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Why is Greenland's ice loss accelerating?

Posted on 17 November 2009 by John Cook

Recently, we looked at satellite gravity measurements of the Greenland ice sheet, finding that Greenland is losing ice at an accelerating rate. This has now been confirmed by independent calculations of surface mass balance and ice discharge rates (van den Broeke et al 2009). More importantly, the extra data offers keen insights into the contributing factors of Greenland's accelerating ice loss.

Greenland's ice mass balance is governed by two factors: Surface Mass Balance (SMB) and ice Discharge (D). Surface Mass Balance is the result of ice gain from precipitation (rain & snow) minus ice loss from sublimation and runoff. In van den Broeke et al 2009, Surface Mass Balance is calculated by the Regional Atmospheric Climate Model which is then confirmed by on-site observations. Ice discharge is the loss of ice as glaciers calve into the ocean. Satellite measure the velocity of ice as it moves towards the coastline as well as the thickness of the glaciers. This data is combined to calculate the total amount of ice discharge (Rignot 2008).

Greenland's total mass change is then calculated by taking the difference between Surface Mass Balance and Discharge. This result SMB - D can be directly compared to Greenland's total mass change as determined by the GRACE satellites' gravity measurements. The two independent time series show strong agreement as seen in Figure 1:


Figure 1: Surface Mass Balance - Discharge (red) compared with GRACE data (blue). GRACE data is offset vertically. Short horizontal lines indicate GRACE uncertainty. Dashed lines indicate linear trends. The scatter plot in the inset shows a direct linear regression between monthly GRACE values as a function of the cumulative SMB - D anomaly (
van den Broeke et al 2009).

The Surface Mass Balance reconstruction also offers deeper insight into the various components that cause Greenland's mass loss. Figure 2 shows Greenland's total mass balance broken into its two components: Surface Mass Balance and Discharge. Don't be put off by the Discharge showing a positive upturn after 2000 - this is actually meant to indicate that Greenland is losing more mass due to ice discharge. We see that not only is Discharge increasing sharply due to faster moving glaciers, the Surface Mass Balance also fell sharply. Greenland's total mass loss over 2000 to 2008 is equally split between Surface Mass Balance changes and Ice Discharge.


Figure 2: Greenland mass balance and its components Surface Mass Balance (SMB) and Discharge (D). Before 1996, D and hence SMB - D, are poorly constrained and therefore not shown.

What caused Surface Mass Balance to fall? There are three components to Surface Mass Balance: precipitation (rain & snow), sublimation and runoff (primarily ice melting with the water running off into the ocean). Figure 3 shows the trends for each component along with the resultant Surface Mass Balance in blue. Again, note that while run-off (orange) is displayed as a strongly positive trend, this actually removes ice mass. On the other hand, precipitation adds ice mass through rain and snow. Between 1996 and 2004, runoff and precipitation anomalies both increase simultaneously, roughly cancelling each other out so that Surface Mass Balance remains relatively steady. After 2004, precipitation levels out while runoff remains high. This caused Surface Mass Balance to fall sharply.


Figure 3: Surface Mass Balance (blue) and its components precipitation (red), runoff (orange) and sublimation (green).

So Surface Mass Balance reconstructions tell us that the acceleration of Greenland ice mass loss is largely due to increased ice discharge and increased run-off. In other words, glaciers are moving faster into the ocean and more ice is melting. Currently, Greenland ice loss is contributing 0.74 mm of sea level rise per year. This is around 23% of the total sea level rise (3.1 mm per year).

Thanks to Ernst Schrama (co-author of the paper), John Cross, Chip Fletcher and chris who all let me know about this paper within a few hours of each other. Obviously I was meant to write a post about this paper!

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Comments

Comments 1 to 24:

  1. Greenland's climate is strongly influenced by the NAO; during periods when this is positive, Greenland and Canada experience colder winters but the N America east coast has milder winters.
    Conversely, when NAO is negative, Greenland has milder winters and eastern US colder ones. Europe and the mediterranean climates are also affected.

