Understanding sea level rise

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RICK WEISS: Hello, everybody. Welcome to our media briefing on understanding sea level rise and the modeling of sea level rise. I want to just give a quick moment to introduce you to SciLine so you know who’s bringing you this media briefing today. SciLine was just last fall. It’s an editorial independent operation located at the American Association for the Advancement of Science. It’s funded completely by philanthropies. And the soul goal of this operation is to help reporters who are covering stories on environment, health, and science.

And we do that in response in part to a trend that I think everyone’s familiar with in journalism today, which is that newsrooms are shrinking. Specialty reporters are shrinking as a core. More and more science stories, and health and environment stories, are being done by reporters, like many of you online today, who cover big beats, who are not necessarily specialists, and who could use some help getting the evidence and the expert sources that you know can be so important to make your stories better. Especially given the increasing deadline pressures that everybody’s under today doing sometimes two or three stories a day instead of just one.

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So, our goal is to get more evidence into news stories and to help you do that. One of the ways we do it is to provide a matching service where, if you’re working on a story, you can get in touch with us, tell us what kind of expert you need. We have a huge database of scientists who are very good in their fields and good communicators. And we have a concierge service of sorts that will hook you up with the expert you need to help you get more evidence into your story. We also do news briefings like this. We’ve got fact sheets that we’re starting to populate our website with and some other services. So, you can check out our website at www.sciline.org and learn more about what we’re up to.

The issue of sea level rise and the modeling that goes into trying to understand sea level rise is the perfect kind of a topic for what we’re seeking to do here. It’s one of those things that seems to be of great potential consequence. But it’s also an area of a lot of confusion. So, we’d like to try to clear that up since it’s important and confusing. I think we’ve all heard that sea levels are rising, and we’ve all heard that climate change seems to be one of the, if not the major reason that sea levels are rising.

But I know for myself that the details about that can be confusing and it’s hard to know what to make of it. We hear stories about millimeters per year which, frankly, doesn’t sound like that big a deal compared to the size of ripples washing up on the shore, and the waves, and the tides. But I also have read that if Greenland were to melt, that’s 20 feet of sea level rise. And it seems like things are melting fast. And Antarctica’s a lot bigger than Greenland. So, what to make of all that?

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I know I’m mixing some frozen apples and thawed oranges here. But the basic idea is we ought to get straight as reporters how to talk about this. And not make the mistake of feeding into climate denial. Or making the mistake of feeding into being what some people have come to call climate bullies. Or maybe feeding into the worse crime of all, I think, which is this namby-pamby false equivalence that comes off a lot where equal space is given to people with weak science. So, to help unpack all this and to make sure that you all can write clearly and accurately about this topic and inform the public and decision makers about what the facts are, and what we know and what we don’t know and how we know it, we have four experts today with great credentials who are going to explain this to us.

I’m not going to go through the full bios with you right now because they’re all available on the web landing page that you folks have seen. But I’ll just give you the batting order, and that is that you’ll first hear from Michael Oppenheimer of Princeton who will give an overview of what we know today about sea level rise and about the models that scientists use to understand what’s happening. Because it never hurts to have the basics spelled out authoritatively. Next, you’ll hear from Andrea Dutton from the University of Florida who will take us back in time to talk about the paleo climate and how studies of climate and sea levels in the distant past can inform and allow us to calibrate our models and predictions today.

Sophie Nowicki of NASA Goddard will tell us what’s happening with the world’s major ice sheets today and how all that feeds into the modeling and prediction process that we’ve been talking about. And finally, Ben Strauss of Climate Central will tie all this together miraculously to help paint the picture of take-homes that you want to have, including maybe literally the take-homes. Like, what’s going to be happening to some of our homes and hometowns in the years to come and what to make of all that. So with that, let me just turn it over to Michael to get things started. Thanks again for joining us.

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MICHAEL OPPENHEIMER: Good morning, everyone. I’m going to give you some general points about sea level rise and its consequences. And then let my colleagues give you the detail. Sea level rose about six inches during the 20th century, mostly due to anthropogenic climate change. We know this from flotation devices near the coast called tide gauges, as well as global-scale satellite altimetry measurements.

We also know that over the preceding centuries long-term ups and downs of sea level generally followed changes in global average temperature. And Andrea will talk about that a little later. Six inches might not sound like a lot but it’s enough to permanently narrow the typical east coast beach by about 50 feet through erosion and submergence. Recently, the rate of rise has accelerated from about six inches per century to about double that, more than an inch every decade, a foot per century.

We know why global sea level rises. Due to the way warming effects ocean water and ice. First, ocean water, like most fluids, expands when it’s heated. Second, mountain glaciers melt when warm. Third, the large ice sheets in Greenland and Antarctica melt and disintegrate into the ocean with warmth, either with warming from above from the atmosphere or warming from a warmer ocean below. Currently these three sources – expansion of sea water, melting glaciers, and disintegrating ice sheets – contribute about equally to sea level rise.

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In the future, we expect the ice sheet contribution to become the largest factor. And that’s a great concern because just the vulnerable parts of Antarctica and the Greenland ice sheet contain 40 feet worth of sea level rise. The harder part is projecting sea level rise into the future due to uncertainties in our understanding, especially with respect to modeling of the two ice sheets. And Sophie will have a few things to say about that later.

