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October 20, 2013

Sea Level, on the Rise

Every once in a while, I do something that’s serious. Okay, often, I do something that’s serious. This is one of them. I played journalist a few weeks ago and interviewed Tad Pfeffer, a glaciologist at CU-Boulder, about the recently released IPCC’s Fifth Assessment Report. They’re somewhat few and far between, and this is the year. What do they tell us? The state of climate science and what we understand about climate past, present, and future—as best as we know at this time. I loved the process of doing the interview in the way that I first thought I would love doing journalism—I loved the learning. I learned as much as I could in the time I had allotted so I could actually conduct a somewhat intelligent interview with Pfeffer. That was the goal. Whether I accomplished it or not doesn’t really matter so much at this point, because I loved feeling like a journalist. And I learned about the IPCC report, knowledge that I can now build on moving forward.

Guyot glacier in Icy Bay, Alaska
[A once-flat, hundreds-of-meters thick Guyot glacier in Icy Bay, Alaska, now cracked as bedrock emerges in Guyot's rapid retreat. Had to use this image after I saw it was taken by a former co-worker, who was a PhD student of Pfeffer's. (Photo: Shad O'Neel, USGS)]

But enough about my process. Curious about sea level changes? Wondering when Miami is going to be gulping for air? We didn’t talk about the latter, but we did talk about some very interesting dynamics. Here are the most salient tidbits, and then you can read on if you want more:



Interest piqued? Read on (and see some cool graphics). Or, take a listen, if you’d rather, to the original interview, with full “um-ness” on KGNU’s How On Earth archives.

Tuesday, October 1, 2013
Tad Pfeffer, University of Colorado at Boulder
For How On Earth

B: I'd like to start broad, talking about this report, and talk about the implications of the report--what does the report try to achieve, and what does it matter. Does it matter?

T: Yeah, it’s achieved I think two really worthwhile and remarkable things. One of them is to really solidify (...) what is happening with climate and sea level rise, the area I work in, and also [provide] better projections of what we expect to have happen in the future.

Our knowledge of warming and of the causes of warming has really evolved over the years. It’s really interesting. I was teaching at CU back in the early 90s, and at that time—so this is right around the release of the first IPCC report—what I was telling my classes was, you know, we’re almost positive that we can see a coherent warming signal in atmospheric temperatures. We’re not totally a hundred percent sure, but you know, a little bit more data, we’ll probably have it.

And then, a couple years later, I was saying okay, well now we know it’s warming, but we don’t really know why, and then it was, now we know it’s warming, and we’re pretty sure that it’s human-caused, but it was like the warming itself several years before. There wasn’t quite enough data to really know if it was us. And now, we really know it’s us. That’s one of the top conclusions of the Working Group I report.

And then, also, projecting forward into the future--and again, sea level’s the part that I really know about--we have better projections of where we think sea level is going to be by 2100, and that is, at the upper level of what we project, just shy of a meter, or about three feet.

B: Let’s get back to that in a second. Could you tell us more about your role in writing this report? Your role was as a lead author, one of fourteen lead authors on this chapter. What did it mean to be a lead author in this international collaboration?

T: Well, yeah, as you said, I’m a lead author, one of a group—each of the fourteen chapters in the Working Group I report has a group of lead authors, not all the same number—it’s on the order of a dozen--and then two coordinating lead authors which are sort of the, you know, orchestral conductors that tries to keep everybody pointing in the same direction. And we come into that group with various different kinds of relative expertise, and since the IPCC isn’t a body that does new science, that’s not our job—our job is to report on, synthesize, and make assessments of the existing science that’s out there—the lead authors need to have a broad awareness of what’s going on in their scientific community around them. And so for me, my job, if I had a specific task, was to represent the knowledge of what the smaller glaciers and ice caps around the world are doing. So this is all the ice in the world with the exception of the two big ice sheets on Greenland and Antarctica. And those [—the small glaciers—] have been the primary contributors of ice adding to sea level through the observational record up till now and will continue to be so through most of the 21st century. But the ice sheets are catching up rapidly. But that was really, that was my first job, was to be the person who knew what was going on with the glaciers and ice caps and contribute that knowledge to the group.

satellite image of Columbia glacier in Alaska
[A satellite image of Columbia glacier in Alaska, one of Pfeffer's field sites. NASA put together a beautiful series showing the glacier's retreat that's well worth checking out.]

B: Could you speak more to that? So your focus is on the glaciers, mainly in the Arctic, specifically in Alaska, and often when we think of big contributors to sea level rise, in terms of ice, we think of the Greenland ice sheet, we think of the Antarctic ice sheet—could you tell us more about what role these smaller glaciers that you study do play? You said they’re a major contributor, so—could you tell us how that works?

T: Yeah, yeah. The small glaciers of the world are—we don’t even really know how many there are, in part because you have to decide how small a glacier are you going to get to before you stop counting, but it’s on the order of maybe 200,000, something like that, and they’re very poorly observed, simply because there are so many of them. And they are a very large contributor of water to sea level rise right now, but at the same time, they’re a very small reservoir. In other words, their total potential for sea level rise is actually extremely low.

