. He’s at UC Berkeley.

Another thing that needs to be paid attention there — the related problem with migration. I think it’s sometimes framed incorrectly as, we’re gonna have tens of millions migrants globally. That’s not, to my mind, the scale you should look at. We need to look at particular border regions.

Migrants are absorbed very well in some countries for long periods of time without creating difficulty, as has been the case in the United States periodically. What you want to look at is regions where the countries either have difficult absorbing the migrants, as we saw in Europe over the last few years, or where countries that used to be peopled efficiently lose too many of their resources and become unstable. There’s a lot of literature — I contributed a lot to this literature — which shows that in many regions warming has in the past stimulated movement of populations.

There’s the case of Bangladesh where there’s just going to be so many people that it’s hard to imagine.

It depends how fast sea level rises. Bangladesh has already got big problems. Bangladeshis already get shot when they try to cross the border into India. That could turn into a disaster. Again, the complexity of social situations is not just that they’re threatening in some way, because climate changes causes threats that destabilize them. You could reach a tipping point when things get out of control in certain areas. But the opposite can happen. In some cases human societies sometimes do address in appropriate ways.

And I would speculate that sometimes we’ll be surprised in the positive direction, but too often we’ll be surprised in the negative direction. The social tipping points are a key about which we know very little. And there’s got to be an intense focus of expert research in the next decade or two so we can at least get a feeling of where the hot spots may be and we can try and mobilize resources to minimize the risks.

But if you talk to Sol, there’s an even broader issue than just the question of conflict arising from climate change. Some of Sol’s work and the work of others has shown that economic productivity should be on your mind as temperature increases. And the numbers give economic damages due to climate change, which is much higher than any of the other previous models suggested.

Again, this is an emerging area. We don’t have final answers. But if that work is even close to being right, the damage from climate change even in wealthy countries could be much higher than previously imagined.

And they may also undermine our ability to take large-scale action as a result.

Absolutely. And again that’s an emerging area that we pretty quickly need to be able to land on some numbers — to get a consensus. Each of these is an area where IPCC for instance has looked at it in their assessments previously, but in some cases did not spend enough focus on it. It started moving the ice sheet and sea-level question into high focus somewhat in the fourth assessment, and a lot in the fifth assessment where they did a much better job. I anticipate this might be a point of very intense focus for the sixth, although we don’t know yet what the sixth assessment report is going to look like.

That’s not scheduled until 2022 or something?

That’s right. There was just a scoping meeting, which is where an outline was drafted. I was at the meeting. The outline won’t become final until the whole panel, at the next meeting. I’m not sure when the next whole meeting of the panel is. We’ll approve an outline and then we’ll have some idea.

Well, thanks so much for taking the time to talk to me.

If I were you and I was going to emphasize something, I would certainly spend time on the physical parts of the system. But it’s these socioeconomic responses that scientists knew— but there hasn’t been enough attention to it in the past. It’s really emerging as the area where we’re gonna understand whether humans are gonna be able to manage climate change or whether we’re about to really go over a cliff. That’s where the big unknowns are gonna be. That’s where the big surprises are gonna be.

This interview has been condensed and edited.