But we are certain that additional sea level rise is baked into the system regardless of future greenhouse gas emissions because of various lags in the system. The time it takes for emissions changes has significantly slowed warming. The time for heat to penetrate deep into the ocean waters. The time for ice to warm and melt. Stabilizing sea level would take centuries under even optimistic emission scenarios. So, emissions reductions, even rapid ones, as envisioned under for instance the two-degree global target, don’t make a very big difference to sea level until after mid-century. But for subsequent decades, the sea level rise difference between low emissions and high emissions scenarios becomes very large.

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IPCC’s Fifth Assessment said that by 2050 global average sea level could rise by more than a foot compared to what it was in year 2000. By 2100 these numbers increase to as much as three feet. Some recent literature published since the Fifth Assessment Report foresees an even faster and larger rise. New projections will be forthcoming in September 2019 with the release of IPCC’s Special Report on oceans, cryosphere, and climate change. In addition to submergence and erosion the effect of sea level rise is felt most notably during the surge that accompanies coastal storms, like hurricane or nor’easters, and also during high tide. Sea level rise means that even if storms don’t strengthen with warming the surge will push water further inland than previously, causing much more extensive flooding.

For example, by 2050, floods that now occur once every 50 years in first, let’s say, Charleston, South Carolina, could occur more than once every three years. More than a factor can increase in the frequency. For New York City in year 2100, floods that now occur once per century – which in the past have shut down the city – could occur almost once every other month. Hurricane Sandy, that caused significant additional damage up and down the northeast coast. And it would have had it arrived a century earlier, even if the city looked the same because it was riding on a higher sea level. And I think Ben will have some things to say about that later.

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Tidal or nuisance flooding, as it’s sometimes called, which is less damaging – translate that as flooded basements rather than running to the nearest shelter – but which occur much more frequently than the large storms have also increased markedly. Areas like the Carolinas that formerly experience such flooding only a few days a year now experience it up to 40 days a year. And Ben can also comment on estimates for the future.

I’ll close by reminding everyone that our history of defending infrastructure and people against coastal flooding is not particularly encouraging. We’re always playing catchup fighting the last flood. In the meantime, sea level rise and the flood heights are only going to increase in the future – for the foreseeable future. This is a battle that we are currently losing. Thank you.

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RICK WEISS: Thank you, Michael, for that inspiring ending. I think actually potentially truly inspiring. Its good to know that something needs to get done, if it does. Let’s move on to Andrea and take a look back in time and see how that contributes to the issue.

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ANDREA DUTTON: Okay. Thanks. So, I’m a geologist and I study past sea level so I often get asked the question of how looking at past sea level has really any relevance to the conversation that we’re having here today about what’s going to happen to sea level in the future. So, today I’m going to answer that question for you. And I’m going to share two different stories with you. The first is about what we have learned about future sea level rise by looking into the deep past. And the second story is about some work that I have gotten involved with more recently looking at sea level rise over the past century using records from tide gauges.

So, first I’m going to tell you what we’ve learned about sea level rise by looking at past warm periods in Earth’s history. And as Michael mentioned earlier, one of our largest uncertainties is how these large polar ice sheets in Greenland and Antarctica are going to respond when we’re thinking about suggesting changes in sea level in the future. So, to answer that question one thing we can do is look into the past because to some extent the earth has done some of these experiments for us. It has been warmer before. Those polar ice sheets have retreated. And so, we can look to those examples to see how much they retreated under warming and how quickly that happened. When we’re modeling into the future, one of the challenges we have is we’ve never been around to observe that particular process before. And so, we don’t have a full understanding of the physics involved in that ice sheet retreat. And that’s why we need to go into the past to put some constraints on that.

So, I’m going to talk about a period of time about 125,000 years ago. And so, this is before the last ice age. So, in the last Ice Age we had large ice sheets advance over, for example, North America. Sea levels dropped because a lot of that water was tied up in the ice sheets. And before that there was another warm period we call the last interglacial. And the best estimates we have right now of what the global average sea level was during this time period is about six to nine meters higher. So, that’s about 20 to 30 feet higher than today. The reason that finding is really important is that the global average temperature at its warmest during that time period was about the same as the global average temperature today, although the poles were a few degrees warmer. But those are temperatures that we are destined to reach in the poles in a matter of a couple of decades.

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So, why is this important? It means that if you had that much warming during that past time period, we saw a significant retreat of ice from Greenland and Antarctica. We can, of course, get some sea level rise by melting the rest of the mountain glaciers and also as water warms it expands. Those two factors, though, can only give us about one meter of sea level rise. We know that the Greenland ice sheet retreated partially during that time period, contributing maybe something about two meters above of average sea level rise around the globe. So, to get up to six to nine meters, or that 20 to 30 feet level, we really would have had to tap into the Antarctic ice sheet. And that’s something that you may have read about a lot in the news. In fact, people are concerned about what’s going to happen, particularly in the west Antarctic ice sheet which is thought to be most vulnerable. Because a large section of that ice sheet is based below sea level, so it’s susceptible to this ocean warming that Michael talked about before and it could retreat very rapidly because of the physics involved there.

