Melting Ice and Sea Level Rise

ResearchBlogging.orgIf all the water currently trapped in all the glaciers across the entire world melted, the sea level would rise far more than most people imagine. Almost everyone living anywhere in the world at an elevation of below about 500 feet with a direct drainage to the sea would be directly affected; The sea level rise itself might be a bit over 300 feet, but oceans tend to migrate horizontally when they rise onto previously uninnundated land surfaces. So if you lived at 500 feet above sea level in most of Maine, you’d have a much shorter walk to the rocky shoreline, but if you lived at 500 feet across much of the Gulf Coast it would only be a matter of time until the eroding sea cliff reached you incorporated you into the offshore sediments.

Having said that, Anthropogenic Global Warming has resulted in only modest sea level rise to date, and it is at this point probably true that warming of the ocean causing thermal expansion has been at the same level of magnitude (or greater) than seas rising because of the influx of melted glacial water.

The problem is, it is very difficult to measure either sea level rise or ice loss very accurately, for a number of reasons. But there is a saving grace. Or should I say, GRACE. GRACE is a NASA project; Twin satellites measure changes in the Earth’s gravity field in such a way that it is possible to identify changes in the distribution of water. From the GRACE overview statement:

GRACE will be able to map the Earth’s gravity fields by making accurate measurements of the distance between the two satellites, using GPS and a microwave ranging system. It will provide scientists from all over the world with an efficient and cost-effective way to map the Earth’s gravity fields with unprecedented accuracy. The results from this mission will yield crucial information about the distribution and flow of mass within the Earth and it’s surroundings.

The gravity variations that GRACE will study include: changes due to surface and deep currents in the ocean; runoff and ground water storage on land masses; exchanges between ice sheets or glaciers and the oceans; and variations of mass within the Earth. Another goal of the mission is to create a better profile of the Earth’s atmosphere. The results from GRACE will make a huge contribution to the goals of NASA’s Earth Science Enterprise, Earth Observation System (EOS) and global climate change studies.

A major study of GRACE data by Thomas Jacob, John Wahr, W. Tad Pheffer and Sean Swenson was published moments ago in Nature. Jonathan Bamber supplies a summary:

Understanding, and closing, the sea-level budget (the relative contributions of mass and thermal expansion to ocean-volume change) is crucial for testing predictions of future sea-level rise. Estimates of the future response of [glaciers and ice caps] to climate change are, in general, based on what we know about how they have responded in the past. A better estimate of past behaviour, such as that obtained by Jacob and colleagues, will therefore result in better estimates of future behaviour. Discussion of the demise of the Himalayan glaciers has been mired in controversy, partly because of basic errors, but also because of the dearth of reliable data on past trends. Given their role as a water supply for so many people1, this has been a cause for concern and an outstanding issue.

The method works like this: Ice bearing region are defined as “mascons” (sampling units in fancytalk) where mass values are taken by the satellite. Factors that are not ice are subtracted from the changes in gravity in each mascon and a fancy modelling program that Global Warming Denialists will not understand and therefore claim to be a hoax is applied to the data. The result is something like this:
i-92692b8fed055f81f7d9a3d1330d69f7-nature10847-f2.2-thumb-500x1459-72449.jpg
This shows the change in mass for numerous regions around the world.

Over the seven year period of this study, mountain glaciers and ice (outside of Greenland and Antarctica) contributed to about 0.041 cm of sea level rise a year, and the major ice sheets of Greenland and Antarctica produced about 0.106 cm of sea level rise a year, for a total of 0.147, according to this study. This is in the same order of magnitude as other studies, though the new research tends to show less melt than previously thought. There is a major discrepancy between the new GRACE study and earlier work; The Tibetan Plateau is shown in other studies to have lost much more ice than GRACE indicates. The authors are convinced that they’ve got the number right. The main problem here is that gravitational studies (and the discrepancies are between different gravitational studies) measure the movement of major land masses. You can think of the Tibetan plateau as a big pile of stuff that is currently being shoved up into a bigger pile of stuff by tectonic movements. It is dyanamically moving for that reason. Then, glaciers grow and shrink on this elevated mass. That complicates the movement of the plateau over time. It is a little like measuring the exact location of a fishing bobber on a rippling pond.

