Season 3/Episode 8: Torbjörn Törnqvist and Sönke Dangendorf: Sea Level Rise and Coastal Restoration
Sönke Dangendorf
Torbjörn Törnqvist
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Tulane professors Torbjörn (Tor) Törnqvist, a geologist, and Sönke Dangendorf, a coastal engineer and physical oceanographer, join John to talk about sea level rise and coastal restoration, and what could happen to coastal communities if we pass the Paris Agreement global temperature threshold of 1.5-degree Celsius. Sönke has more than 15 years of experience researching mean and extreme sea levels, ocean tides and storm surges and the impact on coastal flooding, and is a member of the NASA Sea-Level Change Team. Tor has been researching the evolution of rivers, deltas, coasts, and shallow oceans in response to climate and sea-level change for more than 20 years.
Show Notes
Read their paper on ecosystem collapse due to anthropogenic climate change
IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
Sönke says that since 1900, “sea level has been rising by approximately 1.5 millimeters per year on average,” which is about 19 centimeters, or eight inches. Read the preprint abstract: Probabilistic reconstruction of sea-level changes and their causes since 1900
Sönke discusses projections should we reach the cap set out in the Paris Agreement of 1.5 degrees Celsius. Read more about the Paris Agreement here.
Sönke says that nuisance flooding costs can equal that of a hurricane over time. Read more here.
Sönke reports that Katrina would have been much less destructive before sea-level rise over the 20th century. Read the study here.
Sönke discusses that locally, additional processes affect the rates of sea level rise, such as vertical land motion, which “is a huge factor” in New Orleans and the Gulf of Mexico. Learn more here.
Sönke mentions that sea-level rise is “six times as fast as a global mean sea-level rise over the same period,” in part due to fluid withdrawals. Read one study here.
Tor Törnqvist says “most of our wetlands are going to disappear. That basically a done deal now.” Read the study here.
Tor said that nearly half of the top 10 counties in the nation that are losing population were in Louisiana and mostly right along the coast. See the Census results he’s referring to here.
Tor says that sea-level rise could put the shoreline of Louisiana “somewhere near Baton Rouge.” View NOAA’s interactive Sea Level Rise Viewer.
Tor mentioned that in the warm period before ice age, the global temperature wasn’t much higher than the temperature now. “Global sea level was somewhere between six and 10 meters higher than present,” he says. Read more about ice sheets and sea level rise in the past.
Transcript
START (DANGENDORF AND TÖRNQVIST INTERVIEW)
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JOHN: Welcome to Audacious Water, the podcast about how to create a world of water abundance for everyone. I'm John Sabo, director of the ByWater Institute at Tulane University.
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JOHN: On today's show, sea-level rise and coastal restoration. I have two guests, Sönke Dangendorf and Tor Törnqvist. Sönke is a coastal engineer and physical oceanographer, and is currently a member of the NASA Sea-Level Change team. He was a contributing co-author to the Special Report on the Ocean and Cryosphere in a Changing Climate of the Intergovernmental Panel on Climate Change, or IPCC. Tor is a geologist and has been researching the evolution of rivers, deltas, coasts, and shallow oceans in response to climate and sea-level change for more than 20 years. He was a co-author of the recent paper about the widespread retreat of coastal habitat. Coming up, Sönke and I talk about sea-level rise and what the future might look like for coastal communities due to rising seas.
JOHN: Tor and Sönke, welcome to the show.
TOR: Thanks.
SÖNKE: Hi, John. Thank you for the invitation.
JOHN: Great. Well, let's start with Sönke and talk a little bit about sea-level rise, and just some basic sea-level rise data and questions. How much sea-level rise has already happened, and how much is predicted to happen if we overshoot 1.5?