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So, okay. That happened during that one time period. So, was that a fluke? Or is that a repeated experiment? So, we’ve looked at several at several past warm periods over the last three million years and we saw repeatedly that sea levels rose by at least 20 feet with anywhere from one to three degrees Celsius of warming where each degree Celsius is about 1.8 degrees Fahrenheit. So, that’s a 1.8 to about 5.5 degrees Fahrenheit. So, that’s the repeated experiment that we see over and over again. Now, the good news about this is that that type of sea level rise is not going to happen overnight. We’re not going to wake up tomorrow to dramatic flooding. Right? As mentioned before, it takes some time for that ice to heat up and to respond to that. So, I often talk to people about kind of like throwing an ice cube into a warm room and watching it melt. And you can think of these polar ice sheets as if we’ve thrown them into a warm and now we’re sitting back and watching what’s happened and how quickly they’re going to melt. That is one of the remaining big questions to answer though. How quickly will this process unfold? And that is a very active area of research. One, in fact, that my own research group is focused on at the moment.

We do know something about the volumes of ice involved though. So, the west Antarctic ice sheet, I mentioned before, if we were to collapse that marine-based portion of it that would lead to something like five meters or about 16 feet on average around the globe. But there’s a spatial pattern to that sea level rise. Some areas would be higher; so here on the U.S. east coast, unfortunately for you, you will see a greater amount of sea level rise there if that melt water comes from the west Antarctic ice sheet.

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So, how does this help us to improve our projections into the future? The same models that get run into the future to project how the ice sheets will respond are also run during these past time periods. So, I work closely with these modelers and they try to get their models to fit the data that we’re producing based on our observations of what happened in the past. If they can get their models to fit our data, then they have a better constraint on the physics involved and they can use that physics in the model to help make a more robust projection of how sea level rise will unfold in the future.

So in other words, they’re using our data to calibrate those models that get used to do the sea level projections. How does this help us address uncertainty and plan for the future in communities dealing with this? A lot of people focus on the uncertainty. I’ve mentioned there’s some uncertainty about how quickly that Antarctic ice sheet will retreat. But look at the flipside of the coin. We are certain that sea level rise is going to continue to rise and that what we’re looking at now is just the first step in a very long journey. So, what we need to do is kind of redefine our relationship to the coastline as it continues to evolve and march landward.

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The second story is a little bit shorter that I’m going to tell you today and that’s about the work that we’ve been doing with tide gauges. And we have been focused specifically looking at the region along the U.S. Atlantic coast. And I got involved in this because from the year 2011 to 2015 we saw dramatic acceleration in the rate of sea level rise south of Cape Hatteras all the way down to Miami. And we saw about five inches of sea level rise in five years. So, you heard some numbers before. Three millimeters a year, the global average rate of sea level rise. That equates to about one foot per century, as Michael said, and all of a sudden, we got five inches in five years?

So, what caused that to happen? It turns out there is some natural variability in the rate of sea level rise. It will have some short-term accelerations and people have referred to these as hotspots of sea level rise. The name is a little bit of a misnomer. It has nothing to do with the temperature of the water but just the focused area where you see an acceleration of sea level rise. And it’s due to natural variability that we see between the ocean and the atmosphere. Part of this is due to the El Niño cycle and part of it is due to something that we call the North Atlantic Oscillation, which looks at the pressure of the atmosphere on the surface of the ocean and acts kind of like a seesaw over the North Atlantic basin, changing the ocean currents and causing the water to pile up against the U.S. east coast.

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So, what we’ve found is that these rates of, like, five inches per year were not unprecedented if we look back over the last century. And so, this is a natural variability of the system that will superimpose on the gradual sea level rise that is projected into the future. So, how does that change our projection into the future? We need to prepare for the fact that we could get these short-term, multiyear accelerations in sea level rise. So, we need to plan for having an envelope around our sea level rise projections that we could get these short-term accelerations. But it’s most likely we’re going to be unable to predict exactly when those are going to happen. So, it’s just something we need to build in as a buffer as we move into the future. Thank you.

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RICK WEISS: Thanks, Andrea. Super clear. Sophie, tell us what you’re up to in Antarctica.

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SOPHIE NOWICKI: Right. So, I’m an ice sheet modeler and I use models to make projections of future sea level in a timescale of every 100 years, roughly. And before we start, I just want to remind us that … timescale is actually not so far. I’ve been very fortunate to have my grandmother to live to the age of 105 years old. She just passed away this December. But we just celebrated the five-years-old birthday of my youngest son. And so, it means that for many years, I had to deal with a difference in generation over 100 years. So, when I think about the 100-year projection, this is something that my children get to see.

So, what does it mean? I mean, using a model? What is a model? It sounds a bit scary but actually you use models every day all the time in your life. When bridges are being built, you don’t just build a bridge and then let trucks go along it. And then hope that the bridge doesn’t collapse. And then if the bridge collapses, build a stronger bridge. No. You use actually physics and mathematics to basically design the best bridge. And the design that you’re using reflects the condition on what you’re building your bridge along. So, then if you need to have a bridge that has many roads, you make it a different way, and you get the idea. So, basically, bridges are being built based on a rough estimate of physics and mathematics that takes in account all the facts that are then important.

