RealClimate logo

Under and over the ice

Filed under: — gavin @ 10 March 2011

I really like the fact that there is still so much to discover about important parts of the climate system. The Bell et al paper in Science Express this week (final version in Science) reporting on the surprising results from airborne ground-penetrating radar studies of the Antarctic Ice Sheet is a great example. The ice sheets themselves are the biggest challenge for climate modelling since we don’t have direct evidence of the many of the key processes that occur at the ice sheet base (for obvious reasons), nor even of what the topography or conditions are at the base itself. And of course, the future fate of the ice sheets and how they will dynamically respond to climate warming is hugely important for projections of sea level rise and polar hydrology. The fact that ice sheets will respond to warming is not in doubt (note the 4-6 m sea level rise during the last interglacial), but the speed at which that might happen is highly uncertain, though the other story this week shows it is ongoing.

The radar results (shown on the right) picked up on some really weird looking features that look to be related to pressure-related freezing of basal meltwater as it is pushed uphill by the weight of the ice sheet above. If that sounds odd, it is because it is.

How can water flow uphill in the first place? This is a function of the pressure gradients. If there is a lot of ice above a valley, but it tapers off towards a mountain range, the pressure on any liquid water at the bottom of the valley will be greater than the pressure up the side of the mountain. This will force water uphill. Incidentally, there are many sub-glacial geomorphological features that show this effect in places affected by the LGM ice sheets.

The freezing point of water is also pressure dependent. With 3km of ice pressure above it, water freezes at about -2ºC (the change is -7.5*10-8 ºC/Pa). Water below 0ºC can therefore exist at the base of the ice sheet (and can also be seen emerging from under ice shelves). When the pressure forces this water upwards to lower pressure areas, this can promote instant freezing, and this seems to be the explanation for the structures seen in the radar.

The surprise is how large these structures are, one shown in the paper is 10’s of km long and 100s of meters thick – certainly large enough to be important in ice flow locally, though probably not at the continental scale.

However, at the continental scale, there is a new assessment of the net mass balance of Antarctica and Greenland. Rignot et al have updated results, including those from the GRACE gravity measurement satellite, to the end of 2010 and show that the downward trend in ice mass is continuing (stronger in Greenland than in Antarctica). The net rise in sea level associated with this decline is about 1.3 mm/yr, which will likely accelerate with further warming. Complementary analyses of the surface mass balance of Greenland (Tedesco et al, 2011) also show that 2010 was a record year for melt area extent.

This rate of melting is more than was figured into the tabulated IPCC AR4 estimates of sea level rise, and any further acceleration will obviously make the discrepancy worse. Indeed, even in the highest forcing A1F1 scenario, the IPCC calculated only a 0.3 mm/year contribution from the ice sheets averaged over the whole 21st Century! This was clearly a gross underestimate.

Extrapolating these melt rates forward to 2050, “the cumulative loss could raise sea level by 15 cm by 2050” for a total of 32 cm (adding in 8 cm from glacial ice caps and 9 cm from thermal expansion) – a number very close to the best estimate of Vermeer & Rahmstorf (2009), derived by linking the observed rate of sea level rise to the observed warming.

There is certainly more to learn about ice sheets, and more of a reason than ever to do that as fast as possible.

119 Responses to “Under and over the ice”

  1. 101
    Dan H. says:

    First off I do not that kind of money with which to gamble. Secondly, these are opinions of which I firmly believe, and I support with facts.
    I do not know where you are getting your numbers, but the recent ice minimum is still over 50% of what it was in the 1980s. At the current rate, sea will remain for at least another 30 years, not to mention that the remaining ice is located further northward, and in even colder waters than that which has declined over the past 30 years.
    I did not cherry-pick anything, as you are obviously trying to do with your ridiculous statement about disappearing sea ice by 2016. No data supports that statement. My choice of years were examples to show that winter sea ice maximum does not necessarily correlated with summer minima.
    I am not in denial about anything. I have a real beef with people using short-term data or single incidents to “prove” something. If we break the sea ice minimum this year, it will just be one more data point to establish the long-term trend. I am not “denying” the trend; simply pointing out that the trend over the last 30 years is -80,000 square km / yr. At that rate, it will take more than three years to melt 3 million square km.

