Monday, 3 December 2012

Finally quantified

“Sea-level rise finally quantified”. That was a header on the BBC news website this week; it looked nicely definitive. Now we know how much sea level rise there is! Or do we only know that BBC news headers are short?

The header of the actual article was a bit more precise: “Sea-level rise from polar ice melt finally quantified”. It still sounded definitive. And it also sounded like it had never done before; finally, that lack of knowledge has been resolved. Is that true? Or do we now only know that 53 characters is still too little to say anything scientific?

The article gets to the point quickly; the first sentence is “Melting of polar ice sheets has added 11mm to global sea levels over the past two decades, according to the most definitive assessment so far.” A sentence only slightly bigger than a large tweet, but it carries the essence of the journal article in Science it discusses. There had been estimates of how much polar ice sheets contribute to global sea level before, but this time researchers from 26 institutes had pooled their knowledge, and together produced a result that was much more reliable than all individual efforts. What they come up with, though, still has a large uncertainty: the figure presented is 11.2 +/- 3.8 mm.

 A beautiful picture of the Antarctic Ice Sheet, found on Wikipedia, and taken by my dear old colleague Stephen Hudson.

Why is quantifying the mass balance of ice caps so difficult? There are two main methods of measuring this: one is satellite altimetry, and the other one satellite gravimetry. Satellite altimetry simply measures the top of the ice sheet and calculates its mass from that. And there are two main challenges associated with that: the first concerns the top of the ice sheet. Are you measuring the top of the ice? Or perhaps the top of several metres deep a pack of snow? It matters a lot for the mass you will calculate from your height measurements. 

The other problem concerns the bottom of the ice: you can’t measure that with your altimeter. You have to estimate that from other data, which might be somewhat imprecise. So if last year your measurement of the top of the ice at some point on an ice sheet was 200m above sea level, and this year it is 201m, does that mean there is 1m of ice more? Or has 5m of ice melted off, the whole continent bounced up half a metre as a result, and 5.5 m of fresh snow fallen on top? It’s an extreme example but it does illustrate the difficulties involved.

Satellite gravimetry measures the gravitational pull of the ice sheet concerned, so it needs not distinguish between snow and ice. But it sure needs to distinguish between ice and rock. This can in practice only be done with modelling, and that produces some of the uncertainty that is hard to get rid of. 

Another difference between altimetry and gravimetry, which can be used to one’s advantage, is that altimetry is localised, while gravimetry gives by definition a regional figure. If you use the one to verify the other, the accuracy of your estimates increases. 

A picture of Greenland, also from Wikipedia

The authors of this Science paper combined not only these different approaches, but also pooled the data from all the institutes they represent. That way they acquired much longer time series, and thus higher accuracy. If you have overlap in time and space you can calibrate the various data sets with each other. Their figure of 11.2 +/- 3.8 mm sea level equivalent mass loss over the period 1992-2011 can be considered the best available. 

In the Science paper, numbers are also given for the individual ice sheets, and for various time periods. These results show that East Antarctica is mainly gaining mass; that makes sense, as warmer seas tend to produce more snowfall over the continent. All the other ice masses (West Antarctica, the Antarctic Peninsula, and Greenland) are consistently losing mass. And all of them are accelerating; both Greenland and the Antarctic Peninsula display a four-fold increase in annual mass loss between 1992-2000 and 2000-2011. West Antarctica melt doubles between these periods. The mass that East Antarctica gains in the latter period is outweighed (by a factor of almost 2.5) by the loss of the rest of the continent. 

The authors of this work do not discuss the future, but one can hardly resist mentally extrapolating the graphs shown. A rise of 11 mm in 20 years may in itself not be much, but half of that has happened in the last 5 years. And this is only the polar ice caps; there is of course also the influence of factors like thermal expansion and melting low latitude glaciers. It would be nice if a similar effort was made to reconcile all records of the remaining components of sea level rise, and bring the uncertainty of these to a minimum as well. The real pressing question, on what the future will hold, can only be begun to be reliably answered when we know what is going on right now…

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