Demystifying Sea Level Rise, with Robert Kopp

Notable Quotes

• “Over the last several decades, the increase in the amount of water being added to the ocean and the increase in the volume of water that started out in the ocean through thermal expansion and land ice factors have been roughly balanced. But the land ice portion of that is becoming an ever growing share.” (5:20)
• “If we're in a high emissions trajectory, then it becomes increasingly uncertain how much sea level rise there's going to be.” (9:50)
• “I think we can get on a course that will stabilize the climate and cut the scale of emissions and make smart investments to manage those changes that we have already committed us to.” (25:57)

Top of the Stack

References and recommendations made by Gilbert Metcalf

• "We Need Courage, Not Hope, To Face Climate Change" by Kate Marvel
• The Sixth Extinction: An Unnatural History by Elizabeth Kolbert
• New York 2140 by Kim Stanley Robinson

The Full Transcript

Daniel Raimi: Bob Kopp from Rutgers University—thank you so much for joining us today on Resources Radio.

Bob Kopp: My pleasure.

Daniel Raimi: So Bob, we're going to talk today mostly about sea-level rise and, in particular, about some of the latest research on that topic. But before we do that, we always like to learn from our guests how they got into the world of environmental research. How did it happen for you? How did you end up working on climate change and learning about sea level in particular?

Bob Kopp: Well, I actually started out wanting to study climate change on Mars. My undergraduate work was on Martian meteorites, particularly one meteorite that had putative evidence of signs of life in it. And from there, I went on to study sort of the long-term evolution of life and climate on our own planet. So my PhD work at Caltech focused in part on the fossils of bacteria that make magnets and how we can use those to reconstruct past environments, and in part on past global glaciations, something known as “Snowball Earth.”

After finishing up at Caltech, I wanted to sort of take what I had learned about how to look at the Earth's system as an integrative whole that evolves over time and apply it to some sort of pressing societal challenges. So that's sort of how I started working on climate change and sea-level rise. I did a postdoc at Princeton with Michael Oppenheimer, where I started working primarily on past sea-level changes and what we can learn from them about how sensitive sea level and ice sheets are to future warming.

From there I then went to the Department of Energy, where I spent two years in the climate policy office as an AAAS [American Association for the Advancement of Science] Science and Technology Policy Fellow. So I've worked partially on international energy efficiency cooperation and partially on the first US government estimates of the social cost of carbon, which is sort of how I got in the orbit of Resources for the Future in the first place. And then I came to Rutgers after finishing up there and I sort of continued to work both on sea-level change and on economic impact of climate change in my time here.

Daniel Raimi: Yeah. That's great. So lots of experience and lots of sort of interrelated pieces of this puzzle, which is fantastic. So we're going to focus in mostly on one of those pieces today, which is sea level. And the reason we're talking to you this week is because there have been a few papers coming out in the last several weeks and months, and I guess there are always new papers coming out. But it struck me that a couple papers were particularly interesting. One of them was by Frank Pattyn et al., in Nature Climate Change. The title of the paper, so people can look it up is “The Greenland and Antarctic ice sheets under 1.5 degree Celsius global warming.”

So we thought this would just be a useful time to check in about the state of the science on sea-level rise and try to understand kind of where we are in this new world of research. Before we dive into the latest findings, can you give us a brief overview of the key drivers of sea-level rise in the context of a changing climate, and also what some of the major areas of uncertainty are?

Bob Kopp: Sure. First of all, we have to distinguish between what's happening at the global average and what's happening in particular places, because sea level isn't uniform. So at the global average, there are two main things that are going on. One, the ocean is the main sink for heat on the planet. So it's the thing that slows down the warming of the planet. It absorbs heat and, as it does so, the ocean warms and it expands. And so thermal expansion, as we call it, is one of the key drivers of global mean sea-level rise. And the other is the change in the amount of water that's actually in the ocean.

And so the largest driving factors of changes in the amount of water in the ocean is melting of ice on land. And that's both ice in mountain glaciers and also ice in or adjacent to the planet's large ice sheets—the Greenland ice sheets in the north—which if it were all to melt would raise global average sea level by about 7 meters. And the East Antarctic and West Antarctic ice sheets in the south—which if they were to melt would raise global average sea levels by about 57 meters in total.