If you were to look at the relative sizes of the Antarctic ice sheet, the Greenland ice sheet, and everything else, the Antarctic ice sheet has something approaching, just using round numbers, 70 meters of sea level. More than 210 feet of sea level rise, if all the ice in Antarctica was to be put in the ocean. Greenland would have about 7 meters if it was put into the ocean. And everything else in the world, all of these other glaciers that I work on would only put in about half a meter. So, it’s a tiny, tiny reservoir, but as I said before, talking to some NASA people, the glaciers are like a little bucket with a huge hole in the bottom, because even though they’re such a small reservoir, they’re losing at a tremendous rate because there are so many of them, they’re so low in elevation compared to the ice sheets, and they’re in places where they’re affected first by climate change. So the actual contribution to sea level from the little glaciers has been greater than the combined contribution of the two ice sheets throughout the observational record up to now.

And now, the glaciers’ [contributions] are still a little bit more than the two ice sheets [contributions] together, but the ice sheets have been changing very rapidly. They’re accelerating. The glaciers are very noisy. They speed up for five years, they slow down—that is, their rate of contribution speed up and slows down—but it’s still a very important number to keep track of if you want to know what the total sea level rise is actually going to be. You can’t just focus on the biggest contribution or the contribution that has the potential to be biggest in the future, or the contribution that looks most interesting from a scientific perspective. If you want to produce a useful projection, you have to look at everything. And that includes these 200,000-odd glaciers, even though it’s a difficult task.

Me: And when we’re looking at contributors to sea level rise, we’re looking at the contribution of these smaller glaciers, the contribution of these ice sheets, but we’re also looking at ocean expansion due to warming. Could you explain a little bit about how that works, and how big of a contribution does ocean expansion have?

T: Yeah, that’s actually the largest single contributor, is thermal expansion. So, this is water expanding as it gets warmer. If you take a kettle and fill it right up to the brim, and put it on the stove, and turn the burner on, long before the kettle approaches boiling the water will start to spill out the top, and that’s because the density of the water actually drops. So the same mass takes up a larger volume as it warms up. And that’s happening in the ocean.

It happens mostly in the top layers of the ocean, because that’s where most of the heat from the warming atmosphere is going. So, then, modelers and oceanographers measuring thermal expansion look at it, they divide the ocean into the top 700 meters, which would be treated as a top layer, and then everything else below that, and most of the thermal expansion’s taking place in this top layer.

And of course, it’s not uniform globally because the ocean is warming at different rates. And that’s an important point to make about any of these global numbers that we talk about. Working Group 1 mostly, not entirely, but mostly talks about global averages. And all of these things have significant variations from one place to another, and that especially applies to sea level. The global rate of sea level rise has to be interpreted regionally, from one place to another. If you want to find out what’s going to happen to you if you live in San Francisco or London or Bangladesh, you can’t just look at the global average number. There’s a whole lot more going on local. Thermal expansion is one of those.

B: This is something that piqued my interest. In my naive view of the world, I think of the oceans as all one connected basin, but that’s not how it works.

T: Yeah, yeah—if you look at a map of the topography of the ocean surface, which we can do these days, with satellites that can measure the elevation of the ocean surface at very high resolution, it’s very lumpy. It’s not a nice, smooth dome, at all. And that lumpiness—these are vertical displacements on the order of a couple of meters. You know, six feet one way or another, over horizontal distances of tens of kilometers. When you’re going around in the ocean, you don’t see this. But if you look at a satellite image, it’s very, very pronounced, and it occurs because the water’s being pushed around by ocean currents, by winds, and also because mass on land is attracting water toward itself.

So, if you have a big mass sitting on land adjacent to the ocean, the ocean is going to feel the tug of this extra mass off to one side, and it’s going to pull the water toward it a little bit. And in calculating what sea level’s going to do in the future, it has to take that into account because there’s this very odd other effect of a shrinking ice sheet or a large glacier system where as that ice sheet shrinks in size, it’s gravitational attraction shrinks as well. So, a big Greenland ice sheet can pull a lot of water up around itself, and as the ice sheet shrinks, it does two things: It adds water to the ocean, which raises sea level, but also its gravitational attraction shrinks, and so in the vicinity of the ice sheets, sea level actually falls, for that reason, and you have to look at the sum of those two.

And so, as the Greenland ice sheet diminishes in size, or glaciers in Alaska diminish in size, sea level actually falls immediately adjacent to them, which is sort of counter-intuitive.

ocean surface topography
[Ocean surface topography from the TOPEX/Poseidon space mission in 1992. For more about measuring ocean surface topography, check out the GRACE website, or, learn more about data and images from NASA's TOPEX/Poseidon and Jason altimeters on their Ocean Surface Topography from Space page. (From Wikimedia Commons, via Wikipedia)]

B: Tad, let’s talk some numbers. You touched on it at the beginning of the interview—could you tell us a bit about the history of sea level change and what we’re looking at moving forward? You threw out the word “accelerating” in there.

