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The Greenland Ice

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by Rasmus Benestad, Eric Steig and Gavin Schmidt

In a recent paper in Science, Eric Rignot and Pannir Kanagaratnam present new satellite observations of the speed of glaciers of Greenland, and find that they are sliding towards the sea almost twice as fast as previously thought. Additionally, between 1996 and 2005, they detected a widespread glacier acceleration and consequently an increased rate of ice discharge from the Greenland ice sheet. However, previous papers have recently noted an increase in snow accumulation in the interior (i.e. Johannessen et al., 2005), so how do these different measurements fit into the larger picture of Greenland’s net mass balance?

2004/150 - 05/29 at 14 :25 UTC Southern Greenland Satellite: Terra The measurements by Rignot and Kanagartnam were made with interferometers which measure the movement of the surface horizontally, and so is complimentary to the altimeter data published previously (which measures the absolute height of the ice). Overall, they found widespread increases in glacier speeds, and increases of about 30% in ice discharge rates. (Note that the satellite image shows that the glaciers in the east tend to slide far into the sea whereas on the western coast that happens less).

The higher velocity of the ice is thought to be related to higher temperatures causing increased melt-water which can penetrate to the base of the glacier and hence reduce the ground friction. However, this accelerated movement is not necessarily tied to an increased rate of melting of the Greenland ice, although it can be related. Surges of ice streams from the ice sheet can also occur due to increased accumulation at the head of the glacier. However, when the increased ice velocity is matched to a decreasing thickness that can be sign of net mass loss. These ideas are consistent with observations of surface melting which had a record extent in 2005, and has been increasing steadily (though with significant interannual variability) since 1993. Using the analysis of Hanna et al (2005) (based on the reanalysis datasets) for the surface mass balance, Rignot & Kanagartnam estimate that Greenland is on balance losing mass, and over the period of their study the ice sheet mass deficit (the amount of ice lost to the sea) has doubled increasing from 90 to 220 km3/year (an increase of 0.23 to 0.57 mm/yr sea level equivalent – SLE).

In the earlier Science paper, Johanessen et al. found increased snow accumulation on the top of the interior Greenland ice sheet between 1992 and 2003. Above 1500m a.s.l in much of the interior Greenland they estimated an increase of 6.4 ± 0.2 cm/year and below 1500m they observed a decreasing trend of -2.0 ± 0.9 cm/year. Hence, growth in the interior parts and a thinning of the ice nearer the edges. However, Johanessen et al. were not able to measure all of the coastal ranges. Indeed, the thinning of the margins and growth in the interior Greenland is an expected response to increased temperatures and more precipitation in a warmer climate. These results present no contradiction to the accelerated sliding near the coasts, but both will affect the ice/snow (fresh water) mass estimate. Whereas the finding of Rignot and Kanagaratnam suggests a larger sink of the frozen Greenland fresh water budget (the ice is dumped into the sea), the snow deposition in Greenland interiors is a source term (increases the amount of frozen fresh water). It does not matter for the general sea level in which form the water exists (liguid or solid/frozen) when it is discharged into the sea: The same mass of liquid water and immersed ice affect the water level equally (Archimede’s principle).

A third relevant study is a recent paper in the Journal of Glaciology by Zwally et al. (2005) on the ice mass changes on Greenland and Antarctica. They use the same satellite obsevations (ERS 1 and 2) as Johanessen et al. and again find that the Greenland ice sheet is thinning at the margins (-42 ± 2 Gt/year = -46 ± 2 km3/year below the equilibrium-line altitude – ELA), but growing in the inland (+53 ± 2 Gt/year = 58 ± 2 km3/year). The mass estimates have been converted to volume estimates here, assuming the density of ice is 0.917 g/cm3 at 0°C, so that the mass of one Gt of ice is roughly equivalent to 1.1km3 ice*. This means that the Greenland ice has an overall mass gain by +11 ± 3 Gt/year (=10 ± 2.7 km3/year) which they estimated implied a -0.03 mm/year SLE over the period 1992-2002.

The critical point for Greenland is whether the increased rate of glacier motion more than compensates for the greater accumulation on the surface. While the broad picture of what is happening is consistent between these papers, the bottom-line value for Greenland’s mass balance is different in all three cases. Looking just at the dynamical changes observed by Rignot & Kanagaratnam, there is an increased discharge of about 0.28 mm/year SLE from 1996 to 2005, well outside the range of error bars. This is substantially more than the opposing changes in accumulation estimated by Johannessen et al and Zwally et al, and is unlikely to have been included in their assessments. Thus, the probability is that Greenland has been losing ice in the last decade. We should be careful to point out though that this is only for one decade, and doesn’t prove anything about the longer term. As many of the studies make clear, there is a significant degree of interannual variability (related to the North Atlantic Oscillation, or the response to the cooling associated with Mt. Pinatubo) such that discerning longer term trends is hard.