So, those are the main drivers. Over the last several decades, the increase in the amount of water being added to the ocean and the increase in the volume of water that started out in the ocean through thermal expansion and land-ice factors have been roughly balanced. But the land-ice portion of that is becoming an ever-growing share of that. Mountain glaciers are important but what we're really concerned about are the contributions from the ice sheets, just because there's so much potential sea level locked up in the ice there.

Daniel Raimi: Right. And the contribution of those ice sheets in the North and in the South that we're going to talk about—am I right in thinking that those are kind of the areas where the biggest uncertainty ranges are as well?

Bob Kopp: Yeah. And really, well, on the longer timescale anyways—yes, really, it's the ice sheets, but particularly the Antarctic ice sheets for reasons we can get into. But remember that I said there's a difference between global average sea level and local sea level—and that's important because, when we're talking the global average, that's not necessarily what [...] if you were to go to your local tide gauge in the Potomac River, you wouldn't be seeing a signal that matches global sea level precisely. You'd be seeing a signal that differs from global sea level, both because of factors that may have to do with the solid earth.

So, in DC you're adjacent to the edge of an ice sheet that once covered North America. And the land is sinking because that ice sheet, when it was there, pushed the land up. And you're also exposed to shifts in large-scale ocean circulation. So when the Gulf Stream shifts, that causes a sea-level change—sea level is a different height on different sides of the Gulf Stream. And so that's actually an important factor driving both variability along the coast of North America (the East Coast of North America) and also long-term changes.

Winds are another important driver. So we have to keep in mind that in the short term, it's actually not the ice sheets that are the dominant source of uncertainty. It's largely the sort of atmosphere and ocean dynamics. But in the long term, it is the ice sheets that are the dominant driver of uncertainty.

Daniel Raimi: Right. That makes sense. And so we're going to talk about different estimates of sea-level rise over different time periods. But before that, can you put that in a little bit of context by giving us some of the recent estimates on global mean sea-level rise by the end of the twenty-first century—or other timeframes that we're interested in? What are the consensus ranges that are out there today?

Bob Kopp: So sea level has been rising in the global average since the late nineteenth century. And current estimates are that it's risen by about 7 to 8 inches since 1900, with a good chunk of that (roughly 3 inches) happening in the last two and a half decades. So that's a context. There's also a growing body of evidence that directly ties most of that rise—particularly the rise since 1970—to human caused greenhouse gas emissions. Looking forward, if we look to 2050, we have relatively well-defined distributions of future projections.

And those distributions are relatively insensitive to how much greenhouse gas we emit. So the ice sheets and the oceans are sort of slow and sluggish systems—so just not long enough for differences between a future of high emissions and low emissions to kick in. And so in the global average, 2050, relative to where we were in the year 2000, our numbers are somewhere between about 0.4 feet and 1.3 feet of sea-level rise. So you can think of that as in the ballpark of about a foot.

As we go beyond 2050, the uncertainty in the projections and the sensitivity of the projections to greenhouse gas emissions grow quite a bit. So if we look to 2100, and if we're looking in a low-emission scenario—one that's sort of consistent with the Paris goal of bringing net greenhouse gas emissions to zero in the second half of the century—our projections are roughly in the range of 1 to 3 feet of global average sea-level rise. But if we're in a high emissions trajectory, then it becomes increasingly uncertain how much sea-level rise there's going to be.

If you look at projections that are consistent with sort of where the IPCC [Intergovernmental Panel on Climate Change] thought we were in 2013, you end up with numbers sort of roughly in the range of about 2 to 4 feet. There was one paper that made a big splash a couple of years ago about the potential instability of the Antarctic—driven by something called “marine ice clip instability.” And if you took that paper at face value, under a high emissions scenario of continued fossil fuel growth, you might be looking at a range of 3 to 8 feet.

That's probably a little overly aggressive compared to where most people in the community would land. But I think most people in the community would also land somewhat higher than the 2 to 4 foot range. We actually did a expert elicitation study with some colleagues at Resources for the Future a few months ago. And the key results of that are yet to be published, but they are sort of in the middle between those two distributions.