SÖNKE: Yeah, over the 20th Century this is basically when we got our first observations from (type-gauge) networks (available). Sea level has been rising by approximately 1.5 millimeter per year on average, over the global oceans. So that's about 19 centimeters or 8 inches since 1900. This rate of 1.5 millimeter per year, to put it into historical perspective, is unprecedented over at least the last 3000 years. So that means an acceleration from what we have seen before. This rate has also further accelerated since the 1960s, and reaching recently rates of approximately 4 millimeters per year. When we look into the future we of course don't really know what sea levels will do--that depends on greenhouse-gas emissions and mitigation strategies. But if we look into different scenarios of that these projections indicate an additional rise that will likely fall between 27 centimeters or a meter by 2100, depending on the greenhouse-gas-emission scenarios. I have to note though that there are what we call "low-confidence processes" that are linked to uncertainties in our understanding of the large ice sheets in Greenland, and particularly Antarctica, and that means that we cannot exclude, in particular, under the high greenhouse-gas-emissions scenario an additional sea-level rise of approximately one meter. So in total two meters cannot be excluded. If we reach the Paris goal, which would mean that we would limit global warming to 1.5 degrees Celsius, then we would end up very likely with a sea-level rise of approximately not more than 50 centimeters. So you would basically keep it below 50 centimeters.
JOHN: Let's unpack that a little bit. You said the recent rate over the last century was one centimeter per year I think, and then you said four since 1960. So that's a quadrupling in recent times, is that right? Did I hear that right?
SÖNKE: Yes, but it was millimeters, not centimeters.
JOHN: Ah, millimeters, okay.
SÖNKE: So yes. So we have been talking about 1.5 over the 20th Century, and then 4 millimeters most recently over the last decade.
JOHN: And then talk to me a little bit about those low-uncertainty processes. Tell me about the ice sheets and what causes the high uncertainty.
SÖNKE: Well, we have to understand that the two ice sheets in Greenland, and in particular Antarctica, are very remote locations. And they, compared to other parts of - or the other contributions of seal level, they are not that well-understood just because we don't have so many observations. One of the key uncertainties that we have with the Antarctic ice sheet, for instance, is that the Antarctic ice sheet sits on - of course on topography, but is surrounded by ice shelves. And Antarctica is so cold that if you warm it by a couple of degrees, which we would expect with global warming, that would just end up with more snowfall, so it would basically increase the mass of the ice sheet. However, the way how Antarctica loses mass is by the ocean that surrounds it. The ocean can interact with the ice shelves basically, and there are many areas around Antarctica in near-coastal areas that are below sea level. You can see them as ice (inaudible) or something like that. And if the ocean actually overtops what we call the "grounding line" at some point, then all the ice that is stored behind that point is forced to retreat at some point. But we don't really understand how long that may take. That may take several centuries, and that basically determines this uncertainty that we have. And we think that it primarily kicks in with the high-emission scenarios.
JOHN: So it's kind of like complex topography and morphology of the ice sheet itself, and uncertainty in how those microregions are going to interact with the ocean?
SÖNKE: Yes, exactly.
JOHN: Let's bring it home a little bit. When you talk about accelerating rates of sea-level rise what does that mean for coastal communities, especially those that we're familiar with in the Gulf?
SÖNKE: We have to understand that, in particular over the past one or two centuries, as a society we have been moving into coastal locations. We have basically moved into those locations and adapted to average conditions that were usual one- or two-hundred years ago. And maybe at that time they were relatively stable. However, over the past decade we have seen accelerating sea levels, and that has created several issues. For instance, in particular along the U.S. East Coast, and more recently also along the Gulf Coast, we have experienced minor flood events. So we call that "nuisance flooding." So just a high tide due to sea-level rise can now flood coastal areas. This is not usually - these are non-fatal events so they are not really dangerous, but they can hinder you commuting to work, they create of course flooding, it can mean that you are stuck at home, so you can't leave home for several days or something, and can block infrastructure. So that has serious impacts. And to put it into perspective again, there are some locations that have accumulated costs over the last couple of years, per year, that are equal to a landfalling hurricane. So that is a big burden for coastal communities. So that's one of the, let's call it, "minor" impacts, but that really disrupts our daily life. The other impact is of course that storm surges that we know from landfalling hurricanes, for instance, become way more destructive if you have a higher-base sea level below it. And if sea levels accelerate that means that those hurricanes, even if they don't become more frequent or intense, their storm surge becomes way more destructive.