  2. 102
    Rob says:

    Dan :
    “I am not “denying” the trend; simply pointing out that the trend over the last 30 years is -80,000 square km / yr. At that rate, it will take more than three years to melt 3 million square km”

    Is it just the timing of breaking the 2007 record, or the timing of achieving an ice free Arctic ocean that you are disputing ?

    Why do you think there is any Arctic sea ice downtrend at all ?

    As for betting: Since I already mentioned Intrade ($100 bets) twice now, your evasive answer to betting is revealing at multiple levels.

  3. 103

    Dan H,

    Your unwillingness to even address the question speaks volumes to the mindset of the denial crowd. It’s not about truth, or facts, or reason. It’s about a total and complete inability to deal with truth, facts and reason. Your mind just shuts down at even the hint of an outcome which is not in line with your predefined and desired conclusions, to the extent that you cannot even answer a hypothetical concerning possible future events.

    You can wander into the most complex train of rational thought, as long as it inexorably chugs towards the destination you’ve chosen. Nothing can derail it, not even that fact that the tracks ahead, let alone the destination, could be purely imaginary.

    You repeatedly simply could not bring yourself to answer a simple question which is at odds with your hardened world view.

    You really should seriously consider what this means.

  4. 104
    Dan H. says:

    Really Bob,
    I have no clue what you are talking about or what you think I believe. I suggest you talk a few minutes to calm down and check out the science surrounding the Arctic. As I have already pointed out, the sea ice is declining at about 80,000 km2/year, even though Rob above does not seem to accept that. Are you denying that also? The trend line is not projected to pass the 2007 low for about 8 years, why do think a single year might? As I seaid earlier, single year values are virtually meaningless in the long run, however they are used by alarmists and deniers to “prove” thier case. You appear to be among them. If the current trend were to continue, it indicates that the sea ice would disappear completely in about 75 years. Are you unable to accept that? Neither of you have shown anything that indicates otherwise, except for what the gamblers believe. Sorry, but I will stick with the science.

  5. 105
  6. 106

    #104–“The trend line is not projected to pass the 2007 low for about 8 years, why do think a single year might?”

    Again, you’re looking purely at extent. There’s also this thing called volume, remember? The volume data aren’t as good as we’d like–us ‘alarmists’ are really, really eager to get better numbers from Cryosat–but paying due attention to the third dimension is not “unscientific.”

    The most detailed attempts to model the Arctic sea ice decline show seasonally ice-free Arctic waters around 2030, IIRC–and the modeling has so far consistently under-predicted the decline. The suggestion that the ice cap has 75 years left is simply unrealistic.

    Even so, Dan, the Arctic ice is not declining as fast as your credibility.

  7. 107

    104, Dan,

    …it indicates that the sea ice would disappear completely in about 75 years. Are you unable to accept that?

    Why do you keep dodging the question? I proposed a hypothetical, asked what your reaction would be, and you repeatedly keep arguing about the sea ice. I’m not arguing about that. You can make any prediction you want, and so can I. I don’t care about predictions.

    The question I asked which you are so studiously avoiding, and which by your silence screams to the world the true character of deniers such as yourself, is:

    How would your position on global warming change if, contrary to your own predictions, the summer sea ice minimum in the Arctic continued to fall in the next three years, rather than to show signs of recovery?

    It’s a simple question. It has nothing to do with what will happen or is likely to happen. I don’t care about numbers or trends or this or that.

    I care about how you, as a person, will behave given a particular scenario.

    If, despite all of your logic and protestations to the contrary, sea ice in the Arctic continues to decline, will you re-evaluate your stance on global warming?

    Answer the question!

  8. 108
    Dan H. says:

    Once again, my position would not change. If the sea ice continues to decline at the current rate, there is nothing to change.
    If the sea ice minimum were to decline faster than recently observed, then I would rethink my position and conclude that warming is occurring faster than expected. Similarly, if the decline slows significantly, I would conclude the opposite.
    I simply look at the most recent data in line with the long term results and ask myself, is their reason to conclude that a change has occurred? Remember, my position is not that sea ice would not continue to fall, but rather that it would not break the 2007 minimum in three years.
    Does that satisfactorily answer your question?