The largest contributions to sea level rise so far are estimated to have come from thermal expansion, with the melting of mountain glaciers and icecaps being of second order. Looking forward, the current (small) imbalance (whether positive or negative) of the Greenland ice sheet is not terribly important. What matters is if the melting were to increase significantly. Ongoing observations (most promisingly from the GRACE gravity measurements, Velicogna et al, 2005) will be useful in monitoring trends, but in order to have reasonable projections into the future, we would like to be able to rely on ice sheet models. Unfortunately, the physics of basal lubrication and the importance of ice dynamics highlighted in the Rignot & Kanagaratnam results are very poorly understood and not fully accounted for in current ice sheet models. Until those models include these effects, there is a danger that we may be under-appreciating the dynamic nature of the ice sheets.

References:

Hanna, E; Huybrechts, P; Janssens, I; Cappelen, J; Steffen, K; Stephens, A (2005) J. Geophys. Res.Vo. 110, D13108, doi:10.1029/2004JD005641

Johanessen, O.M; Khvorostovsky, K; Miles, M.W; Bobylev, L.P. (2005) ScienceVo. 310 no. 5750, pp 1013-1016

Ringnot, E; Kanagaratnam, P (2006) ScienceVo. 311 no. 5763, pp 986-990

Velicogna, I; Wahr, J; Hanna, E; Huybrechts, P. (2005) Geophys. Res. Lett.Vo. 32, L05501, doi:10.1029/2004GL021948

Zwally, H. Jay; Giovinetto, Mario B.; Li, Jun; Cornejo, Helen G.; Beckley, Matthew A.; Brenner, Anita C.; Saba, Jack L.; Yi, Donghui (2005), Journal of Glaciology, Volume 51, Number 175, December, pp. 509-527(19)

*Update: Correction for arithmetic error in orginal post in converting Gt to km3, see comments.

Reader Interactions

175 Responses to "The Greenland Ice"

  1. Re # 149 – **On the science, IMHO Gerald misinterprets the post. **
    I do not believe that I have misrepresented the post. There is no conclusion or proof of what will happen. Several studies are quoted.
    **Gerald’s response seems to assume that we can discount to zero impacts on future generations.**
    I have not said the impact is zero, but I have said that before we have significant rise we will pollute ourselves, among the other things not discussed on this blog.
    **Exactly how quickly the collapse could really occur has yet to be established, but the signs (the recent speed-up of glaciers in southern Greenland and the apparent progress of this speed-up into the north, plus the equally rapid growth in the surface melt zone) are very bad.**
    And I do not buy the alarmist collapse of the Greenland Ice Cap. Here is where I indicate that scientists are not sure if warming is actually contributing to an increase in ice mass through more precipitation as warmer air holds more moisture.
    **I have yet to hear a scientist working on these issues say, e.g., we can categorically exclude the possibility that Greenland amd/or the WAIS might substantially collapse within a century or so.**
    I do not believe that too many scientists actually believe that Greenland would see enough melting to even raise the sea level even half a metre in a century. I still say “let’s work on the pollution”

  2. Re #151: I think you’re being a little evasive, but in any case there’s no point in you and I continuing to go around on these issues. However, I am curious as to what you personally are doing about air pollution. Details, please.

  3. 146 onwards. Thanks to all and yes I have read the master post at the top.

    My point was more to do with hotspots. As with preferring ozone holes to steer clear of populated areas I wondered if having a carbon hotspot at one pole was very healthy for us : it cant make the current small (? and rate of change uncertain) negative mass balance for the Greenland sheet any better.

    Ozone holes, I understand, are having significant problems repairing themselves and for carbon hotspots I havent a clue.

    This seems to me like a really technical question to do with source(s), sinks, mixing or lack of it in the atmosphere and no doubt other issues.

    [Response:CO2 in the atmosphere is well mixed. It doesn’t matter where you add it – William]

  4. Thanks William but your answer does beg some questions.

    For example, you refer to CO2 but not methane : does that matter?

    For example, if I am to take Hank’s 146 comment then it will be only a matter of time (how long?) before the carbon is dispersed around the globe or absorbed or both. However I cant really believe that the Swedes would report on carbon levels which are that innocuous.

    If on the other hand the reported carbon levels are not local then what are the implications for that?