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And so for ice sheet models, it’s just the same thing. We build an ice sheet model based on what is important for ice sheets and some of the physics and mathematics. So, currently we have about 13 ice sheet models that I use to make projections of future sea level rise. And nine of them have been coupled to climate models. And there’s about five models that are really being developed in the U.S. They are different. And why are they different? Well, it’s because usually when they’ve been created at the start, they might have had a very slight different interest. So, for example, the ice sheet system model built at NASA GTL is really designed to make the full use of the observations that NASA collects. But then you have another ice sheet model called the Community Earth System Model that’s specifically being built to be coupled to the community system model. And so, it has very different needs. So, different flavors. But at their core, all of the models use strong physics and mathematics that we know can be used to describe the ice behaviors and its interaction with the earth system. And by the earth system, I mean that the ice reacts with the atmosphere, the ocean, and the solid earth.

So, what do we know about ice sheets when I build a model? Well, we know that they grow to snowfall. So, if you go on an ice sheet and you take a snow pick, or if you fly with a snow radar of an ice sheet, you can just see there’s like very thin layers of snowfall each year. And so, the youngest layer being on top of the ice sheet, and as you dig deeper or you see deeper with your radar the ice layers as being older. Just like when you cut a tree and then you kind of count the trimmings? Basically, you can see you have those layers that tell you how old the tree is and how fast the tree grew. Well, we can go back to an ice sheet and kind of count the layers and kind of see how much accumulation has been in the past.

But you shouldn’t think of an ice sheet as just being that snow falling and making this big massive snowball and nothing happening. Basically, an ice sheet actually is a very dynamic system. It flows, just like if you were making a pancake. When you’re making a pancake in a pan, you put the dough in the middle and it’s going to spread slowly towards the edge of the pan, making a nice shape of a pancake. Well, ice sheets behave that way. You model them as a fluid that flows very slowly. How fast the ice is going to flow really depends on what’s at the bottom of the very thick ice. If the ice – the entirety of the ice sheets usually the ice is flowing over a really, really hard bedrock and it’s frozen. And so, it’s kind of a very, very, very slow flow. But as the ice spreads towards the continent towards the ocean, it starts flowing over sediment of water. And it forms those rivers of ice sheets, making ice streams. And, basically, the ice flows very, very rapidly and can take speed of the order of kilometers per year.

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So, what else do we know about ice sheets? Well, we know it can crack. It can crack. It can make a massive iceberg. A good example is the one that was formed last year in 2017 from the Pine Island Glacier. It was the size of Manhattan. It was a big chunk of ice that cracked. And after an iceberg calving event, observations of ice thickness and ice velocities, show us that the ice inland starts to speed up. So, now we also know that ice can lose mass at the surface of the ice sheet, just at the top of the ice sheet, basically where the temperature gets really hot. And so, the snow can melt at the surface. And also, ice physically also loses mass when it stops flowing over the ocean because the ocean can start melting the ice. So basically, ice sheets grow because of snowfall, spreads towards the coast and towards the ocean. They will lose mass by iceberg calving or by melting underneath the ice shelf or on top of the ice sheet. And so, a lot of the knowledge of ice sheet behavior is linked to the observation that the satellites have made or the observation that we’ve made on the field just by people walking on the field bringing stuff with it. And as new processes have been implemented we basically includes those into our model.

So, what do we know not so well about projections? Like, why are there so many models and why when you’re going to read something the number seems to be different? Basically anything that happens underneath the ice sheet we don’t really know. Because basically the ice sheets are so thick that you can’t necessarily be able to see what’s underneath them. And so, for example, we don’t really know where the ice is basically frozen and sticking to the bedrock or where the ice can flow rapidly over soft sediment. When you have water that’s melting at the surface, you can see it’s crack on the surface that water can go deep underneath the ice sheet and reach the bed and make the ice flow faster. But we don’t really know how much of that ice goes, and whether it’s a true effect of this water hitting the bed, making the ice easier to slip away.

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Finally, it is important that we don’t really know the shape of the bedrock or where the ice flows over. So, as part of Operation IceBridge – it was NASA’s campaign that investigates by aircraft. And one of the things that they were able to see with the radar is the shape of the bedrock. And so, in 2013 they flew over a glacier in northern Greenland and they discovered a canyon that was the size of the Grand Canyon or bigger. And what it meant is that this canyon could link the ocean to the interior. So, it meant that if something happened at the boundary where the ice basically met the ocean and the ice sheet tried collapse if it does. But now, basically because it ice sheet can basically go right back in and it cannot be stopped. And that’s something that we didn’t know two years ago until we had those measurements.

Another good example is for in Antarctica to get better knowledge of the bed. That there have been some studies that shows that using two different bedrocks can lead to a difference of 20 percent of future sea level rise and in 100 years. And in that study, 20 percent was 0.1 meters. So, the bed really matters, and unfortunately, the bed is hard to see. And so basically, some of the uncertainty in suitable projection is just because we don’t what the bed looks like. Another good example is that ice can change directions where the ice sheet floats over the ocean. Again, we can’t observe it. It’s very hard to look underneath the these ice sheets and so ice sheet modellers cannot make good modeling. We try our best to make a model about how the ice and the ocean interact. And so, a few years ago there were like three studies about Thwaites Glacier that gave quite different numbers. And the only reason that they gave different numbers on how Thwaites was going to react in the future was just the way that modellers worked the equation about how the ice and the ocean reacted and interacted.

