Scientists forecast metre rise in sea levels this century says The Grauniad. I strongly suspect they have garbled things, though I admit I haven’t read the original Science paper. I have read the NCAAR/UCAR press release Arctic, Antarctic Melting May Raise Sea Levels Faster than Expected.

The Grauniad continues Half of Greenland and vast areas of Antarctica are destined to melt if global warming continues at the same pace until the end of the century, scientists warned yesterday. Their research shows that the loss of so much ice will trigger dramatic rises in sea levels, ultimately swamping low-lying regions of Essex, Lincolnshire and Norfolk and threatening the flood defences of cities such as London, Liverpool and Bristol. The last time so much ice was lost from the poles – in a period between ice ages 129,000 years ago – global sea levels rose by four to six metres.

Now, as far as I can see, this is – deliberately or accidentally – mixing up two things. If you read this quickly, you expect half of greenland to melt *within the century*. But thats *not* what it is saying. It is saying (and this is clearer, but still not very clear to the casual reader) from the NCAR press release that by 2100 temperatures will be as high as they were during the last glacial, and that given enough time this would be expected to melt the ice sheets to the degree that they melted then. What it *doesn’t* mention is that this is likely to take a thousand years or more (dependent on the exact degree of T rise, of course).

However, that simply comes into the “heard it all before” category – what exactly is new about this?

Working from the press release, the modelling component says that GW could wam the arctic by 3-5°C by 2100, and that this is roughly as warm as it was 130,000 years ago in the last interglacial (maybe, maybe, I’d have said the last interglacial was perhaps a shade cooler than that, but am not sure. No matter, continue…). Then The CCSM suggests that during the interglacial period, meltwater from Greenland and other Arctic sources raised sea level by as much as 11 feet (3.5 meters), says Otto-Bliesner. However, coral records indicate that the sea level actually rose 13 to 20 feet (4-6 meters) or more. Overpeck concludes that Antarctic melting must have produced the remainder of the sea-level rise. OK, so this is starting to make sense: where exactly previous sea level rise came from (arctic, antarctic, whatever) is a research topic. It looks like these people were maybe working on partitioning this rise between the different ice sheets (which fits the abstract Simulating Arctic Climate Warmth and Icefield Retreat in the Last Interglaciation). So, the CCSm people have got round to coupling their AOGCM to an ice sheet, only a few years after the Hadley Centre did the same. They’re doing well to get this into Science, though! Then there is the palaeo paper, Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise, which has an abstract that looks positively designed to sow the confusion that the papers have fallen into, Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable.

So this is (once again) the idea that once CO2 levels get high enough, we will be committed to melting Greenland. Which is fair enough, but it *isn’t news*. Is it something to worry about? That depends on a lot… suppose we assume this melt happens linearly over 1kyr; thats 5m/1kyr = 5mm/yr, or 0.5m/century. And you can add on thermal expansion and maybe some Antarctic melt and other stuff on to that. But still, 0.5m/century is not *fast*. It is, though, cumulative. How will our descendants cope with 5+m of SLR? I would guess that this would cause large problems over many parts of the world, if we were at todays tech levels. But we won’t be: we’ll be back in the stone age by then… or we’ll have reached the nearer stars… who knows, really?

Comments

  1. #1 Jon
    2006/03/24

    Thanks William, I’ll put link to Artic, Antartic Melting May Rise Sea Levels faster than expected on Homepage – Environment section

  2. #2 coby
    2006/03/24

    Do you think a linear melting is a good first order approximation? I don’t have that impression. Plus 1K yrs is old school, even James Hansen (admitedly speculating) is now of the opinion that we will see very rapid (one to a few centuries) collapsing of the vulnerable ice. I know the recent studies of Greenland melt accelerating are new, but does that, and if not when will it, change your thinking on this “1K yr” timeframe?

    [I can see no evidence at all for a 100y collapse, or any reason to revise the 1kyr timescale. In fact 1kyr is on the short side, if you want the full 5m. The reason "acceleration" stuff is still quite small, and by no means conclusive.

    Linear: dunno, it was the best I could do on the spur of the moment. Oh, also: http://www.grida.no/climate/ipcc_tar/wg1/fig11-16.htm -W]

  3. #3 sidd
    2006/03/24

    What do you think of Bindschadler’s piece, where he points out
    that while Greenland glaciers could retreat to land above sea level there would be no escape for large areas of the Antarctic sheets
    since they are bedded in basins below sea level ?

    [Thats W Ant, not all of it. Than can get you another 6m ish, but again the timescale is *probably* long -W]

    Alley has an interesting comment in
    http://www.csmonitor.com/2006/0324/p01s03-sten.html
    “Only five years ago, he notes, climate scientists expected the ice sheets to gain mass through 2100, then begin to melt. “We’re now 100 years ahead of schedule,”

    [Dubious, IMHO. Greenland was expected to melt, Ant to gain mass. Ant probably is gaining mass, so I'm not sure what Alley is on about... -W]

    Would you agree with Alley’s characterization of the consensus five years ago ?

    [Sure, its the bog-std IPCC: http://www.grida.no/climate/ipcc_tar/wg1/427.htm -W]
    sidd

  4. #4 sidd
    2006/03/24

    The last line in Bindschadler’s article:
    “This might limit the retreat in Greenland,but will save neither
    West Antarctica, nor the equally large subglacial basin in East
    Antarctica where submarine beds extend to the center of the ice sheet.”