To give you an idea of the relative magnitude of 0.147 cm a year of sea level rise, consdier that the total sea level rise since the last glacial (ice age), if averaged over 12,000 years (roughly the time since intensive melting started) would be about 1 cm a year. Of course, the actual rate of sea level rise was much greater than that when the glaciers first started melting and then slowed considerably later.

Current sea level rise is not very significant, but we expect the rate of glacial melting to increase if Anthropocentric Global Warming is not curbed. In fact, this study does not help us at all in estimating the rate of future melting. What it does do is refine our ability to measure ice wasting as it happens, in a way that is probably more accurate than previous method and more comprehensive, including all the ice globally.

Jacob, T., Wahr, J., Pfeffer, W., & Swenson, S. (2012). Recent contributions of glaciers and ice caps to sea level rise Nature DOI: 10.1038/nature10847

See also this press release from NASA.

Comments

  1. #1 Dave R
    February 8, 2012

    Just under the graph I think you should have 0.041 instead of 0.41. And the first sentence is missing the word “melted” before the comma.

  2. #2 Greg Laden
    February 8, 2012

    Thanks very much. This is what happens when I think Imma copy and paste a leader from a NASA press release directions my readers to an interesting story and end up doing a more extensive blog post with numbers and graphics and 8×10 glossies with arrows ….

  3. #3 Russell
    February 8, 2012

    And here, I’m planning on building a house about 30 feet above sea level. Let’s hope I have another 50 years!

  4. #4 Greg Laden
    February 8, 2012

    Seriously? I hope you are not in the North Atlantic! You’re probably OK for 50 years in the subtropical regions or where the ocean is nice and wide. Not counting storms, of course. Also, try not to face any major subduction zones or transverse faults, even if they are across a pretty large ocean!

  5. #5 Russell
    February 8, 2012

    Gulf coast, Greg. No worry about tides or tsunami, but I’d like to stay above storm surges.

  6. #6 SplatterPatterns
    February 9, 2012

    The results are probably true and the trends likely more so for alpine glaciers I’d think. The biggest changes based on the Grace observations are at high northern latitudes, and the sizable declines are in the areas of the northeastern Canadian Arctic Archipelago (plots 14 and 15)/northwestern Greenland, and, in the northern Gulf of Alaska (plot 12). The ones in the Gulf are tidewater type, which are quite variable in mass due to the interplay of several factors. From the write-up by Wynne Parry in Live Science, the U of Bristol Glaciology Centre’s Jonathan Bamber notes in “a commentary published along with the study …. that the study period was too brief to capture large fluctuations in melting from some areas, such as in the Gulf of Alaska and the high Asian mountains.”

    Glaciers edging the Gulf have the same tectonic forces at work that Linden explained occur in the Himalayas, with the Pacific Plate subducting under them, as well as large fluctuations in the very heavy precipitation they receive.

    The photos I’ve seen of Baffin and Ellesmere Islands look to have the same mountainous coasts as the Gulf, but the atmospheric circulation, precipitation patterns, and temperature regimes are enough different that I can’t speculate (at least on the basis of knowing much detail) about the eastern side of the continent, though I think it more likely that the warming signal there is less prone to confounding factors.

  7. #7 Greg Laden
    February 9, 2012

    Doug, the mention of the Tibetan water supply is not about the water at the mouths of the rivers a very, very long way away. Most of the water in the headwaters, and all of the high stage water that is used in local irrigation, is from meltwater. You have distorted and cherrypicked the data again. Can’t help yourself, can you!

  8. #8 Brian
    February 9, 2012

    The Sky is Falling Syndrome

    In order to incite people to action, a problem needs to be presented in a way that induces an emotion (fear, anxiety, excitement, …).

    In this article the first paragraph talks about large rises of the sea (300 feet) and as if that wasn’t enough talks about the level to which the impact would rise (500 feet that it mentions 3 times).

    After some really interesting graphs is buried the actual number: .147cm per year. This is the average over the last 7 years (2003-2011). Interesting that this number is in cm, but the other in feet. Doing some simple math, we can find out that .147 centimeters is .00482 feet (rounding). At this rate how long would it take the ocean to rise just 1 foot? 207 years. And how long would it take it to rise 300 feet? 62 thousand years.

