Sea level in the Arctic

A recent conference presentation at AGU (reported here) while confirming that global sea level is indeed rising (in line with other estimates), showed that Arctic sea levels may actually be falling. On the face of it these preliminary results are a little puzzling (though note that this isn’t yet a properly peer reviewed paper, and so may not reflect what ends up in the journal), but it does reveal some of the complexities in analysing sea level in relatively small enclosed basins and so a brief overview of the different factors involved is probably useful.

Firstly, where is this new data from? The ERS-2 satellite has a ‘radar altimeter’ similar to that flown on the TOPEX/ POSEIDON (and now JASON) satellites and it works by beaming down a radar pulse and seeing how long it takes to come back. These techniques are remarkably accurate (down to a few cm’s and can give accuracy in trends to a few tenths of mm per year averaged over large areas). However the Arctic and Antarctic present special problems due to the poorer coverage near the pole (nothing above 82ÂșN for instance) and, of course, the presence of sea ice. The ‘sea level’ being talked about is that of the water in between the ice floes, and so the data must be very carefully sorted to yield only the open water values (i.e. where there is no floating ice). This clearly removes the majority of the data and must make any Arctic-wide estimates much more uncertain than the global numbers outside of the polar regions). There is also the possibilty that the analysis has still not removed all sources of contamination in the trend – there may be a seasonal bias (since open water has increased more in summer than winter) or the thinning of the ice cover means that any residual ice contamination could make it look incorrectly like ‘sea’ levels were falling. These issues will obviously be considered in greater depth in any actual publication.

But what if the numbers are roughly correct? What would that mean?

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