    I presume that he is speaking of the Aurora and Wilkes basin in the
    East. Increased melting has been observed in the Amery shelf
    region and a speedup in the Lamberth glacier. A fascinating
    (though large) web page with some relevant data may be found at
    www-bprc.mps.ohio-state.edu/rsl/radarsat/radarsat.html

    The Vellicogna and Wahr GRAVIS results, also discussed in the
    same issue of Science indicate that both East and West
    Antarctica are losing mass. Do you feel this result (especially
    for the East) is not well founded ?

    sidd

  5. #5 coby
    2006/03/24

    William, you will like this one even more!

    ====================================
    The Irish Times, Fri, Mar 24, 06
    http://www.ireland.com/newspaper/world/2006/0324/1116351484FR24SEALEVEL.html

    Six-metre sea level rise predicted

    Two new studies claim the rise could take place in less than 100 years, writes Dick Ahlstrom, Science Editor

    The world could be facing a staggering six-metre rise in sea level in less than 100 years if the results of two new studies are correct. This would swamp a third of the world’s population and, closer to home, leave cities such as Dublin, Cork and Belfast under water.

    The two studies from the US combine the latest in computer modelling with ancient data on climate recovered from corals and other sources.

    These show that the northern hemisphere is now almost as warm as it was 130,000 years ago, when the sea level was six metres higher than it is today.”

    I can cutnpaste more but you get the drift.

    [Oh dear, they fell for it -W]

  6. #6 hank
    2006/03/29

    Good grief. And did “Dick Ahlstrom, Science Editor” retract that story? (It’s a subscription-fee site.)

  7. #7 Chris
    2006/03/29

    I thought that I had read that melting was largely insignificant to sea level rise and almost all the projected rise was thermal expansion of seawater. (Disclaimer: I am not an oceanographer.)

    It’s easy to see that melting of free-floating ice doesn’t cause any sea level change whatsoever (the water it is already displacing is precisely equal to the volume it will occupy once it melts). Land-based ice could, but there just isn’t that much land with ice on it, compared to the huge area of the oceans. Antarctica and Greenland have a combined land area of 16 million km^2 compared to 361 million km^2 for the combined oceans of the world – that means you have to melt over *22m* of Antarctic and Greenland ice to get enough water to raise sea level by 1m. (Actually slightly more because of the density difference.) Most parts of Greenland don’t even have 22m of ice on them, let alone the 132m you would need for a 6m rise in overall ocean level based on melting.

    On the other hand, if large portions of the Antarctic ice sheet fell into the ocean, the displaced water might cause a sea level rise even if they didn’t melt; but again we’re talking dozens of meters of ice over the whole area of the continent, or hundreds of meters if the landslide took place over a more limited area, per meter of resultant sea level change. One meter of sea level change = 361,000 km^3 of water (call it 400,000 km^3 of ice) = 3.6 x 10^14 tons of water/ice if it’s coming from new water/ice entering the ocean rather than from thermal expansion = a little under ten billion tons of new water/ice per day, if it’s spread out evenly over the whole century.

    I don’t know the details of the physics on thermal expansion of water, but since the oceans are big and deep, with a volume of 1.3 *billion* km^3, less than a percent change could produce a rise in water level that humans might consider quite significant. 1% increase in volume would be an extra 13 million km^3 of ocean volume; dividing that by 361 million km^2 of ocean area gives 36m of sea-level rise. Most of the ocean isn’t near the surface though, so a rise in surface temperatures may not immediately cause thermal expansion of the deep ocean.

    Yet another source would be subsidence of the continents and/or displacement of water due to upwelling of the ocean floor, but that’s not anthropogenic and we can’t do anything about it. In any case there’s no a priori reason to believe that it would dominate over *upward* movement of continental plates (literally lifting them out of the ocean) and/or *sinking* of the ocean floor (allowing more water to rush into the deepening trenches). (Disclaimer: I am not a geologist specializing in plate tectonics either. I don’t even know the *word* for a geologist specializing in plate tectonics.) Probably all four processes are going on in different parts of the earth.

    I wouldn’t be at all surprised if anthropogenic sources of sea level change (or for that matter, climate change) were trivial compared to non-anthropogenic sources. We’re a conceited species, but the world doesn’t *really* revolve around us. There were lots of periods of global climate change before there were humans around to cause them.

    All of these articles ignore the fact that “a third of the world’s population” is not immovably rooted; in fact, probably 90% of the population that will be here in 100 years hasn’t even been born yet, and the houses that they will live in haven’t been built yet, etc. Some cultural artifacts that are located right on the waterline might need relocation or special protection (I suggest consulting the Dutch), but otherwise, I don’t see why this is being called such a huge deal. The same or larger changes have happened before, as they point out; even within the existence of the human species. And we survived it that time – without any advance warning or high technology.

    There are troublesome aspects of potential global climate change, but I don’t see sea level change as a major factor. Its effects are just too local to really matter.

  8. #8 Bob Little
    2006/07/30

    I’m lost. Are we to believe that the uncontrolled factory smokestacks and auto exhausts of 127,000 yrs ago caused the GW that caused the oceans to rise 5 m or 10 m or whatever? Or if not, is it clear that there’s absolutely nothing we can do to prevent its happening again? And if carpooling with my neighbor can do nothing but defer the inevitable for ? yrs why should I, the man on the street, particularly care about non- anthropogenic GW. “The moving finger writes, and having writ –”

    PoeticPundit website

  9. So this is (once again) the idea that once CO2 levels get high enough, we will be committed to melting Greenland.