JOHN: I think that's insightful, because I think a lot of times, especially if you don't live next to the ocean, you hear about sea-level rise but don't really - you kind of think of it as being a phenomenon in 2100 or something like that. And, you know, I know I won't be here in 2100 so it kind of puts it out-of-sight-out-of-mind, but the context of having daily interruptions, not every day, but things that affect your daily life now I think is really poignant. And certainly we saw in Ian the increased destructive power of storm surge, right?
SÖNKE: Yes, that's true. And it is important to understand. For instance, I mean, floodings have been happening along this coastline before as well. But, like, in the 1950s, 1960s most of the locations have not experienced more than maybe two or three events as a maximum per year, if at all. So in the recent years there are many locations where these events now happen, like, 30, 40 times. And at many locations we would expect in a couple of years that number increases to, like, more than 100, 200, or even 300 days per year. And that makes many places (un)inhabitable basically, in particular along a coastline like in Louisiana; that is very fragmented. And yeah, so the storm surges, the same thing. There have been investigations, for instance, with Katrina. Katrina occurred in 2006, before we have experienced a significant sea-level rise over the 20th Century. And without that sea-level rise Katrina would have been way less destructive. So that is something where we can immediately see the impact.
JOHN: Thanks for putting that in perspective, especially in context to Katrina because it is, you know, that sort of baseline that we live with in New Orleans.
SÖNKE: Yeah, that's true.
JOHN: Let's go to the sea-level rise in the Gulf. And I’m thinking about some of your recent work and recent papers. Is the situation better or worse in the Gulf than globally? Because you were giving global statistics before. Focus in on the Gulf if you can.
SÖNKE: Yeah, sure. I mean, it is important to understand that sea levels globally are rising. Two processes--it's either the melting of the ice sheets, which put more mass into the ocean, or it's what we call the "thermal expansion," so the heating of the ocean leads to an expansion and therefore to sea-level rise. However, regionally or locally there are additional processes that can affect the rates of sea-level rise that can be circulation systems or wind systems that push water masses from one side of the ocean to the other. There are processes like vertical-(inaudible) motion, for instance, so it's not only the ocean that is changing but also the land on which we build is moving up and down. And here in the Gulf of Mexico that is a huge factor that makes the situation way worse than anywhere else. In particular along the Louisiana and Texas coastlines, where we have observed rapid subsidence over the last 100 years. That was, in case of Louisiana, of course partially related to the fact that we live in the Mississippi Delta, and that much of the land is cut off from sediments that usually come with river floods. But there have been additional processes like fluid-withdrawals, oil, gas in Texas for drinking water purposes. For instance in Grand Isle, to put that again into perspective, over the same period sea levels have been rising by more than nine millimeters per year. So that's approximately six times as fast as global mean sea-level rise over the same period. So that hopefully puts it a little bit into perspective.
JOHN: That's stunning. Let's close on the sea-level piece with going back to kind of the global scope and... I read recently in AR6 report, that even with perfect mitigation sea-level rise will continue for a millennium. And I may have the exacts a little bit off, but the idea is that no matter what we do it's going to continue. Can you give us a little bit of 101 on the physics of that?
SÖNKE: I mean, we have to understand that the ocean is probably the most persistent part of the Earth system, or the ice sheets and the ocean, so it reacts way slower than any other system that we know. So basically you set something in motion, it takes a while until it is measurable, but then it continues for a long time. Just as an example, I sometimes tell my students to understand that if you would go to the Mississippi River and try to push a little boat you could move it very quickly, but it would stop at some point. If you would like to push a big tanker that requires way more pushing, but once you get it moved it is way, way, harder to slow it down, right? And that is happening in the ocean as well. It has a lot of inertia. Basically even if we were to get to net-zero greenhouse-gas emissions right now then we would still be committed to sea-level rise over the next centuries to come. But if we were to stop emitting greenhouse gases that would certainly prevent dangerous sea-level rise in the future. So it would probably keep it to within limits that are manageable for society. Independently of that, that also means that there is a need for adaptation. So we need to adapt to sea-level rise no matter what. Locally we need mitigation on a global-policy level, but on a local level we need adaptation for sure.
