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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Why do glaciers lose ice?

What the science says...

Antarctica is losing ice because its glaciers are speeding up. This is due to melt water lubricating the base of the glaciers and the removal of ice shelves which act as a "speed bump" slowing the glacier flow. The ice shelves are thinning due to warming ocean waters.

Climate Myth...

Antarctica is too cold to lose ice
"The real issue is absolute temperatures. Some of the regions in which GRACE claims ice loss in East Antarctica average colder than -30°C during the summer, and never, ever get above freezing. How can you melt ice at those temperatures?" (Steve Goddard)

Glaciers are large viscous masses of ice which creep naturally through a process called internal deformation. This “creep” or movement is caused by gravity and the weight of accumulated snow and ice forcing the ice to deform like plastic.


(Figure 1: Midtsdalsbreen, Southern Norway, Personal Photograph)

Glaciers gain mass through accumulation of snowfall and through re-freezing of meltwater but lose mass (termed ablation) through surface melt, basal melt, sublimation and iceberg calving (Figure 2, right). The accumulation of ice primarily occurs in the glacier’s accumulation zone and ice loss generally originates in the glacier’s ablation zone (Figure 2, left).


(Figure2, accumulation/ablation zones, www.physicalgeography.net, image of calving glacier on right)

For an ideal glacier, ice flow through a cross-section must exactly balance the accumulation and ablation taking place (Benn and Evans, 1998, 142). The difference between the total gains and losses measured over a specified time refers to the mass balance. Mass balance is usually measured over the course of a year which computes the sum of all the annual accumulation and ablation (Benn and Evans, 1998, 75). The velocity at which a glacier moves whereby its Mass Balance is 0 represents the point at which its inputs (through accumulation) equals its outputs (through ablation) and is termed the Balance Velocity (Figure 3).


Figure 3: (Left) Balance Velocities for Antarctica as illustrated in Bamber et al (2009). (Right) Actual velocities across the ice sheet as measured by Rignot and Thomas (2002).

As every individual basin is rarely in balance, the actual velocities of glaciers/ice streams across Antarctica shows that many glaciers have velocities in excess of their balance velocity and many are less than their balance velocities (Figure 3, right).

The question of balance velocities brings us to one of the most important points of this post. When a glacier is in balance or flowing at its balance velocity, net mass will remain balanced. However, when a glacier accelerates while near or at its balance velocity, the outputs resultantly increase but the inputs do not, thereby shifting the glacier regime to one of negative mass balance or net ice loss. This situation is particularly important because accelerated ice flow is the key method through which the Antarctic ice sheets incur a net ice loss. Accelerations such as these occur through two primary mechanisms. The first of which is caused by surface melt water reaching the glacial bed causing basal lubrication therefore reducing the frictional forces at the bed and thus increasing ice flow (Bell 2008).

The second mechanism refers to when the forces at the downstream terminus of a glacier or ice stream are disturbed or altered. This can occur through removing buttressing ice shelves or by shifting the glacier’s grounding line (point where glacier ice reaches floatation). The presence of an ice shelf provides a longitudinal compressive force which slows the flow of ice streams. If removal of this compressive force occurs, velocity of ice streams increase. This has been observed directly by Scambos et al (2004) and Rignot et al (2004) through both visual observations (Scambos) and radar interferometry (Rignot).

In terms of a grounding line retreat, an inland shift of the grounding line causes less backpressure through increased calving and basal melting. This process results in increased glacier velocities and subsequent inland thinning as more ice is being pulled from the accumulation zone (Bell 2008). In a warmer climate, one would expect that surface melting would increase, making the first mechanism more likely, however because of Antarctica’s climate and the omnipresence of ice shelves and calving glaciers there, the second mechanism actually dictates the ice losses from Antarctica. Evidence has already been presented which supports the theory that it is warm ocean water in West Antarctica which is in actuality enabling this second mechanism (Shepherd, Wingham and Rignot, 2002).

We should all now at least remotely understand that mass balance changes in Antarctica aren’t reliant on surface melting but rather depend on dynamic responses such as the 2nd mechanism.

Last updated on 14 July 2010 by robert way.

