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Arctic Sea Ice: Why Do Skeptics Think in Only Two Dimensions?

August 24, 2010

Has Arctic sea ice returned to normal?

Discussions about the amount of sea ice in the Arctic often confuse two very different measures of how much ice there is. One measure is sea-ice extent which, as the name implies, is a measure of coverage of the ocean where ice covers 15% or more of the surface. It is a two-dimensional measurement; extent does not tell us how thick the ice is. The other measure of Arctic ice, using all three dimensions, is volume, the measure of how much ice there really is.

Sea-ice consists of first-year ice, which is thin, and older ice which has accumulated volume, called multi-year ice. Multi-year ice is very important because it comprises most of the volume of ice at the North Pole. Volume is also the important measure when it comes to climate change, because it is the volume of the ice – the sheer amount of the stuff – that science is concerned about, rather than how much of the sea is covered in a thin layer of ice*.

Over time, sea ice reflects the fast-changing circumstances of weather. It is driven principally by changes in surface temperature, forming and melting according to the seasons, the winds, cloud cover and ocean currents. In 2010, for example, sea ice extent recovered dramatically in March, only to melt again by May.
Sea-ice is subject to powerful short-term effects so while we can’t conclude anything about the health of the ice from just a few years’ data, an obvious trend emerges over the space of a decade or more, showing a decrease of about 5% of average sea-ice cover per decade.


Source: Rayner, 2004, updated

Where has the thick ice gone?

When we consider the multi-year ice and look at the various measurements of it, we see a steep decline in this thick ice. As you might imagine, thick ice takes a lot more heat to melt, so the fact that it is disappearing so fast is of great concern.

Source: Polar Science Centre, University of Washington

It is clear from the various data sets, terrestrial and satellite, that both the sea ice extent and multi-year ice volume are reducing. Sea ice extent recovered slightly during the Arctic winters of 2008-09, but the full extent of annual ice reduction or gain is seen in September of each year, at the end of the Arctic summer. The volume of multi-year ice has not recovered at all, and is showing a steeply negative trend.

* Footnote: Although a thin layer of ice doesn’t tell us much about the overall state of ice loss at the Arctic, it does tell us a great deal about Albedo, the property of ice to reflect heat back into space. When the sea ice diminishes, more heat passes into the oceans. That heat melts the thick ice and speeds up the melting of thinner sea ice, which in turns allows more heat to accumulate in the oceans. This is an example of a positive feedback.

This post was written for SkepticalScience as part of an ongoing project to add ‘basic’ rebuttals of common climate change denial arguments. References for all statements can be found in the intermediate discussion on Skepticalscience.

8 Comments leave one →
  1. August 25, 2010 4:24 pm

    Obviously, my critical comments on SkSci were based on the misunderstanding explained there about getting intermediate and basic posts mixed up.

    This is very good.

  2. Graham Wayne permalink*
    August 25, 2010 5:18 pm

    Cheers Byron – it gave me a weird moment, reading the post on SkS and thinking “I don’t remember writing this…” 🙂

  3. elsa nasser permalink
    August 25, 2010 5:32 pm

    And can we have the same charts for the Antarctic please?

  4. adelady permalink
    August 26, 2010 2:07 pm

    elsa, I’m not sure that this kind of graph is all that useful or interesting for the Antarctic.

    I’ve been indulging in a bit of car-crash voyeurism with the Arctic ice melting these last few weeks. I gather that sea ice in the Antarctic just doesn’t have multi-year accumulation in the same way as the Arctic. The better northern versus southern hemisphere comparison is between Greenland and the Antarctic. Any big areas of thick ice in the Antarctic is almost guaranteed to be from glacier calving rather than from sea ice breaking up.

    The interesting graphs for the Antarctic are those showing the rate of loss of ice mass on land.

  5. Graham Wayne permalink*
    August 26, 2010 3:06 pm

    I’m just trying to figure out if I can post graphs in the comments…

    Figure 2: Ice mass changes for the Antarctic ice sheet from April 2002 to February 2009. Unfiltered data are blue crosses. Data filtered for the seasonal dependence are red crosses. The best-fitting quadratic trend is shown as the green line (Velicogna 2009).

    Apparently I can -but it’s tedious. So Elsa, perhaps you could do your own research instead of expecting other people to find information for you? I have to work for a living too, you know…

  6. elsa nasser permalink
    August 27, 2010 10:11 am

    But it only starts at 2003.

  7. Graham Wayne permalink*
    August 27, 2010 12:13 pm

    Ok Elsa, now you go and find one that goes as far back as you want…like I said, do your own research.

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