Arguments
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!
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.
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
Yes, but not all round Greenland, whereas the ice melt is.
RSVP, I think the "gloomandoomer" comment was uncalled for.
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) :-)
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.
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
However my main point was that no one is avoiding anything.
Regards,
John
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.
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
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.
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.
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!
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.
"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!
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.
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!
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?
"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?
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
The Yooper