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A basic overview of Antarctic ice

Posted on 29 November 2010 by robert way

Skeptic arguments that Antarctica is gaining ice frequently hinge on an error of omission, namely ignoring the difference between land ice and sea ice.

In glaciology and particularly with respect to Antarctic ice, not all things are created equal. Let us consider the following differences. Antarctic land ice is the ice which has accumulated over thousands of years on the Antarctica landmass itself through snowfall. This land ice therefore is actually stored ocean water that once fell as precipitation. Sea ice in Antarctica is quite different as it is generally considered to be ice which forms in salt water primarily during the winter months.

In Antarctica, sea ice grows quite extensively during winter but nearly completely melts away during the summer (Figure 1). That is where the important difference between antarctic and arctic sea ice exists. Arctic sea ice lasts all the year round, there are increases during the winter months and decreases during the summer months but an ice cover does in fact remain in the North which includes quite a bit of ice from previous years (Figure 1). Essentially Arctic sea ice is more important for the earth's energy balance because when it melts, more sunlight is absorbed by the oceans whereas Antarctic sea ice normally melts each summer leaving the earth's energy balance largely unchanged.

Figure 1: Coverage of sea ice in both the Arctic (Top) and Antarctica (Bottom) for both summer minimums and winter maximums
Source: National Snow and Ice Data Center

One must also be careful how you interpret trends in Antarctic sea ice. Currently this ice is increasing and has been for years but is this the smoking gun against climate change? Not quite. Antarctic sea ice is gaining because of many different reasons but the most accepted recent explanations are listed below:

i) Ozone levels over Antarctica have dropped causing stratospheric cooling and increasing winds which lead to more areas of open water that can be frozen (Gillet 2003, Thompson 2002, Turner 2009).

and

ii) The Southern Ocean is freshening because of increased rain, glacial run-off and snowfall. This changes the composition of the different layers in the ocean there causing less mixing between warm and cold layers and thus less melted sea ice (Zhang 2007).

All the sea ice talk aside, it is quite clear that really when it comes to Antarctic ice, sea ice is not the most important thing to measure. In Antarctica, the most important ice mass is the land ice sitting on the West Antarctic Ice Sheet and the East Antarctic Ice Sheet.

Therefore, how is Antarctic Land-ice doing?

Figure 2: Estimates of Total Antarctic Land Ice Changes and approximate sea level contributions using many different measurement techniques. Adapted from The Copenhagen Diagnosis. (CH= Chen et al. 2006, WH= Wingham et al. 2006, R= Rignot et al. 2008b, CZ= Cazenave et al. 2009 and V=Velicogna 2009)

Estimates of recent changes in Antarctic land ice (Figure 2) range from losing 100 Gt/year to over 300 Gt/year. Because 360 Gt/year represents an annual sea level rise of 1 mm/year, recent estimates indicate a contribution of between 0.27 mm/year and 0.83 mm/year coming from Antarctica. There is of course uncertainty in the estimations methods but multiple different types of measurement techniques (explained here) all show the same thing, Antarctica is losing land ice as a whole, and these losses are accelerating quickly.

NOTE: This post is the Basic Version (written by Robert Way) of the skeptic argument "Antarctica is gaining ice". This post means we now have basic rebuttals for all of the top ten skeptic arguments. More impressively, the prolific Skeptical Science authors have written basic rebuttals to 65 skeptic arguments in total. Go team!

Lastly, just a reminder that Robert Way has also written a comprehensive post on the measurement of land ice loss from Antarctica which puts the recent study by Wu et al into perspective (anyone wishing to cite the Wu study is recommended to read Robert's post first).

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Comments 101 to 108 out of 108:

  1. There is no such thing as combined polar ice, because these are asynchronous occurences. The evolution over time of the true global sea ice coverage is what matters.
    And can you elaborate on the year round Arctic albedo idea?
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  2. Even using your artificial ways of looking at the data, there is more than just significance. A significant negative change means just that it is statistically signficant. If there is a negative change that is also significant but much larger, the difference between the two will be a negative change that will still be statistically significant. Considering how large the difference is between the Arctic loss and the Antarctic "gain" it is quite easy to see that the difference will inevitably be a statistically significant loss.
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  3. Philippe Chantreau at 03:45 AM on 7 December, 2010
    "There is no such thing as combined polar ice, because these are asynchronous occurences. The evolution over time of the true global sea ice coverage is what matters.
    And can you elaborate on the year round Arctic albedo idea?"

    So what is your point? Global sea ice coverage is global sea ice coverage. The fact they are asynchronous doesn't seem to be making any point.
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  4. # 103
    Global sea ice is declining at a statistical significant rate. My results from fitting a linear trend to the data from NSIDC:
    Global sea ice area, linear trend: -0.0334 ± 0.0129
    Global sea ice area, linear trend: -0.0367 ± 0.0112
    Units are mio. square km pr year, the trend are given with AR1 corrected 95% confidence intervals.

    For more info check my post earlier in this thread
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  5. Re: 104:
    Typo error, the second trend estimate is for extent not area
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  6. Thanks for clarifying that SRJ. Bill, the fact that the maximum extents for each pole are asynchronous means that any calculation putting the 2 together does not correspond to any physical reality. When looking at global coverage, there is a certain amount of coverage at any given time. That amount is the physical reality and is what should be studied.

    As for Arctic albedo, it is obviously negligible at and above the polar circle for any time between the fall and spring equinoxes. One reason why the decrease in Arctic sea ice is significant is because the lower summer coverage allows for large amounts of energy to be absorbed by the ocean.
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  7. Has Antarctic Ice been similiar in the recent past? Yes it has, and in fact has prob been lower.

    http://geology.gsapubs.org/content/38/7/635.abstract?sid=0f422966-63d5-4963-895f-b8a3935beb2d
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  8. Re #107,

    So on the same thread we have "skeptics" arguing that the (statistically insignificant) increase in Antarctic sea ice means that we have nothing to worry about concerning the dramatic loss of Arctic ice. Other "skeptics" seem to be suggesting that the statistically insignificant increase in Antarctic sea ice runs contrary to the theory of AGW-- which it does not.

    And then, lo and behold, we have someone at #107 claiming that Antarctic ice extent was less in the past-- not clear from the abstract whether or not they also include sea ice. The mind continues to boggle at the inconsistency and incoherence of "skeptic" arguments. This one can be filed under, "climate has changed in the past" strawman.
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