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.
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.
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 km2 in the antarctic summer and grows to around 18 million km2 in the antarctic winter i.e. around 15 million km2 of sea ice re-freezes every year.
- In the northern hemisphere the sea ice falls to around 5 million km2 in the arctic summer and grows to around 15 million km2 in the arctic winter i.e. around 10 million km2 of 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.
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.
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 km2 whereas 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 km2 low. 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 thing. But I have learned a little so the exercise has not been quite as useless as a useless thing.