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So, another great example, too, is basically iceberg calving. We know that the ice cracks. But it’s really hard to actually implement that into a model. And it’s really hard – like, we don’t really have a good understanding about what causes this ice to crack. So basically, there is lots of factors that we don’t really know how to implement in that model. But the thing is that whenever there’s a … we don’t really how to implement them. As you modeller’s know, we take a lot of thought on trying to describe the processes for our model. And so, we might get there a different way. If we all get the same answers, it means that the process that we’re trying to describe that we don’t really know is really captured into our models. But if we all get a very different answer to a response of the same process, then it means that we just have more work to do to try and understand this process better. And this work will be helped with – that we also need to have more observation or more people doing experiments in the field or in the lab.

So finally, one of the factors about why it’s so hard to make projections of future sea level rise is because us humans have an impact on future climate. And we’re going to impact with the choices we make every day by impacting how much the oceans and the atmosphere is going to warm. And it’s something that myself, as the modeller, I have to try to incorporate into our models.

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RICK WEISS: Thanks, Sophie, for clarifying a lot of the factors that go into understanding all those variables. And reminding us that we are, behaviorally, one of those variables. Ben, why don’t you bring it home?

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BEN STRAUSS: Perfect. Thanks so much, Rick. Pleasure to be here. So, I thought I would start by trying to recap some of the main points from Michael, Andrea, and Sophie. To begin, we’ve seen around six inches of sea level rise over the course of the 20th century, and that’s a global average. Local values vary. But about six inches, which is more important that it seems, and I’ll return to that. We know in terms of looking towards future sea level rise that ice sheets – the ice sheets in Greenland and Antarctica – are going to be key in the equation. They hold the vast majority of potential sea level rise. And we know from the long-term, ancient geological evidence that those ice sheets are extremely sensitive to changes in temperature. So that temperatures similar to today or very slightly warmer are associated with sea levels. And they have been 20 or 30 feet higher in the past.

It is much harder for us to project how quickly sea level will rise, as compared to how much it will rise. One thing is to look back in the record and see the kind of resting place sensitivity of how high the oceans might get compared to a given warmth. But we can’t resolve that historical record down to the decade level that’s interesting for our projections today to give us guidance around how quickly sea level might rise. And even if we could resolve the historical record that well, of course conditions then were a little bit different from conditions today. And we have tremendous challenges in understanding how the ice sheets will respond in detail that Sophie described.

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These are massive formations in very remote and harsh locations where we have relatively little data. Almost no long-term observational data to develop trendlines and a sense of behavior over a multi-decade or century timescales in recent time. And it’s very expensive and difficult to make observations. The ice sheets move slowly. We don’t have the benefit of being able to run experiments, except for the one that we’re running right now on a planetary scale. So, there are a lot of challenges and a lot of reason to focus our attention there. Some research and researchers looking at the west Antarctic ice sheets suggest that while today those ice sheets are contributing on the order inches per century to sea level, there are certain thresholds that can be tripped after which we might expect them to contribute feet per decade for a short period, as the basins empty out once basically ice dams and blockades have been removed and the flow of glaciers and ice sheets into the ocean can be released.

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So, now I want to put this into a little bit more context of what it means for people who live on this planet and near the coasts. And first, I thought I’d give a global perspective very briefly. If we could show the first slide, there’s a map of the globe that was published in the New York Times several years ago based on an analysis that I did with colleagues. And the point here is not to look at the details but rather the size of the squares indicates the number of people living on land that could be permanently inundated or subject to chronic flooding by the end of the century. And you can see that on a global scale, this problem is very strongly concentrated in Asia. Five out of six of the world’s people who live on vulnerable land live in Asia. And that’s a great concern in terms of prompting migration over time and instability in the region.

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Of course, I have to put an important caveat on that research if we move to the next slide. It’s based on a global elevation data set that is extremely poor and which is another source of uncertainty in our understanding of the impact of sea level rise. Fortunately, we have much better elevation data in the United States and the panel shows two pictures. Land less than five feet above the local high tide line that is in blue on one side of the right-hand panel. I think that’s more filled in. That’s based on high-quality, laser rangefinder data called lidar taken in the United States.

And on the left panel is the satellite-based data that we use for global research and, for example, for those numbers in Asia. And you’ll see that much less land area appears to be affected. And the reason is that the satellite-based data that the entire research community uses to understand global threats and threats in Asia are measuring treetops and building tops and averaging those into our ground to elevations. So, on average, the elevations are two to three meters too high in the coastal zone. And we’re talking about one to two meters of sea level rise in this century. So, that error is actually a major source of uncertainty that we have not really invested in or focused on. And unfortunately, the direction of the arrow we understand quite clearly that as we improve elevation data, we will come to see that many more people are at threat than we currently recognize.

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Moving into the United States, I want to talk about some of the other factors that lead to local impacts. There’s a lot of variation in the rate of sea level rise if we move to the next slide. You’ll see kind of recent sea level rise rates, historic sea level rise rates from around the United States as tracked by NOAA. And there are many causes for that variation that you should be aware of if you’re doing a local story. In some places in particular, the land is sinking. In other places, more rarely – particularly Alaska – land is rising. But in Louisiana and the Chesapeake Bay area, and in fact – in those two areas, land is sinking particularly fast and in many other areas of the United States it’s sinking slowly. Over the last century the sinking rates were, in places, comparable to sea level rise from climate change and global warming. But in this coming and in the current century we expect the rates due to warming to dwarf the rates from sinking land in those locations.