T: Yeah. Sure. During the 20th Century, so 1900 up to the present more or less, we got really quite good measurements of global sea level rise. The rate of sea level rise was about a millimeter and a half per year. So that’s from, like, 1901 to 1990. But if you look at it much more recently, like 1993 to 2010, that rate has almost tripled. It’s gone up to 3.2 millimeters per year. And it’s not a smooth curve. It’s very choppy and jumpy and you have to average over pretty long time spans, more than a decade, to see those average rates. So you can find areas where sea level actually drops for a few years, then starts going up again. Five or six years ago or so there was a brief decline in the rate of sea level rise, and some skeptics were latching onto that (...), but you can find little patches like that in virtually any of these records, where if you look at a small enough time scale you can find the trend going in the opposite direction from the longer-term trend, but of course it’s the long-term trends that we all have to think about.

So, that was during the 20th century up to now, an increase from about one and a half millimeters per year over the entirety of the 20th century up to a little more than three millimeters per year over the last two decades or so. Incidentally, those numbers come from our tide gauge measurements earlier in that period of time, before we had satellites, so you were measuring the height of the ocean relative to a fixed point on land. But, of course, fixed points on land aren’t really fixed, because the land can go up and down, for a whole variety of reasons. And there’s a whole branch of science and geodesy in working out what those land motions are, because your reference point isn’t fixed, either. So using tide gauges is actually pretty complicated. And for many years, particularly in earlier times, there weren’t very many of them. So it’s hard to get a good global average.

Once satellites came in, it was possible to get a very comprehensive picture of sea level rise through satellite altimetry. So, the satellite is simply measuring the elevation of the sea surface, and this has all kinds of advantages, including the fact that you get coverage everywhere, not just along the coastline where you’ve installed the tide gauge. And also, you can get very dense spatial coverage across the oceans—that’s where you start to see the sea level topography. And also it can be done over and over and over again, so you get very high time resolution.

So that’s what the present looks like.

B: And moving forward…?

T: And moving forward, into the future, there’s an interesting story because, you know, we could talk for an hour about this by itself, because (...) We’ve been doing the sea level projections since the early 1970s. Essentially, since we had climate models that told us enough about what future atmospheric temperatures might do, we can start thinking about what the ocean might do as a consequence. But for the future, right now, the official IPCC projections range from about 26 cm up to just shy of a meter. 98 centimeters.

One meter sea level rise
[The Southeastern U.S. sans what would be lost to one meter of sea level rise. (Screen shot from NASA's Climate Time Machine)]

B: Why is that range so big?

T: Okay, well there are two reasons for it. One of them is that we use a variety of different future forcing scenarios. In other words, what might climate do? That’s what is ultimately driving sea level. And if there’s uncertainty in what the atmosphere is gonna do, that carries right over into uncertainty in what sea level is gonna do. So we actually have five different future pathways for global temperature, and each one of those produces a slightly different range of sea level rise. But then, within each one of those scenarios, there’s a lot of uncertainty, and that’s because we don’t understand how the entire system works.

But the most volatile bit of the business of projecting sea level rise really lies in rapid ice dynamics, which is a fancy word that essentially boils down to calving of icebergs.

Glaciers and ice sheets that end in the ocean can get rid of ice much faster, much more efficiently, by calving icebergs, and that’s controlled primarily by how rapidly ice has moved down to the sea, to the perimeter of the ice where it ends in the ocean, and there’s a lot of complicated internal physics there, which we’ve made significant progress in since the last IPCC report, but it’s still kind of rudimentary. So, there’s a lot of uncertainty for that reason still.

B: So one of the main differences, and we might have to end on this point, but one of the main differences that’s been cited in the media between the last IPCC report, which was in 2007, so it’s been six years, and this year, is--a lot of people are talking about a difference in projected temperature range by the end of the century. Before, it was from a 2 to 4 degree Celsius increase, I believe, and now it’s 1.5 to 4, so that lower bound has lowered, the upper bound of potential temperature increase has stayed the same, but yet the sea level projections have gone up on their upper bound.

T: Yeah. Yeah. And that is because—well, again, the uncertainty is large compared to what uncertainty is, or, really, the change in the projection from that slightly lower bound on future warming. But also, it’s because this time, compared to AR4, we have the dynamics worked out more reliably. And this is the business of icebergs calving into the ocean rapidly. And that doesn’t depend very directly on temperature. That’s a process which is triggered by temperature, so warming can push glaciers and ice sheets into the state where they’re dumping icebergs like crazy into the ocean, but that rate of loss to iceberg calving doesn’t vary up and down with the temperature. So all of these processes that happen at the other end, the high end of the spectrum, don’t really care too much about the details of what the temperature is doing, as long as it’s pushing the ice system above this certain threshold.

Another thing that’s happening is we’re getting a better idea of what the small glaciers are doing.

And that's where we had to cut it off, for want of time. End. Didn't get to my questions about any difficulties in building consensus, which is required for the report, or the effects of the sea level rise. The latter can easily be explored on the Web, although I swear that there was a great tool for looking at sea level rise that I just can't for the life of me find. The ones that came up were all too geographically limited, too rough of a scale, or just plain ugly. Sigh. Explore. Let me know what you find.

Sea level. It's coming up.

Posted by beth at October 20, 2013 3:08 AM

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