    So here is the problem I’m having. I really like being able to breath and thus I think clean air, and protecting the environment are things that we as a society should be striving for. The problem is that people that are trying to promote change are distorting reality. I realize that they are trying to get people’s attention and push for change, but this distortion is having the opposite effect on people who take the time to understand the facts.

    I realize that saying “The sky is falling, at a rate of 1 foot every 200 years” isn’t a good way to try and get people to change, but without this restraint all you convince are the idiots. Worse you deafen people to the real problem. Basically you end up “crying wolf.”

  9. #9 Greg Laden
    February 9, 2012

    Brian, the problem you are having is mainly with reading comprehension. I can’t believe you actually think it’s OK to misrepresent and cherry pick a piece of writing that is sitting right there for people to actually read, to see what you did.

  10. #10 Quantico Cat
    February 9, 2012

    “but if you lived at 500 feet across much of the Gulf Coast it would only be a matter of time until the eroding sea cliff reached you incorporated you into the offshore sediments.”

    It’s possible that wording like the above is affecting Brian’s comprehension. Whatever the case, it is difficult to discern the point trying to be made in that first paragraph.

    As an anthropologist, maybe you could speculate about what it is about the human condition that compels us universally to invent end-of-days mythologies?

  11. #11 Amused
    February 9, 2012

    I don’t dispute the melting, but how does one accurately adjust out the underlying mass of the earth crust? I would think that variabilities in the densiities of the underlying strata would be multiplies of the volume of ice on the surface.

  12. #12 Amused
    February 9, 2012

    Duh! Just realized my mistake. We’re talking about less then a decade….

  13. #13 Greg Laden
    February 9, 2012

    Amused, yeah, people have thought of things like that!

    I’m ruthlessly deleating the obnoxious denier comments, but I do want to respond to a sentiment I’ve seen there as well as on comments on G+ and elsewhere regarding this: People seem to not be finding it interesting that if all the land-ice on the planet melted, that the sea level would rise over 300 feet and carve out significantly more landscape than the 300+foot contour line.

    How can you not find that interesting?

    “Why would you say that” … “What relevance is this” … “I did not know that therefore it annoys me…”

    Think about it. Just let your brain have the thought for a while, unwrapped in whatever it is that makes you not want to know new things or learn new things, and notice something: It is in fact very very interesting that if all the land ice on the planet melted, the shoreline would move to some point between the current 300 foot contour and above (I’m using 500 feet as a thumbsuck).

    Go look at a map with top data on it, say of the New Orleans area or New York City or your favoriate coastal location.

    When you’re done failing to figure out what that is ABSOLUTELY FUCKING AMAZING go and find a map of North America or wherever you happen to live of the late Jurasic and Cretaceous periods and look at the shoreline. The basic continental masses have not changed much since then, though they’ve moved and mountain building and erosion has happened. But during periods of high seas, THAT IS WHAT IS HAPPENING. The ice is all melted.

    Now, if you happen to live in one of the places that was under the high seas during one of those periods, go dig a hole or look at a road cut or a stream embankment or talk to a local geologist and find out what kind of sedimentary rocks are in your area.

    It will likely be the sediments that formed under those high seas.

    Is this still not interesting? If not, please get a job!

    Thank you very much, that is all for now.

  14. #14 steve
    February 10, 2012

    Greg; You state that you are “ruthlessly deleting” denier comments. Since there are so far so few comments on the thread is it fair to assume that most comments are critical?

  15. #15 Greg Laden
    February 10, 2012

    Steve, no. I’ve never used the term “ruthless” to indicate quantity. Why do you ask?

    Also, I would not call the denier comments critical. They’re just crazytalk.

  16. #16 Russell
    February 11, 2012

    Of course, “if you lived at 500 feet across much of the Gulf Coast,” you would be committing a navigation error, and confused about your location. On most of the Gulf Coast, the ground elevation is on the order of 5 to 10 feet. I know only two towns on the Gulf Coast that have more than 30 feet of elevation in parts.

  17. #17 Quantico Cat
    February 11, 2012

    One thing we can all probably agree on is that the evidence suggest that sea levels have much been higher in the past (before industrialization, before people). Sea levels are not static, they change due to global temperature changes, to one degree or the other. The question becomes then, is this a argument for or against Anthropogenic Global Warming?