    For Gerald, I am now going to go stone wall building and leave the Ark for another day. Stone wall building has more Zen attached but it is very exhausting.

  5. Methane is also well mixed in the atmosphere. The only greenhouse gas that varies in concentration is water vapor, which we only affect indirectly. The implication is that local emissions of greenhouse gases have a global effect.

    [Response: Not quite true, ozone is also a GHG, is very heterogeneous and is an important forcing that is mostly driven by local emissions (of ozone precursors like NOx, VOCs, CO etc.). – gavin]

  6. Re # 152 – Evasive? Not sure, but I do not accept everything I read without data.
    Pollution – Well, I worked for the gov – they do not listen, so my message is not getting thru. They “approved” Kyoto, but are doing nothing. They falsify information(will not go into details).
    Me – drive fuel efficient vehicle, Electical heating in house – insulated long ago to well above new standards – beats gas heating. That is enough.

  7. Re #156: Ah. So, broadly speaking, let others (me, e.g.) work on the air pollution problem while you spend your spare time on the internet developing arguments against solving global warming. Fair enough.

  8. Re #144 — Correcting the Greenland ice cap sea stand equivalent.
    According to T.M. Cronin in “Principles of Paleoclimatology”, page 30, the rise, should it all melt, would be 7.4 meters.

  9. Re #156, #157 – If you think that only getting rid of CO2 will solve global warming, you are dreaming. Look at the history of temperature fluctuations over the past thousands of years. There is more to the variation than CO2. Melting Ice Caps and rising sea levels are only a possibility – pollution is here now. My son is in Japan where people wear face masks on the street. California had(and still has some) a problem, so they imposed stronger standards for cars. It is time to go much farther. If we develop the technology to clean up pollution (in land, sea, air, and food), there will be related technologies which will also take care of CO2.
    If you only talk CO2, there is less chance of a gov response. The last Canadian Minister of the Environment was fron Victoria, British Columbia. His own city has been spewing raw sewage into the Strait for years.
    The new government under Stephen Harper in their campaign indicated they would redo Kyoto into a workable program. We will see if they carry it out, but we can try a test – write to the minister and see if she responds to our communications more than the last one. I am doing that.

  10. RE 157, 159:

    Gerald has a good point that needs to be contextualized. It is absolutely true that – implicit in Gerald’s comments – is that human land use/landcover (LU/LC)changes have impacted the environment. China has started their dust storms this year and the cloud should be in Japan soon (if it isn’t now). And I moved from Sacramento, CA because I couldn’t take the air pollution any more.

    But focusing on LU/LC changes or short-term air pollution concerns shouldn’t dismiss our focus on CO2 – the argument in 159 implies the two are separable. They are not.

    Best,

    D

  11. Trying to have a qualitative understanding of the glacier processes.

    A frequent mention is made of lubrication by meltwater at the base of a glacier, as well as of growing pressure from above due to increasing snowfall. There are other factors as well.

    I do not know exactly how the satellites measure the extent of summer melt in Greenland. Anyway, a major change seems to be underway, reflected in the large change in this parameter. Some related components I could identify are:

    1) Change in snow surface layer properties and in the process that converts snow into ice. This might impact the measurements (sensor calibration), for instance. This forms mainly slush that do not flow.

    2) In places, meltwater may flow and form pools, in extreme cases even lakes that do not freeze fully during the winter season. The terrain could get more even than before. A small part of meltwater may even flow into the ocean.

    3) Summer meltwater fills existing cracks in the ice, then re-freezes there at some depth as the interior of the glacier is well below zero temperature. This is heat transport, as the water latent heat is released the glacier inside temperature increases.

    4. Re-freezing also generates tremendous mechanical strains. Water expands as it freezes. Old cracks close, but new ones are formed and cracks deeper down are widened. (Incidentally, this process was sometime used in stone quarries. A row of holes was drilled, filled with water and the forming ice cracked the granite. In nature it can be observed on the slopes of fells, where layered sedimentary rock continues to be broken into stone flows and cascades. Maybe there is an analogy to the flowing glaciers?)

    One might propose that the coastal glaciers now get broken into smaller blocks as these processes get stronger, which would facilitate the flow. This also would impact the tongues extending into the ocean. Icebergs would then break free easier than before, but would have a bit smaller volumes.

    Maybe these phenomena are included somehow in the models, though it seems to me that they are quite difficult to quantify in any detail.

    B.R.