JOHN: Thanks for bringing those two up, adaptation and mitigation, and also putting that into the context for - from the standpoint of mitigation would prevent catastrophic impacts, even if sea-level rise continues with perfect mitigation. I think that's good context. And maybe off the record, I like your analogy to big ships and small ships. And I always say it--I'm in Mexico City right now--I always say that drinking mezcal is like a big ship leaving port. Takes a while to get up to speed, but once you get up to speed it's really hard to slow down.
SÖNKE: Yeah, that's true [LAUGHTER].
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JOHN: Up next, Tor and I discuss the state of rising seas in the Gulf, what mitigation and restoration can actually do on a local scale, and why an earlier retreat might be the best option.
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JOHN: Let's turn to Tor now and talk about coastal restoration and subsident sediment processes and the interaction of that with sea-level rise in the context of adaptation measures that the state is taking on through CPRA to protect our coastline and to keep our coastline intact. Tor, we're, like, kind of in a Red Queen situation, you know, where mitigation, or lack thereof, is creating more need for coastal production. But at the same time we've got sea-level rise happening right in our back yard. How are we doing, and is it possible to keep up with sea-level rise in doing these projects?
TOR: I would say to a certain extent, but we have to be very realistic about what they can do and what they cannot do. And I think it's increasingly recognized also at the state level here by Coastal Protection of Restoration Authority that these coastal restoration efforts that are underway or that are in the planning, that they could be helpful at the local level. But if you look at the entire coast here in Louisiana we are in the process of what you could call a pretty large-scale retreat. You know, most of our wetlands are going to disappear; that's basically a done deal now. And unfortunately we know from studies, both here in the region and also worldwide, that coastal wetlands here in the Gulf of Mexico are among the most vulnerable in the world. A lot of that has to do with the fact that our tides are incredibly low. As a result these wetlands are sitting just right above sea level. And as a result it doesn't take very much for them to basically drown and disappear. And unfortunately we now have a much better understanding of the threshold rates of sea level that basically set wetlands up for drowning, even though it may not happen right away. And based on that we know that threshold has already been passed. We just heard Sönke talk about adaptation. We are starting to adapt at a very small scale, but we're going to have to ramp that up. At the same time, to answer your question about restoration, there are certainly things we can do to slow down the process at the local scale. This is where, for example, we hope to do these river diversions that can actually rebuild some land in certain targeted areas. But the implication of that is that there is many other areas that we are going to have to abandon.
JOHN: That's great. Let's talk about the interaction between sediment and sea-level rise just in really 101 basic terms so that the listeners can understand why this is happening. So you talked about diversions, sediment diversions to rebuild land. And I mentioned the Red Queen before--you know, with sea-level rise accelerating what's the challenge in terms of keeping some of these projects above water?
TOR: The challenge is that the more sea-level rise further accelerates the more sediment you're going to need to maintain let's say the same area you had at the beginning, given that the sediment's availability is not increasing. In fact, in many cases it's decreasing because of all kinds of actions that have been taken further inland. That means that sea level basically creates a lot of space that has to be filled with sediment, right? And if that sediment is not there, yeah, then you're basically going to convert these low-lying wetlands into open water. So yeah, you're right, ultimately it's all the connection between, on one hand, the sediment that is coming in, on the other hand, the rise of sea level, whether it's due to climate change or, in the case of the Gulf Coast, further exacerbated by subsidence.
JOHN: So where sea-level rise and subsidence outstrip sediment input you're going to have drowned wetlands?
TOR: Yep, that's right. Unfortunately we know based on how our coastal wetlands behaved in the geological past when they faced conditions that are similar to the conditions we are going to see later this century, we know that when those rates exceed something like three millimeters per year they may hang in there for quite some time, but eventually they're not going to make it. And as Sönke just explained, the global average of sea-level rise right now is already four millimeters per year, and it's not going to go below three probably for centuries. So yeah, that's the harsh truth, that we're going to lose a lot more in - later this century and beyond.