  9. 109
    Didactylos says:

    “The trend line is not projected to pass the 2007 low for about 8 years, why do think a single year might?”

    That has to be the silliest thing I’ve read so far today. Really, it demonstrates a lot about why Dan is struggling with this whole debate.

    Okay, Dan. First, the decline is non-linear. Predicting what will happen is difficult. Second, there is considerable variability. Just as 2007 was substantially below the trend, 2011 or 2012 may be also.

    Now, you observe that single years are “meaningless” in the long run, in a failed attempt to turn around that argument we keep making to deniers when they lovingly cherry-pick some date they like. But you got it wrong. Single years aren’t meaningless. They are each one data point. And each year that falls substantially below the trend changes the trend. This is what we have observed. September 2007, 2008, 2009 and 2010 were all below the linear trend as computed by NSIDC.

    When we see this on the end of a 30 year downwards slope, it really isn’t hard to see the non-linearity. It also isn’t that hard to analyse just how non-linear it is.

  10. 110
    Hank Roberts says:

    > If the current trend were to continue
    You don’t even know if the current trend is linear.

    As Tamino has taken pains to point out many times, if simply fitting trend lines or curves to past performance were a useful way of anticipating future events, science would be simple–but as changing the physical world changes what happens, it isn’t.

  11. 111
    adelady says:

    Dan@ 108. I presume that you mean Arctic ice =extent= won’t decline below the 2007 minimum.

    If you’re talking about ice volume, it looks very much as though 2007 has already lost its place as the least ever Arctic ice volume.

  12. 112

    Dan H, 108,

    If the current trend were to continue…

    Okay, I’ll phrase it a different way. What if your gross and simplistic estimate of 80,000k2/yr turns out to be wrong? What if by 2020 (or even 2030) the Arctic sea ice minimum is zero?

    What will your position on global warming be then?

  13. 113
    flxible says:

    DanH links to a contrarian forum post that uses graphs from a denialator site called “hide the decline” to try to convince us that declining sea ice volume is irrelevant. Nice try Dan.

  14. 114
    Didactylos says:

    Dan H.: You are getting very tiresome. I’m sure we have been over all this with you before. PIPS is for submarine navigation, so it calculates the thickest ice in a region, not the average. This makes it largely irrelevant for calculating volume.

    2009 didn’t fall on the line back in 2009. But that trend line keeps moving. Next year 2009 may be above the line.

    You still don’t get why cherry-picking is bad, and why I’m not cherry-picking. You have all the data right there, but rather than look at it, you replace it with a single line! As I said, my claim isn’t based on eyeballing, but by looking at the actual analysis. Moreover, we don’t *expect* the ice loss to be linear.

    Okay, enough. Can you please go back to the bore hole now?

  15. 115

    113, Dan H.

    …it is not my calculation, but by the University of Colorado…

    That’s offensively disingenuous. You’re implying that the NSIDC and U. of Colorado agree with your logic of computing a linear trend based on 31 years of non-linear data, which is patently false. The graph in the post was produced by one D Kelly O’Day, the blogger at the site, as attributed on the graph itself, using NSIDC data. To claim that that is the position of the NSIDC because it made use of their data is absurd.

    How about being a little more obvious, or at least clever, in your arguments?

    116, Dan H

    …how would it impact your view?

    Being a logical and open minded person, I’d retreat 100% from my current position and completely rethink things, long, long before the Arctic returned to 1980s levels.

  16. 116
    Eddie says:


    When all the glaciers are melted away the water level will rise by 7 feet. It will occur more tsunamis and natural disasters.

  17. 117
    John E. Pearson says:

    Krugman is worth reading today.

  18. 118
    John E. Pearson says:

    Sorry if this is a double post: Krugman is good today.

    He talks about Muller’s testimony last week.

  19. 119
    Hank Roberts says:

    Aside for anyone new to the subject, “Eddie” (3 April 2011 1:59 pm) says “7 feet” — that’s one estimate for this century for melting isolated glaciers, not the longterm number. Look where sea level was the last time the polar icecaps weren’t there. Same amount of water in the world. but read the history, there are edit wars going on in climate topics most of the time.