    The graph at http://www.cpc.noaa.gov/products/precip/CWlink/pna/nao.timeseries.gif
    (covering 1950 - 2009) shows more negative phases 1950-1971/2, more positive phases from 1971/2 to 1995 , more positive 1995-2005 and more negative 1995- 2009.
    Eyeballing the graph it appears the phase change is increasing in frequency and intensity.
    Since 2005 the NAO has been mostly, and strongly, negative, thus giving Greenland milder ( relatively!!) winters with increased precipitation.
    As the NAO is driven by permanent pressure differential between the Azores HP zone and the Icelandic LP zone, I would expect that differential to increase as the tropics warm in advance of the higher latitudes, so favouring negative phases, thereby accelerating ice loss at the margins.
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  2. With all that has been said about how much ice is melting, I have not seen any mention of how this might cool the oceans. If the volume of ice is so significant, the energy absorption it brings with it should also have some impact (at least in the short term), and thus to some degree provide a cooling feedback mechanism. I can understand how this topic might be avoided as it would not help gloomanddoomer positions.
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  3. RSVP: looking at the Ice Mass Loss graph above and doing some quick calculations I get an average of about 222 Gt a year of ice loss over the past few years (when loss is the most). If you don't mind my long hand math, this works out to 222,000,000,000 tonnes per year of ice melted.

    Now, the latent heat of fusion of ice is about 334 J/gram or (again I apologize for the notation) 334,000 J/kg = 334,000,000 J/ton. we multiply this by our ice loss from above and get an annual heat required to melt the ice of 222,000,000,000*334,000,000=74*10^(18) J.

    Going to a previous post by John we see that the increase in heat content of the ocean is about 18*10^(21) J/year.

    Looking at the ratio it requires about 1/250 of the heat accumulating in the oceans to melt the ice. Nothing more than a drop in the bucket ;)

    Regards,
    John
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  4. 3# "Going to a previous post by John we see that the increase in heat content of the ocean is about 18*10^(21) J/year."

    Yes, but not all round Greenland, whereas the ice melt is.
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  5. A more often put forward idea is that freshwater from the melted ice sits on top of the ocean water, stratifying it more, and it refreezes more readily. This may also be a bit of a negative feedback mechanism. Where did I read about it? http://www.skepticalscience.com/Why-is-Antarctic-sea-ice-increasing.html
    RSVP, I think the "gloomandoomer" comment was uncalled for.
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    Response: Note that the stratification in the Southern Ocean may be unique to the region. When first researching increasing Antarctic sea ice, I encountered Zhang 2007 who explained that the warming Southern Ocean paradoxically could lead to less warm water upwelling to melt sea ice. I wondered why a similar effect didn't occur with Arctic sea ice which is sharply falling and contacted the author about it. He informed me that the stratification of water in the Arctic is different to the Antarctic, hence sea ice reacts in a different way to warming waters.

    Agreed re RSVP's comment - I would've deleted it due to the unconstructive tone but there were several interesting comments posted in response which I didn't want to waste (John Cross went to so much effort with his sums) :-)
  6. The Labrador sea, for example, is warming. We can safely conclude that the influx of cold water from the extra melting of the Greenland ice sheet is not the dominant effect.

    It should be expected, indeed. The Labrador sea is pretty cold and adding water only a little colder does not change much. Also, it should be compared to the concomitant melting of the winter ice of the whole region; it's just a tiny fraction.

    Need to put "significant melting" into context.

    What is true is that it's not included in any of the current GCM models, given that to my knowledge none of them predicted this fast melting.

    What matters, instead, is the different salinity which is the driver of deep circulation. Very much like the whole arctic ocean, the Labrador sea is more salinity stratified than temperature stratified, being the temperature gradient small.
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  7. reply to John Cross
    Isnt it oversimplifying (and a bit unfair) to compare heat of ice melt in localized northern seas to that which is distributed throughout the entire planet's oceans? The energy can only cancel when these waters mix. This doesnt happen instantaneously, and less due to stratification. From any and all references Ive seen on the subject of Artic currents, if anything cold water seems to move outwardly away from poles. And these actually are changing over time as well.