JOHN: Well, it's stunning. Let's turn to maybe something more strategic. If we're - let's pretend like we're state planners and we're thinking about the portfolio of projects that might be available in the strategic plan. Can you give some advice about where those projects might be most impactful in the future if built now?
TOR: We have to shift our priorities much more towards incentivizing people to retreat. One of the things - I'll give you one example. This is obviously a difficult and also controversial issue. Something that was reported fairly recently--I don't know if you saw this--it was kind of an annual census assessment for the entire country, and it looks at the county-level population change just over the last year. And it turned out that if you look at the top ten counties in the nation that are losing population, that about half of them were in Louisiana, and mostly right here along the coast. Generally that is looked at as, you know, something negative, especially if you look at it as an economist. Economists tend to like population growth, right, because that is often associated with economic growth. And this is of course quite the opposite. I think we're going to have to change our mindset on this. And personally even though there is a lot of human suffering that goes along with this, right--there are people who are displaced by hurricanes and are not able to come back--but in the long run I think it's a better situation to have population loss start now than if you think about what's going on in Florida, which is still growing incredibly rapidly, in particular in these very vulnerable coastal zones. You can see the catastrophe playing out in the future. So maybe that's the one little silver lining here in Louisiana, that we are in some ways ahead of the curve, even though it's maybe not planned. But going back to what I said at the beginning, I think we can do a lot more to try to incentivize people to rebuild their lives in a safer place further inland where they're just going to be a lot less vulnerable. I do think it's going to happen no matter what, because we already see what's happening with insurance rates that become completely unaffordable for a lot of people. Yeah, that's going to drive people out. And that's probably one of the reasons for the population loss that's already happening.
JOHN: You went straight to wearing the economist hat with the insurance, so that's the primary driver. And, you know, I wasn't expecting you to go there that fast, but I'm glad you did. Like, I was thinking about more, like, where are the wetlands going to be in the future? But I think that's kind of what you're saying is, like, the population will shift inland. And so the follow-up question is should we be building to protect those places where the retreat is going to happen, like the recipient communities of those people who are retreating?
TOR: That's what we have to start thinking about, right? Where is this going? And realistically, given what Sönke said earlier, that the ocean is very slow, the response - even if we stop emitting greenhouse gases tomorrow chances are that we're going to see continued rise for a few more centuries. Realistically given where we are now we're probably looking at a shoreline that may be eventually stabilizing somewhere near Baton Rouge. And that means we're talking about immense numbers of people who have to relocate. And I would take it a little further and say that we need to meet the Paris goals to ensure that the shoreline will eventually stabilize in Baton Rouge and not go even further. We know that we go back to the last warm period, before the last ice age, we had a global temperature that was not that much higher than the temperature we have right now. And global sea level was somewhere between 6 and 10 meters higher than present. So if we don't meet the Paris goals then the probability that we're eventually going to end up with that situation is going to be pretty high. Yeah, and then you talk about can we also save Baton Rouge? This may sound unreal right now, but it is not that unreal.
JOHN: I'm glad you put that into perspective. And I would encourage the listeners to look at the sea-level rise viewer that NOAA has published if you want to see what that looks like at two meters, which is one scenario in 2100. The coastline is almost at Baton Rouge at that level, and I think that'll give you some context, both on how much land-loss there is under that scenario, and where the coastline migrates in that scenario. So let's finish, Tor, with an academic question that I think is super interesting. And, you know, I've heard you say things about this question in the media before, which I think are spot-on, and certainly are parallel to things that we've been thinking about at ByWater. Are these projects that are in areas that are going to be gone in 2050 or 2100, are they awash, or can we do something with them to help us in the future in some way?