  18. #18 Greg Laden
    February 11, 2012

    Russell, that is exactly the point. If sea level rose, the coast would move inland, in the case of the Gulf, mostly north. Go find the 300 and 500 foot contours nearest the current coasline!

    Quantico Cat, do tell: When and where was the sea level higher than it is now? I’m not saying it wasn’t, but I’m not entirely sure you’ve got a handle on this. Yes, that is a quiz!

  19. #19 Russell
    February 11, 2012

    In all seriousness, it is really hard to get people to think much about the next thirty years, much less then next three hundred. I’ve seen one-story McMansions built with ground entry on barrier islands right next to older beach houses, built up on piers. If you can’t get people to plan for the next major storm, the probability of which is well-known, how do you get them to plan beyond that?

  20. #20 Greg Laden
    February 11, 2012

    Well, you certainly don’t do it by relying on their common sense.

    In New England, you couldn’t do what you just described. I’m actually amazed one could do it on the Gulf Coast because building in coastal regions is to some extent federally regulated. After the storm of 78, all new coastal construction in certain beach areas of New England (much of the Mass shoreline, as it turns out) was simply not allowed. Some 200 houses were washed out to sea in that storm, and they then provided a nighmare for obstructions at sea and where they re-landed on the shore on national seashores and on various commercial docks, public landings, etc. It was determined that all of the construction in a given zone was a hazard.

    There probably shoulnd’t be any new building on the barrier islands along the Gulf Coast.

  21. #21 NJ
    February 11, 2012

    Greg Laden @ 21:

    There probably shoulnd’t be any new building on the barrier islands along the Gulf Coast anywhere.

    Edited slightly to reflect improved understanding of coastal hazards a la Pilkey.

  22. #22 Greg Laden
    February 11, 2012

    Well, OK, but I mentioned the Gulf Coast specifically becuase I was under the impression that barrier island construction had been mostly curtailed in other areas of the US.

  23. #23 Quantico Cat
    February 11, 2012

    Come on, Greg. Here is your quote: “But during periods of high seas, THAT IS WHAT IS HAPPENING. The ice is all melted.

    Now, if you happen to live in one of the places that was under the high seas during one of those periods, go dig a hole or look at a road cut or a stream embankment or talk to a local geologist and find out what kind of sedimentary rocks are in your area.

    It will likely be the sediments that formed under those high seas.”

    Is this really the point you want to dicker over? I stopped by to see if there was any room here for a rational conversation about these issues. It looks more and more like there isn’t.

  24. #24 NJ
    February 11, 2012

    Greg Laden @ 23:

    I mentioned the Gulf Coast specifically becuase I was under the impression that barrier island construction had been mostly curtailed in other areas of the US.

    As with any extant environmental protection, that curtailment is subject to legal and legislative erosion:

    http://coastalcare.org/2011/07/sandbagged-the-undoing-of-a-quarter-century-of-north-carolina-coastal-conservation/

    Strictly speaking, that isn’t about new construction, but if you push against one set of regulations, it’s a safe bet that a push against related ones isn’t far off.

  25. #25 daedalus2u
    February 12, 2012

    What GRACE is measuring is the gravitational attraction of mass concentrations on the Earth. If, as the ice melted, the surface “rebounded”, the resulting observed mass loss would be reduced.

    There are a couple of ways the surface can “rebound”, one is if the objects the ice is on are “floating” (which they are), but then the speed of rebound depends on the viscosity of the fluid they are floating on. Viscosity of silicate melts is highly dependent on the amount of water present, with more water producing lower viscosity.

    Places where there is subduction are going to have more water and so have lower viscosity magma. Similarly, the places with the highest mountains, likely also have the lowest density magma under neath them (so the mountains float higher) which means higher temperature and likely lower viscosity and (likely) faster rebound from glacial unloading.

    Also, the Three Gorges Dam holds 39 GT of water, and is sort of near the Tibetan plateau. The emptying and filling of melt-fed reservoirs is going to be 180 degrees out of phase with the seasonal deposition and melting of ice, that balancing may obscure the actual deviations.

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