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Comments

Comments 1 to 9:

  1. Forget ice slides and ice shields, that has been happening for millions of years. The question is: Is the Antarctic getting larger because of the cooling that is taking place in this region of the world? Is ice getting larger in this area?
  2. To answer Barney's question, no. Antarctica is experiencing a net loss of land ice. There is also a slight increase in Antarctic sea ice, but that's a different subject (and it's not caused by "cooling" in any case, since the southern ocean is actually warming).

    See Is Antarctica losing or gaining ice?
  3. barney2022, the volume of Antarctic ice is decreasing. Obviously that is the 'most important' issue overall, but the article above explaining WHY the ice volume is now in decline is certainly also relevant... despite involving processes which have existed for a very long time. Also, you say "cooling that is taking place in this region of the world"... actually, Antarctica has warmed considerably in the past few decades.
  4. You can see the mass balance for antarctica at the following link

    http://www.skepticalscience.com/Part-Three-Response-to-Goddard.html
  5. Chapter 10 Figure 10.7 in the IPCC's AR4 shows negative numbers across the board for the contribution of the Antarctic Ice sheet to Sea level rise which would indicate that Antarctica is gaining ice.

    http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch10s10-6-5.html

    All the other numbers in table 10.7 are positive.
  6. 5, Steve,

    I'm not sure what your point is, but it's important to read everything, and not just stop when you find something you like. That's not what I'd call "skeptical."

    I have a lot more research to do, but after just a few minutes work...

    First, remember the report was released in 2007, meaning it was based on data compiled before then. In particular, however, the section you linked to includes these words:
    Further accelerations in ice flow of the kind recently observed in some Greenland outlet glaciers and West Antarctic ice streams could increase the ice sheet contributions substantially, but quantitative projections cannot be made with confidence (see Section 10.6.4.2).


    Just a year later, a study found that ice lose in Antarctica was in fact accelerating:

    Antarctic is losing ice ‘nearly twice as fast as ten years ago’

    In addition, I don't know what GRACE data was available at the time. The satellite was only launched in 2002, and some years would be needed to identify a trend -- a very serious and bad trend, it turned out. From NASA's GRACE pages:
    Gravity data collected from space using NASA's Grace satellite show that Antarctica has been losing more than a hundred cubic kilometers (24 cubic miles) of ice each year since 2002. The latest data reveal that Antarctica is losing ice at an accelerating rate, too.


    At the same time, the numbers in the IPCC AR4 report do not seem to have used GRACE data at all (although they could have). They were instead based on models that apparently predicted mass gain in the Antarctic (if we are interpreting that table correctly).

    But it was based on model runs, and so limited by assumptions made in constructing the models. It would appear that the models then did not take into account what has since been discovered, and explained fairly clearly in the above post.

    Appendix 10.A: Methods for Sea Level Projections for the 21st Century
  7. 5, Steve,

    Just to highlight the obvious to anyone, in case it's not... what all of this means is that the IPCC AR4 underestimated sea level rise, because it counted on a negative rather than a positive contribution from Antarctica.

    This means that things are, once again, worse than expected, and the IPCC was too conservative (= non-alarmist) in its projections.
  8. Based on the post, I get the impression that the causal connection between (CO2 driven) rising global temperature and the decline in Antarctic land ice can be summerazised as follows:
    1) CO2 causes an increase in surface air temperature
    2) From 1) an increase in ocean temperatures follow
    3) The warmer ocean causes ice shelves at the terminus of the glaciers to thinnen, making them lighter and thus given less resistance to the ice flow of the glaciers
    4) From 3) an increase in glacier velocity occurs and this removes more ice from the accumulation zone, the result being that the glacier shrinks.

    Is this an accurate summary of how more CO2 is leading to less antarctic land ice? Suggestions and corrections are more than welcome
  9. Regarding my post #8
    I am most interested in the first 2 points, I think that point 3 and 4 are well explained in Shepherd, Wingham and Rignot, 2002.

    Assumming my points 3 and 4 are correct my question then is if this is correct:
    1) CO2 causes an increase in surface air temperature
    2) From 1) an increase in ocean temperatures follow, specifially ocean currents at thee Western antarctic

    And points 3 and 4 as above

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