Daniel Raimi: Right. That's really helpful. So let's talk a little bit about at least one of these new papers, and you can add more context around them than me (who's only read a few of these things). So the paper that I read recently, the Pattyn et al. paper, makes estimates about how the ice sheets in Greenland and the West Antarctic ice sheets might melt and, in particular, where they might have so-called tipping points, which are the points at which they would completely collapse over some period of time.

So let's talk a little bit about that idea of tipping points for those two major ice sheets, starting with the Greenland ice sheet, which I think we might spend a little less time on than the Antarctic context—can you tell us what we sort of know about tipping points for the Greenland ice sheet, including how much sea-level rise might be likely to occur if we were to reach one of those points and also the timeframes over which that rise could happen?

Bob Kopp: Yeah. So I'm (along with some other colleagues) on record as not liking the term "tipping points," when it's applied to a lot of things in the climate system that many people like applying it to, because I think it's confusing. If you think about where most people talk about tipping points, you're talking about sort of Malcolm Gladwell-type tipping points where you hit some threshold and then things happen quickly. There are tipping points like that in the climate system where you can sort of hit a threshold and then get rapid changes.

But largely when we're talking about ice sheets, we're talking about thresholds that lead to irreversible commitments. But those commitments might still play out over thousands of years. And, you know, if we look at the Pattyn paper, certainly what it finds for the Greenland ice sheet. So one thing tipping points—in general, both sort of Gladwell-type tipping points and the other things that I would just prefer to call critical thresholds—have in common is that they involve feedback, so things that feed upon themselves, that amplify themselves.

In Greenland, the two feedbacks they focus on have to do with something called “elevation feedback.” Basically, as you melt the ice sheet it decreases in altitude. And at lower altitudes it's warmer, so you get more melt. And also the “albedo feedback.” As you shrink the ice sheet, you replace this highly reflective ice with dark land, so you absorb more of the sun's energy and you warm up faster. And those two feedbacks help drive sort of a critical threshold in the Greenland ice sheet which, that paper indicates, is somewhere in the range of 1.5 to 2.5 degrees Celsius of warming.

So, you pass some threshold and you commit yourself to this feedback loop that leads you to a much smaller Greenland ice sheet. And we have some evidence from about 125,000 years ago (the last time global average temperatures were around where they are today) that Greenland may have been smaller than today by enough to raise sea levels somewhere between, basically, 6 and 30 feet. It's actually a little contentious—how small Greenland was during this time period, the last interglacial. But sea level itself was substantially higher, and Greenland seems to be somewhat smaller. So that's sort of the Greenland story.

Daniel Raimi: And when we think about timeframes for those changes in the system, what kind of timeframes are we looking at for Greenland?

Bob Kopp: You're probably looking at several thousand years for a full transition to take place. Right now, Greenland is contributing slightly less than a millimeter per year to global average sea-level rise. And most of the projections for 2100 would put its contribution in the sort of 10 centimeter-ish range.

Daniel Raimi: Yeah, okay. So—thousands of years. That definitely doesn't sound like a tipping point. I'm glad you cleared that up. When I think of a tipping point I think of my five-month-old son who's falling over from a sitting position. And when he starts to go, he goes fast. So that's definitely not what's happening in Greenland.

Bob Kopp: That is also what my five-month-old son does.

Daniel Raimi: We should talk about this more. Let's move south now from Greenland and talk a little bit about Antarctica. Can you tell us a little bit about what the outlook is for Antarctica, in terms of how much it's contributing to sea-level rise today and what the projections are over the coming decades—and then move into some of these critical threshold questions and what we might be seeing in the research for Antarctica in terms of longer-term sea-level rise?

Bob Kopp: Yes. Well, Antarctica is—I may be a little biased but, I think—where some of the most important questions determining future sea-level rise are today. And the reason why it's more Antarctica than Greenland has to do with the nature of the ice sheet. There are many vulnerable regions of the Antarctic, mostly in West Antarctica but also some in the much larger East Antarctic, where the ice sits below sea level. And so the dominant factor determining what happens to the ice isn't, as it is in Greenland, sort of the balance between how much snow you get and how much melt you get.

It's really quite complicated interactions that happen between the ice sheet, the floating ice shelves at the edge of the ice sheet, and the ocean. So there was actually a paper that came out that had an estimate of the Antarctic contribution over the last eight years or so and its estimate was that the Antarctic contribution had grown so that it was almost as large as the Greenland contribution. And it also found that there was a significant contribution over the last several decades from a part of the Antarctic that many people think has actually been fairly stable, the East Antarctic.