TOR: Yeah - no, it's a totally valid question. And one of the reasons I think these big river-diversion projects are incredibly important is that on one hand they can give us more time, right? Because eventually New Orleans is going to have to retreat, right? And the big diversions that are planned are clearly designed to give the New Orleans region more of a buffer. And that could potentially give us maybe a few more decades, which can make the difference between somewhat managed retreat vs. complete chaos, which is of course the last thing anyone should want. But there's another argument in favor of doing these things, even though they are costly, and that is that these are not going to be the last diversions. We're going to do more of those. In all likelihood we're going to do more of those further inland. I imagine at some point--and I'm not going to live to see this; none of us will--but at some point there will probably be a big diversion that will be used to fill Lake Pontchartrain. And so to provide a buffer for the area north of Lake Pontchartrain where, you know, people will live at that time, that's relatively high ground. So that should be, you know, a reasonable spot to move to. But you may want to have a little bit of - a nice buffer of wetlands between that and the open Gulf. So in other words, doing these diversions now will actually give us an opportunity to learn how you actually do that, and how... Because of course there will be mistakes, and there will be things that don't go as well as they maybe could. But that's the only way you're going to learn it, right? It's not going to be perfect in the first try. But it would be very beneficial if we gain enough experience with these things that later down the line we can do it in a more effective, and maybe also cost-effective way.
JOHN: When I'm listening to you I'm thinking of Sherlock Holmes, inductive reasoning, and learning from trial and error, and ruling out things, and improving. And I’m wondering if there's a way that we could fast-track that with experiments. Like, you know, some of these diversions are so big that you could ostensibly split them in half and look at the difference between control and experiment. Any thoughts on that idea?
TOR: That shouldn't be too hard because these diversions, even though they're big, they still affect a somewhat limited area. And the counterparts that you're referring to are all the other areas where we don't do diversions. And we can do a pretty straightforward comparison how those areas are going to fare compared to the area where the diversion is happening. So it is like a big natural experiment.
JOHN: Well, I appreciate your time, Tor and Sönke. Thanks for being on the show and thanks for all your insights.
TOR: Thank you, John.
SÖNKE: Thank you, John. I really appreciate it.
JOHN: Okay, this is a wrap on a very sobering topic of the reality of sea-level rise and coastline retreat. Sea-level rise is happening right now, right in front of our eyes on the frontlines of climate change here in Louisiana and in the Gulf south. Mean sea level is already about 8 inches higher than it was in 1900. Last summer I did a road trip with my 14-year-old daughter Lily, from the mouth of the Mississippi River in Louisiana to the headwaters in Minnesota. Be on the lookout for a bonus episode that chronicles this trip. We started this trip in Venice, Louisiana. Two months later Lily was in Venice, Italy. In both places you can see the ocean overtaking infrastructure and households. In Louisiana we stood at the edge of the sea, where Highway 1 now disappears into a coastal marsh, birds fishing on the underwater road to nowhere, and critical oil and gas infrastructure only 100 yards away. In Italy stairways leading into centuries-old human dwellings are halfway submerged. Unless you've seen it with your own eyes like I have it's hard to understand. To put 8 inches into perspective, Louisiana has lost an estimated 4833 square miles of land in the last century, says generative AI. This is equivalent to the land area of the State of Delaware; a whole state is gone. More sobering, even if we do meet the goals of the Paris Agreement sea-level rise will continue to gobble up coastline for centuries; the IPCC says a millennium. The physics of the ocean are slow to change and equally slow to reverse course. This does not mean we can jettison mitigation. We have to mitigate to Paris Agreement targets or the coastline of Louisiana will be at Baton Rouge, not New Orleans. New Orleans will be an Alcatraz at sea, protected by multibillion-dollar flood- and surge-protection systems, but stranded from the state's capital across a moat of see. More importantly, we need adaptation, science and action. The IPCC AR6 report is clear about the enormous adaptation gap in front of us. Without it there will be people forgotten in this human retreat that corresponds to the physical retreat of the boundary between land and ocean. If we view the chaos that Tor alluded to through an equity lens, the haves will retreat, and the have-nots will be forced to withstand the advance of the sea and the calamity that brings with storms, storm surge, and local flooding. How we reconfigure communities in a climate-adapted future to address this inequity will have large bearing on whether we just survive climate change or get ahead of it enough to thrive.
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JOHN: That's it for this episode of Audacious Water. If you liked the show please rate or review us and tell your colleagues and friends. For more information about Audacious Water visit our website at AudaciousWater.org/podcast. Until next time, I'm John Sabo.
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END (DANGENDORF AND TÖRNQVIST INTERVIEW)