    Ref example, "Arctic ocean currents shown to affect polar climate more than global warming" http://www.tgdaily.com/trendwatch-features/34866-arctic-ocean-currents-shown-to-affect-polar-climate-more-than-global-warming
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  8. RSVP: to me it appears that you are trying to move your goalposts now. You initial post was obviously in regards to the globe and said nothing about the local temperature. I agree, it may have an impact on the local temperature (I would calculate the area of the sea around Greenland and compare it to the area of the whole ocean, but John Cook might make fun of my sums again ;-) ).

    However my main point was that no one is avoiding anything.

    Regards,
    John
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  9. John Cross
    After re-reading what I wrote, I can see why you thought this, since I was not more specific... sorry about that. My mistake. I can assure you that that was not my intent. In any case, I did like your approach.
    I am also aware that my tone as pointed out by others went over the line and I will try hard not to let this happen again.
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  10. RSVP: In my opinion, no harm no foul.

    In regards to my calculations above, while it is probably moot, I will note that in fact, a portion of the melting would actually take place out of the water with the phase change being caused by above zero ambient temperatures (for the short while this actually happens) and what sunshine they get up there.

    I took a quick look to see if I could get a number for sunshine hours, but quickly realized it wouldn't give me what I needed anyway so I gave up.

    Regards
    John
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  11. One more number to put the extra inflow into context. The 400 GTon/yr correspond to 1.3*10^4 m3/s which in turn translates into 0.013 Sv. Both the Labrador current and the east Greenland current are several Sv.
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  12. @RSVP: The mixing of a fresh water pulse is described in a paper written by Detlef Stammer last year in JGR. To summarize, it takes a while before it is mixed and it mixes globally, not locally at suggested here. We are talking about time scales of 100 years or so.
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  13. Your figure 2 seems to indicate that ice-mass loss in Greenland seemed to accelerate around 2000. This is also around the time global atmospheric temperatures seemed to flat-line. Isn't it classic thermodynamics, that the temperature of a system won't increase while a material is changing phase? Is it possible that the 'acceleration' of ice-mass loss from Greenland, and perhaps other ice-areas of earth as well, is responsible for the perceived deceleration in global temperature increase seen in the last 10 years?
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    Response:

    The amount of energy that goes into ice melt is fairly small compared to the amount of energy being absorbed by the oceans. In the figure below, all the energy gone into ice melt is included in the red "Land + Atmosphere" segment:



    Note that the oceans are still absorbing massive amounts of energy even during recent years when surface temperatures have either flattened or shown short term cooling.

  14. What if the ice-atmosphere system is more highly coupled than the ice-ocean system? Suppose we could assume the ocean was off doing its own thing, and that melting ice was mostly absorbing heat coming from the atmosphere? Perhaps then the ice melt could explain the (admittedly small) slowdown in air warming experienced in the last decade.
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  15. ubrew12,
    it is well know that in the arctic summer air temperature cannot be much higher than the melting point of ice; most of the heat goes to latent heat of melting thus limiting the increase in temperature. On the contrary, temperature fluctuations in winter are much larger because ice insulates the atmsophere from the warmer ocean.
    This is to say that atmosphere and ice are indeed strongly coupled.
    Ice and ocean couplig, instead, appears to be particularly relevant when the latter melts the floating ice shelves along the grounding line, speeding up the ice stream feeding the ice shelf.
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  16. Why is there an offset of about 500 Gt between GRACE and van den Broeke et al 2009? To my unskilled view it would suggest that van den Broeke 2009 confirms the GRACE trend but not the amount of ice loss(?). Or is it shown this way just for clarity? Thanks!

    John, your great site is improving one post to the other. I think this is currently the best online resource on climate change for the lay public. Congrats and thank you!
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  17. PeterPan,
    if you are referring to fig.1 in this post, it is stated in the caption "GRACE data is offset vertically".
    But anyway, if you look at anomalies the offset does not matter.
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  18. Thanks, Riccardo. Yes, I was referring to fig. 1 in this post. It seems that my English is worse than I thought :) I had read that sentence, but that sounds merely descriptive to me, kind of "there is a vertical difference compared to GRACE"... I'll check my dictionary when I'm back home this evening ;-)

    "If you look at anomalies the offset does not matter", unless those anomalies refer to the same reference period, isn't it? (eg. mass annomaly with reference to the 1980-1990 average)

    I realize I lack some education on the matter and this is just a small non-important bit in the whole post, so there's no problem if you don't feel like addressing this ;-). Thanks!
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  19. PeterPan,
    my mother language is not english, so i might be wrong as well ;)

    If a have that bit of knowledge, I'm always happy to answer questions from people willing to understand.

