Why do I punish myself?

An image from the GWPF Web SIte purporting to show that there was more sea in December 2013 than at any time in history. Can that really be true?

An image from the GWPF Web Site purporting to show that there was more sea-ice in December 2013 than at any time in history. Can that really be true? (Short Answer: No)

I ought to know better than to visit the web site of the Global Warming Policy Foundation. But I did: these things happen.

And there I saw the graph (above) which purports to show that there is currently (January 2014) more sea ice than there has ever been. The graph looks poorly plotted and it came with no explanation, but I thought that was a very strong claim so I just thought I would check if it was true.

So I popped over to NSIDC and downloaded the relevant data files for the Northern and Southern hemisphere sea-ice indices which record daily satellite measurements of sea-ice extent since 1978. The first graph below shows the data for the Northern and Southern hemisphere indices plotted together. This graph shows just the last few years of data.

Data from the US NSIDC showing Northern and Southern Hemisphere Sea Ice Extent since 2005.

Data from the US NSIDC showing Northern and Southern Hemisphere Sea Ice Extent since 2005. Click for a larger version

The graph below shows all the data taken nearly every day since 23 August 1978. On this graph I have also plotted the yearly averages and fitted a straight line to these to show the long-term trend.

Data from the US NSIDC showing Northern and Southern Hemisphere Sea Ice Extent since 2005.

Data from the US NSIDC showing Northern and Southern Hemisphere Sea Ice Extent since 1978. Click for a larger version.

This data highlights interesting differences between the two poles – these are discussed further on this excellent web page at NSIDC. You can see that both northern and southern hemispheres hold on average about the same amount of sea ice – around 12 million square kilometres (km2), but every year this grows and shrinks with the seasons.

  • In the southern hemisphere the sea ice falls to around 3 million kmin the antarctic summer and grows to around 18 million kmin the antarctic winter i.e. around 15 million kmof sea ice re-freezes every year.
  • In the northern hemisphere the sea ice falls to around 5 million kmin the arctic summer and grows to around 15 million kmin the arctic winter i.e. around 10 million kmof sea ice re-freezes every year.

So what of global sea ice? To find that I added the two data sets together and these are plotted below along with a yearly average. Because the extents of the antarctic and arctic freezes are different, globally the amount of sea-ice oscillates. However the annualised average global sea-ice extent is falling.

Data from US NSIDC showing global sea ice extent versus year. The trend is clearly downward by the difference between the two trends in the previous graphs.

Data from US NSIDC showing global sea ice extent versus year. The trend is downward by the difference between the two trends in the previous graphs.

And on the graph below I show how the extent of sea-ice in the northern and southern hemispheres are phased to yield a global maximum near the start of December.

Data from US NSIDC showing the northern hemisphere, southern hemisphere and global sea ice extent plotted versus the day of the year. The d

Data from US NSIDC showing the northern hemisphere, southern hemisphere and global sea ice extent plotted versus the day of the year. The data is same as that shown in the above two graphs but I have just ignored the ‘year’ label. It shows quite clearly the relative phasing of the sea-ice changes in the two hemispheres, and that December is the month in which global sea-ice extent is the largest.

So where does the data plotted by the GWPF come from? Well, I can’t tell for sure but I think it may simply be mislabelled. You can see that it is between 17 and 18 million kmwhereas the global sea-ice extent for December is around 25 million km2. So it might correspond to simply the Southern Hemisphere data which is indeed growing (slowly) and may well have reached its maximum value, but even that does not tie up precisely.

The GWPF do supply two links. The first one links to a web site called Real Science which just reproduces the graph without any analysis. The second links to the a data set which looks a bit like the Global Sea Ice Index but appears to be about 5 million kmlow. It too shows a trend which falls with time.

And where does the red line plotted on the graph come from? I have no idea.

So overall this is as wrong as wrong thingBut I have learned a little so the exercise has not been quite as useless as a useless thing.

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3 Responses to “Why do I punish myself?”

  1. gareth Says:

    The reference appears to be from the university of Illinois from – arctic.atmos.uiuc.edu/cryosphere (who also cite NSIDC ).Their graphic indicates December ice extent anomaly is arctic.atmos.uiuc.edu/cryosphere

    • protonsforbreakfast Says:

      Thanks Gareth. I hadn’t noticed that they also cite NSIDC. I downloaded the data file they linked to and I couldn’t understand the numbers: they didn’t tie up with my NSIDC calculations or the numbers plotted on the GWPF.

  2. The IPCC 5th Assessment Report: The authors speak | Protons for Breakfast Blog Says:

    […] As I mentioned previously – Antarctic Sea Ice is growing slightly. I don’t know why and when I ask specialists I get answers that seem handwaving. However Arctic Sea is shrinking dramatically, and this is well understood and predicted by climate models. […]

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