Another factor that affects how a locality experiences this threat is simply the pattern of topography and development relative to that topography. So, you have major metro areas, like in south Florida which are on very flat, very low-lying lands with high population density. And then you have in southern California, Los Angeles which has a lot of topographic relief. It’s much hillier and higher. So, very simple but quite important in determining the threat locally. Another less-appreciated factor is the importance of the bedrock under a city or geography. In most cases, the bedrock is solid and largely impermeable. But in south Florida, unfortunately, most of the bedrock is porous, like a sponge. So that, even if protective levees were to be built, water would penetrate underneath those structures and come up through the grounds, making defense measures much more difficult in that geography.

[00:39:31]

I should also add, moving to the next slide, that sea level rise very much amplifies the effect of coastal flooding. And that’s really how we’re experiencing sea level rise today and will in the coming few decades, making coastal floods more frequent, higher, more destructive. So, colleagues and I are currently working on research that indicates that a significant fraction of the damages from superstorm Sandy can be linked to human-caused sea level rise, those last few inches. Looking retrospectively, and this connects to the chart that you see in front of you, we counted up the number of coastal floods at 27 tide gauges around the United States since 1950 as they occurred. Then we subtracted out human caused sea level rise to see how many would have occurred without that effect.

In fact, it’s two-thirds of floods at those 27 tide gauges would not have been classified as floods without human caused sea level rise, in our analysis. Our sea level rise tipped–really, we’re talking about minor floods, but it tipped them over the balance for something that would have been just high water, which is something that the national weather services locally defines as a flood event based on their local observations of roads flooding and infrastructure impact. Even a few inches is already shaping this exponential growth curve in the frequency of coastal floods, that you can see plainly in that graph.

Finally, an important factor in what impact sea level rise will have, besides how much more carbon we put in the atmosphere, is how we respond in the places that we live along the coast, what we do in our cities and rural areas in terms of developing defenses, accommodating more floods, or retreating from rising waters. Maybe I’ll wrap it up by giving a very brief tour around the United States to give a sense of exposures at just a couple of elevations. If you look at the state level populations on land less than half a meter above the high tide line, California actually has the most.

[00:42:06]

This is an elevation range where we can expect this much sea level rise in the second half of this century at some point, under almost any scenario. About 85,000 people in California live less than half a meter, or 1.8 feet, above local high tide. Only around 63,000 people in Florida live on that land, and around 31,000 people in Louisiana. I’m excluding people who are protected by levees that are in place. Otherwise, the number in Louisiana would be much greater. If you jump to one and a half meters, which is in between a lower and higher end estimates of sea level rise we might see by the end of the century, the order really flips with 1.2 million people in Florida living on land that low, so a very dramatic uptick. Versus a number two slot in California at just several hundred thousand, followed by New Jersey at 260,000. Ranking counties at half a meter: San Mateo County is number one in the country, followed by Mirren, followed by Terrebonne in Louisiana.

But at one and a half meters, it’s Miami Dade, Broward, and Pinellas, all in Florida. I should add that if we do that by percentage of the population and not the total number, the really big threat is, first and foremost, in North Carolina, where in Hyde County 36% of the population lives less than half a meter above the high tide line. Seven other counties nationwide have more than 10% of their population there.

[00:43:57]

For more details like this, I want to advertise a tool that colleagues and I have built with input from the research community and coastal managers around the country. It’s called Risk Finder. You can find it at riskfinder.org, and you can look up there, for whatever ZIP Codes, city, county, or state you chose, how many people live on land below different elevation thresholds and look at integrated projections of sea level rise and flood risk, as well, and explore toxic waste sites, infrastructure exposure and a great other diversity of things.

To wrap it up, we already know a lot about sea level rise. We have a lot more to learn, especially around the ice sheets and how quickly they might decay this century. As the South Florida Sun Sentinel put it in an editorial just a month ago or so, at least for some parts of the country, for South Florida, they’re headline to this editorial launching–a very special journalistic project in south Florida, an alliance between three papers, the three largest papers sharing a common pool of reporters and stories for a full year. They’re launching editorial headline was “Sea Level Rise: The Defining Issue of This Century.” There are some places in the United States to which that is absolutely true.

[00:45:34]

RICK WEISS: Thank you, Ben. Fantastic wrap up and thank all four of you, first of all, for really very clear presentations. I’m so glad we got to do this with you. For reporters online, if you’ve got questions as Josh mentioned, you can type them in the Q&A box there, and I will see them here and be able to convey them to the panelists. We already have a couple in, so let me just get started. One is from Mary Landers at the Savannah Morning News. Just a definitional question but a good one to make sure we get our terms right. She’s saying she realizes she doesn’t really know the difference between iceberg, ice sheet, and glacier. I don’t know. Maybe, Sophie, this is for you. Is there any easy way to keep those things straight or are they all synonymous?

[00:46:22]

SOPHIE NOWICKI: Yeah. No, it’s a very good question. Basically, an ice sheet–when I talk about an ice sheet, I talk about Greenland and Antarctica as a whole. It’s basically this ice that flows over the massive continents, and so they are typically a few miles thick and then super, super wide. So the full volume of Greenland that was melted could give rise to seven meters of sea level rise. Antarctica is like 67 or something like that.