So, why are we so concerned about the Antarctic? Well, we have these complex interactions between the ocean and the ice sheet and the ice shelves, and they give rise to sort of two different feedback loops we're concerned about. When you think positive feedback loops, amplifying feedback loops, you always have potential for these critical thresholds. And in the case of the Antarctic, we're really not sure what brakes are and what the speed limits are. But one of those feedback loops involves something called “marine ice sheet instability.”

And the idea there is, basically, you have the ice sheet sort of sitting in a bowl that it's created under its own weight in the Earth. As warm water eats away at the ice sheet from underneath, you actually expose a larger and larger cross-section of the bowl to warm water. So you get this marine ice sheet instability. That's been something that's been studied significantly for a little over a decade and is in most of the models that are used to project future Antarctic change.

And then there's this second, very controversial idea involving something called “marine ice clip instability.” And the idea there is that, if you lose the floating ice shelves—and ice shelves, for those who don't know, they're sort of ice at the edge of the ice sheet that are floating. And so when they melt, they don't affect sea level directly. But by being there, they sort of protect the ice sheet from some of the exposure it might otherwise get.

Daniel Raimi: Right. It's sort of holding back the ice that's currently on land.

Bob Kopp: Holding back the ice that's currently on land. And if you lose those, then there's the potential to get large cliffs of ice that can become unstable under their own weight. And you can see this process happening in sort of narrow fjords in Greenland. But we don't think it's a very important process in the Antarctic right now—but one of the big controversies is how important it might become in the future.

It's that process—the loss of the ice shelves followed by the instability of the ice cliffs—that led this paper by [inaudible 00:19:07] that came out in 2016 to predict very high-end Antarctic contributions that gave rise to that sort of 3 to 8 foot number for total global sea-level rise by the end of the century that I suggested. And, as I said, I think Rob and Dave have a new paper that will delay that a little, so they don't get quite as large numbers. But you're still talking potentially about numbers that are significantly larger than what the IPCC thought five years ago.

Daniel Raimi: Right. And as this new research is coming out, how quickly does it get integrated into these sort of larger Earth system models? Do modelers who work on this type of stuff, are they able to integrate it relatively quickly? Or is there a lot of testing and sort of model inter-comparison that needs to happen before we have much confidence in the models?

Bob Kopp: I guess one of the things that is a frontier, but it only sometimes happens right now. Ice sheets actually aren't in the larger system models, by and large. There are some specific experiments where the modeler tried coupling an ice sheet into an Earth system model. But, by and large, when we're looking at Earth system models—the things that model the oceans and the atmosphere and the land—the ice sheets are just sitting there. They're not dynamically responding. And so when you turn to these Earth system models or global climate models, they can tell you about changes in sea level driven by the uptake of heat by the ocean and by changes in ocean and atmosphere circulation.

But for other contributors to sea-level rise, you actually have to go and take the output from those models and feed it into another model that, say, looks at how glaciers change or looks at how the ice sheets change. And one of the things that's actually emerging from some of the work on the ice sheet is that, sometimes you may actually need to do a full and very tedious and slow coupling between those two because, for instance, the runoff from the ice sheet may have a significant effect back on the oceans and the atmosphere.

Daniel Raimi: Right. That's so interesting. I imagine this is a really active area of research that people are working on.

Bob Kopp: Yes, definitely.

Daniel Raimi: Last question about Antarctica, which is the same as the question I asked about Greenland a few minutes ago: When we are thinking about these long-term projections for reaching these critical thresholds for Antarctica, are we also thinking about timeframes of thousands of years like we are for Greenland?

Bob Kopp: I mean, again, it sort of depends where your story sits. If you were to melt all of the Antarctic, you would be talking about 200 feet of sea-level rise. And we're saying, "Well, let's suppose we're on a high emissions trajectory. Do we commit to 200 feet of sea-level rise?" We may in a couple of centuries. But we are not going to get a couple of hundred feet of sea-level rise for several millennia. But when you have a couple of hundred feet of potential sea-level rise there, it doesn't take that much to have an amount of sea-level rise that is quite significant by human standards.