  12. Re #161: Based on what I know, I think some of what you mention is correct but some is not. There are papers referenced in comments 5 and 11 that would be worth a look. The Journal of Glaciology articles become public after a time, so those may be public access by now. Try Google Scholar for public access versions of the others. You might also try Richard Alley’s site to see if he has any material posted.

  13. Looking much deeper below the surface:
    http://www.ocean.udel.edu/newscenter/meddies.html

    Scientists Use Satellites to Detect Deep-Ocean Whirlpools

    Xiao-Hai Yan (seated) and Young-Heon Jo

    Xiao-Hai Yan (seated), Mary A. S. Lighthipe Professor of Marine Studies at the University of Delaware, and postdoctoral researcher Young-Heon Jo have detected deep-ocean whirlpools called “Meddies” using a new satellite-based technique they developed with researchers at NASA and the Ocean University of China. press release, 2:00 p.m. Eastern, March 20, 2006

  17. This is getting more worst case Hansenian by the week.

    It doesnt seem to me that our poor world has many options other than stopping consumption today.

    I know that William and the rest like to use the word “septik” but it looks more like stumblebum to me.

  18. The Guardian article was confusing: It seems to mix up global temperature rises and the higher Arctic ones. It suggests that temperatures during the Eemian were up to 5 degrees C higher than today – I thought they were only slightly above today’s. Even the NCAR report speaks of a possibility of 3-4 degrees C Arctic warming by 2100. On the other hand, NASA GISS figures show that temperatures in most of the Arctic were already 1.5-3.5 degrees C above the 1950-1980 baseline.

    Could anybody who has read the paper clarify what it says? What levels of Arctic warming and global warming are linked to changes as large as in the Eemian? Many thanks!

    Almuth Ernsting

    [Response: I haven’t read the paper(s). But I believe that Arctic temperatures were supposed to swing more than global ones, so 3-4 oC in the Arctic for the Eemian is possible, though I would have thought a bit on the high side – William]

  19. I have been participating and attending glaciology meetings for 25 years. It is common understanding based on ice sheet dynamics that neither the East Antarctic Ice Sheet or the Greenland Ice Sheet can collapse quickly. They do not have the ability to undergo a large calving retreat, because of basal topography. The ice sheets are grounded and most of the retreat would have to come from in situ, ie. slow, melting. Each does have important outlet glaciers, which could experience a calving retreat, but this is not majority of the ice sheet margin. Again having worked on Pine Island and Jakobshavns Glacier I am very concerned by the substantail acceleration. But be careful of considering a rapid collapse of the Greenland Ice Sheet. Only the West Antarctic Ice Sheet is capable of rapid collapse.

  20. Maurio, wouldn’t the “icequakes” — which would have been well known if they’d been happening all along — suggest more cracks and crevices, and so more water flow, and so more heat transfer, and so more rapid melting all of a sudden? Or is there nothing new here to add to the common understanding from the past 25 years?

    If the “icequakes” are novel in the seismic record, what difference do they make, are there any comparable sites where icequakes have long been documented and aren’t increasing?

  21. Maurio, I’m reading comments at New Scientist, here, that say the Greenland ice sheet is at risk now despite the fact that the ice is, as you say, “grounded” (compared to the Antarctic grounded ice where the surface is not melting).

    Do you disagree with that distinction? It’s new, not part of the 25 year history of studying this.

    Quoting from here:
    http://www.newscientist.com/article/dn8889-glacial-earthquakes-rock-greenland-ice-sheet.html

    QUOTE
    “… analysed glacial seismic records back to 1993, they found a striking increase in the number of quakes recorded in recent years. All 136 of the best-documented slips were traced to glaciated valleys draining the main Greenland ice sheet. A handful of others occurred in Alaskan glaciers or on Antarctica.

    Ekström reports that quakes ranged from six to 15 per year from 1993 to 2002, then jumped to 20 in 2003, 23 in 2004, and 32 in the first 10 months of 2005 � matching an increase in Greenland temperatures.

    The finding adds to evidence that the Greenland ice sheet is far more vulnerable to temperature increases than had been thought. Models that treated glaciers like giant ice cubes had predicted very slow melting. But recent studies of Greenland glaciers have shown much faster effects when meltwater causes glaciers to slip easily over rock.

    “Within a few years after temperature warms, you get a big increase in discharge,” says Ian Joughin of the polar science center at the University of Washington in Seattle, US. “If temperature rises two or three degrees in Greenland, things are going to start falling apart,” Joughin told New Scientist. Antarctica is not as sensitive to rising air temperature because it is too cold for surface melting, which accounts for about half the mass lost from the Greenland ice sheet.”
    END QUOTE

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