A glacier can be in two forms. There are the glaciers of what you’re used to when you go and hike a national park in the US, those little glaciers that flow down the mountains. But ice sheets also have glaciers. When I talk about the glaciers in ice sheets, it’s basically those regions of the ice sheet that are flowing very, very, very fast like a river of ice. An ice berg is basically a chunk of this ice sheet or this glacier from either the same world that basically has been cut off from the main part of the ice sheet. And basically, it’s cut off because there’s been a crack that has developed at the top and at the bottom. The piece, it’s a chunk of its own little ice cube, that basically you chop up an ice cube and that little old thing keeps flowing by itself into the sea. Right now, if you look, I think there’s some beautiful pictures of ice bergs flowing in Canada, amazing pictures that I think are on the BBC website or CNN, like really beautiful icebergs flowing on water. That’s basically just ice that has been chunked off from the ice sheet and is flowing away into water by itself. It could be Manhattan-sized.

[00:48:25]

RICK WEISS: A question from Maryann Mesina, who I believe is a freelancer, wondering what kind of helpful new data are we able to capture when these dramatic events happen where a chunk of ice breaks away off of a glacier or an ice sheet? What can that teach us? I’m not sure if that’s Andrea or Sophie or anyone want to address that?

[00:48:51]

MICHAEL OPPENHEIMER: Let me just make one comment. When larger ice bergs break away, we sometimes can look at the response of the rest of the ice that’s left behind. Sometime ago, early in this century, a set of ice bergs formed at once by the disintegration of an ice shelf, from the Larson B ice shelf. It’s disintegration, this is of the Antarctic peninsula, which is that thumb that points up towards South America.

The function of that ice shelf and other ice shelves sometimes is to hold the ice behind it that’s land based, to keep it on the land because the ice shelves themselves, as they drift away from the land base, they’re like tongues that drift off the land-based ice. They get jammed against geological features like rises on the sea floor or the edges of the bays in which the ice shelves exist. But as they fragment and the ice bergs that form break away, the ice behind those ice shelves that’s based on land starts to move faster, in fact. One of the very important pieces of information that we picked up by watching the disintegration of this Larson ice shelf is that all the land-based ice in back of it started to accelerate rather rapidly after the ice shelf broke into ice bergs and was no longer there. Watching those margins, watching what happens in response to a change like the break off of an ice berg is a very important opportunity to scientists.

[00:50:45]

RICK WEISS: Great. We have a question here from Kathy Kowalski at Science News for Students saying she could benefit from some better distinction between higher than ever sea level rise, generally, and storm surge levels, in particular. Is there one or the other of those that engineers and policy makers should be giving more weight to for planning purposes, and how should journalists distinguish between those two things for their lay readers?

[00:51:20]

BEN STRAUSS: Maybe I’ll take a crack at that. Storm surge is really what you have to worry about today. Sandy had a nine-foot storm surge, and none but the very highest of sea level rise projections get to nine feet anywhere by the end of this century. It’s possible today to have these events that are damaging in a short term. Of course, a permanent rise of nine feet would be drastically more difficult than a temporary flood.

But really, the challenge is in the combination of sea level rise and storm surge because the truth is that most of our cities probably aren’t well designed even for rare big storm surge events as they would naturally occur today with the current sea level, or even sea level five inches below where we are today. We already build a lot in the 100-year flood plain, for example.

That probably is only going to get much worse as the sea level continues to rise and we have floods with more and more frequency in areas that aren’t accustomed to seeing them and therefore probably haven’t been engineered or developed to protect. One metaphor I like to use is think of a damaging flood a little bit like a dunk in a basketball game. Dunks are hard to get.

They don’t happen that often, but now image somehow the level of the basketball floor, and the dunk is the storm surge event–now imagine the level of the basketball floor was slowly rising. The basket got shorter. Suddenly, you would be getting dunks all the time, and you wouldn’t need a lot of rise to accomplish that. That’s how sea level rise and storm surge interact.

[00:53:22]

RICK WEISS: Great metaphor. Thank you. We’re getting close to the end of our time, but I want to get at least one more question in. We might go a couple minutes over if we need to. We do have a question from a freelance journalist in India, asking a good question of to what extent does sea level rise potentially effect marine ecosystems and biodiversity and the ecology of the oceans themselves? Does someone want to pick that up?

[00:53:52]

MICHAEL OPPENHEIMER: One effect on coral reefs is coral reefs have to keep in pops near the ocean surface. If sea level rise–under certain conditions, it can rise too fast for the coral reefs to keep up, so that’s one example. Although coral reefs have a lot of other problems like the warming of the ocean and acidification itself. Another issue is sea level rise drowns costal marshes, and under conditions where there wasn’t a lot of infrastructure and settlement behind the coast, the marshes would probably be able to move inland fast enough. But in the developed world, like much of the United States, Australia and Europe, these marshes have nowhere to go.

[00:54:45]

ANDREA DUTTON: I would add that those marshes are important for biodiversities, is what the question asked about, because the mangroves are nurseries for a lot of the fishes and so forth. If you lose those crucial habitats, it will be a very big issue for the biodiversity.