    You are right, it is the reference period that must be the same. But then you are left with just the linear trend. In other words, the fact that the GRACE anomaly is lower would indicate just a different reference period with no other physics involved. This is the reason why people usually do not bother to specify it and just shift one curve for visual clarity.
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  20. Ok! I think I realize now that we are just interested in how much ice is being lost, and this is just the trend, not the absolute values. The paper itself is even clearer: "GRACE values are not absolute numbers, and the curve has been vertically shifted for clarity".

    The correlation coefficient is 0.99! So it really is a strong independent confirmation of the GRACE-derived estimates (that had recently been questioned).

    *Now I see that your nickname seems Italian, so I guess we may be able to understand each other in any of our mother languages (I'm Spanish ;-) ) (though it would be very bad-mannered in an English forum, of course ;-) )

    Thanks again for your answers. Cheers!
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  21. As shown in the other Greenland post in this site, the best-fit curve of total ice mass loss from GRACE shows that Greenland ice loss is accelerating at a rate of 30 Gigatonnes/yr^2.

    But now results that we have the contributions:

    Ice Discharge: -94 Gt/yr (39,5%)
    Surface Mass Balance: -144 Gt/yr (60,5%)

    So most of ice loss comes from just surface melting!

    This is surprising because surface-melt minus surface-precipitation is something that is very weather-sensitive.

    Now I ask:

    1. How could a weather-sensitive melting follow a quadratic function so closely (i.e. how could the acceleration be so close to a constant value of 30 Gigatonnes/yr^2)?

    2. Can we expect this trend to persist or weather-climate variability will "break" the soft curve here shown at any time?
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  22. The paper states:
    "Our results show that both mass balance components,
    SMB and D (eq. S1), contributed equally to the
    post-1996 cumulative GrIS mass loss (Fig. 2A)."

    But then, Fig.3 shows:

    Ice Discharge: -94 Gt/yr
    Surface Mass Balance: -144 Gt/yr

    Isn't this a contradiction?

    Then comes this statement: "A quadratic decrease (r^2 = 0.97) explains the2000–2008 cumulative mass anomaly better thana linear fit (r^2 = 0.90). Equation S1 implies thatwhen SMB-D is negative but constant in time,
    ice sheet mass will decrease linearly in time. If, however, SMB-D decreases linearly in time, ashas been approximately the case since 2000 (fig.S3), ice sheet mass is indeed expected to decrease quadratically in time"

    What is this "r^2 = 0.97" and how it is related to the equations:

    MB = ∂M/∂t = SMB – D (S1)
    δM = ∫dt (SMB-D) = t (SMB0–D0) + ∫dt (δSMB–δD) (S4)

    Any idea?
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  23. Where did Greenland get its name from? Could have it been "green" at some time when Vikings roamed?

    Is the ice thinning? I have seen evidence that the warming and cooling, the thickening and thinning of the Arctic (readings from the 80th parallel) has remained remarkably consistent over the past century. Naturally there are deviations from the mean but it shows that it has always thinned and thickened at roughly the same time and rate each year.
    http://ocean.dmi.dk/arctic/meant80n.uk.php
    http://www.ijis.iarc.uaf.edu/en/home/seaice_extent.htm

    Best Regards,
    Royce R. Vines

    Unintended consequences rule when busybodies get on their "high horses" - Emmett
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  24. Stop the presses, this just in:
    Danish research scientist Sebastian Mernild of Los Alamos National Laboratory in the US told national daily newspaper Jyllands-Posten that his calculations show that 540 cubic kilometres of inland ice, weighing approx. 500 gigatons, have melted this summer, which is 25-50% more than in a typical year.
    According to Jyllands-Posten, climate researcher Jason E. Box from Byrd Polar Research Center at Ohio State University, USA, is also saying that the inland ice melting has been particular strong this year:
    "It is my assessment that we have had the strongest melting since they started measuring the temperature in Greenland in 1873."
    Let the good times roll.

    The Yooper
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