So, as I said, when we are talking about things like 8 feet of sea-level rise in the century, that's within the realm of physical plausibility. And it's within the realm of physical plausibility largely because you could see a contribution from Antarctica of that scale or of the scale of several feet. Now, if we see an Antarctic contribution that's, say, 3 feet in this century (that you would need in order to get to that 8 foot of global average sea-level rise)—that means we've committed to much more than that. The system is still quite slowly responding, but it can respond quickly enough that we can get a large amount on a timeframe that is economically relevant. And that's just a down payment on what would be entrained if that is what we saw.

Daniel Raimi: Right. That makes sense. So that makes me think of a question that people ask me frequently when they learn that I do at least some research on climate issues. And when they ask me this question, they're not just thinking about sea-level rise. But let me ask it just in the context of sea-level rise. So, if I tell someone I work on climate change, one of the questions that they often ask is, "Are we doomed to some sort of dystopian future, where sea levels are 20, 50, 100 feet higher than they are today?" When people ask you that question, how do you think about it and how do you respond to it?

Bob Kopp: So, the answer is, "No, we're not doomed." So let's start with that. There is sea-level rise that we can't avoid, and it's on the scale of feet in this century, and we will have to take measures to adapt to that. If you ask, "Are there small island states, are there barrier islands that might not make it through this century?" Then I say, "Yes. In that context, there are areas where, probably, relocation is inevitable." But more broadly, one: we can reduce our emissions. And if we bring them down to zero in this century, we significantly cut this sort of tail of possible really high-end outcomes.

Second: humans are smart and we can, I think, make investments to adapt to the changes that are in store. The state of infrastructure in the US sometimes give me pause when I say that, but there certainly have been times in American history where we have made investments with significant foresight. And then I guess the third thing is dystopian futures are [...] that's a product of the social system. The boundary condition may be provided by the climate, but how we respond to that is the choice of individuals and societies and economic systems. And I think we can get on a course that will stabilize the climate and cut the scale of emissions and make smart investments to manage those changes that we have already committed us to.

Daniel Raimi: Yeah. That's really well said. I totally agree with it and wish I could have you at my dinner parties to be more eloquently answering that question to them than I usually am. Thanks so much for sharing all of this great insight. And I know we've really just scratched the surface here on the topic of sea-level rise. But thank you so much for sharing all of your perspectives. I want to close out by asking you the same question that we always ask our guests at the end of an episode, which is: What have you read or watched or heard recently related to energy and the environment that you think is really interesting and that you'd recommend to our listeners?

Bob Kopp: So I was thinking about that question and, to be honest, I had a first son a few months ago—so I haven't really had that much time to do things that aren't sort of technical reading, as you're probably aware.

Daniel Raimi: Yes. I know the feeling.

Bob Kopp: But there are a couple of things that came to mind. So, one, there's this very nice essay that Kate Marvel from Columbia wrote. It was a personal reflection—it gets back to your question about are we doomed. And she was arguing we shouldn't be thinking about this in terms of hope. We should be thinking about this in terms of courage. We are creating a world that is different from the one that we grew up in. And we're going to need courage to deal with that, not just hope that it will be a better place.

That was a very, very interesting reflection. There's a number of popular books that I think have been fairly interesting. Elizabeth Kolbert's Sixth Extinction is one that I liked and have thought about using in teaching, because it sort of places modern changes in this long timescale context. And then I've also sort of been in a little bit of a climate fiction kick, in particular sort of looking for fiction that, again, kind of has that theme of courage or hope—not despair—that sort of presents a view of humans thriving in the face of what is generally, for literary reasons, fairly extreme climate change. So Kim Stanley Robinson had a book a couple of years ago, New York 2140, that I liked a lot. Also, Cory Doctorow had one, Walkaway. So this idea of sort of optimistic visions of the future.

Daniel Raimi: Right. Yeah. New York 2140—a friend lent that to me recently and I'm actually in the middle of it right now. It's funny [that] you mentioned climate-related fiction. We had Fran Moore from UC Davis on the show a few weeks ago, and she actually recommended some of the same books—or at least the same topic, actually, not any of the same books, but the same topic. So it's good to know that we're out there thinking about climate at work and at home. Bob Kopp, thank you so much for joining us today on Resources Radio. It's been really fascinating.

Bob Kopp: My pleasure.