[00:55:03]

RICK WEISS: Let’s take a few more questions and let this stretch a little bit before we close off. Ing Fay Chen, a freelancer writer from the West Coast, has a question about the map that you showed, Ben, showing interestingly that actually there’s some sea level decrease predicted as well along, for example, the coast of Alaska and Western Canada. What’s that about?

[00:55:26]

BEN STRAUSS: That’s about the land there in some areas of Alaska is rising more quickly than the sea level is rising. Even though if you took the absolute elevation of the water surface, it is going up. The elevation of the land is going up faster, and so the effect on a tide gauge, a kind of water level measuring device on the shore, it would make it seem like the water level was going down. The reason behind a lot of vertical land motion in general has to do with the Earth’s slow response to changes since the last ice age.

Ice sheets, and there used to be a massive ice sheet covering most of Canada, with incursions into the United States, 10,000 years ago, are extremely heavy, and they compress the Earth underneath them. I think of it a little bit like a person sitting on a mattress. The person is the ice sheet, and the mattress is the Earth underneath. When a person sits down on a mattress, the mattress compresses under the person, and when the person gets up, the mattress, that section that was under him, springs back up.

That’s what’s happening in Alaska. Now, in other areas that weren’t under the person, when the person sits down on the mattress, the part under him goes down, but the part away from him actually goes up like a lever. When he gets up, that part away sinks back down. That sinking back down is what’s happening in most of the United States which wasn’t under an ice sheet in the last ice age.

[00:57:09]

RICK WEISS: Great. One more question, and we’ll go until about five after here. Mary Landers again at the Savannah Morning News wants to follow up on your statement–I think this was–Michael, this might have been you. Six inches of sea level rise could translate to about 50 feet less beach, at least in some parts of the East Coast. She’s wondering if these kinds of calculations are available for specific areas for reporters who want to report on what’s going to predictably happen right where they are, and how reliable are those kinds of statistics?

[00:57:45]

MICHAEL OPPENHEIMER: So that particular hundred to one ratio is something called the Brood Rule. It’s very rough, and it kind of tries to accommodate all the different situations at all the beaches around the world. In order to really get a usable, local answer, what I would do is talk to the folks at your nearby institute of oceanography, for instance. And some of them have done local estimates of what happens on particular beaches. It’s not easy to do. The models are difficult because they have to take account of many factors at once.

I don’t know of any global–I know global databases on overall beach loss, but when you get down to the particular level that you might be interested in, you really need to go to regional or local experts who may have done the specific model. There’s information on historical loss. If you’re interested in future loss, you have to use the modeling, and that’s what the problem is.

[00:58:56]

BEN STRAUSS: I might add I feel like I’ve read that most beaches that are visited by tourists in Florida have actually been nourished, replenished with sand that’s brought it. I wouldn’t be surprised if the same were the case in Georgia. It’s not a projection, but if you snooped around, you might find that most any beach that is important in your area is already being replenished with sand. If that’s the case, it would be an indicator that a threat is underway, and it’s actually being counteracted. There are other questions about how sustainable and how much we can keep up by replenishing the sand on beaches artificially.

[00:59:42]

RICK WEISS: I’m going to give one more question here, and this is coming from Kevin Loria at Business Insider. It relates to your point, Andrea, earlier, that the records indicate that when temperatures were about what they are now, sea levels were 20 to 30 feet higher than they are. The question is does that indicate that basically that much change is built in, that we’re going to get there, and that the only real question at this point is how quickly? Or is there still room to prevent or change that kind of outcome?

[01:00:18]

ANDREA DUTTON: That’s a great question, and that example I gave from the past, as we mentioned before, is not necessarily going to be a perfect analog for what we see in the future. But we did see a repeated pattern of hitting that. So it’s reasonable to assume that we’re going to get a lot of that sea level rise based on the amount of temperature rise that we have already seen. But, for example, one of the ways in which that past time period is not analogous to today is that right now, we’re heating our atmosphere by adding greenhouse gasses, which increases the temperatures at both polls of the Earth simultaneously.

That was not the same forcing mechanism that we saw in the past where it was kind of one pole at a time was being heated by variations in the orbit of the Earth around the Sun. What I sometimes say to people is actually it could be worse because now we’re forcing Greenland and Antarctica at the same time, whereas before there was a little bit of what we call bio-polar see-saws, the play off between the two poles, one getting heated and the other one.

Certainly, it gives you the idea that these ice sheets are, as we said before, very sensitive to what you might think of as a very small change in global mean temperature. Remember, though, if you drop temperature by four degrees Celsius global mean, that’s what happened during the last ice age. So four degrees doesn’t sound like a lot, unless you could be under a couple miles of ice in New York City or in Canada or wherever you are. Small temperature changes can mean big things for the Earth.

[01:01:48]

RICK WEISS: Gives real new meaning to the Goldilocks zone that we’re living in. I want to thank you all for a fantastic set of presentations. Thanks to the reports for logging in and for doing everything you do to get the best evidence out there into your stories. I want to encourage you all to follow us at SciLine at @RealSciLine and check out our website.

It’s sciline.org, for more information about how we can help you as you do your reporting, finding you the best experts and expertise and contextually information to help you get more of that actual, good scientific evidence into every story that you write. Thanks, everybody, for participating, and we’ll look forward to seeing you again at the next SciLine media briefing.