Monday, 30 July 2012

The Greenland story

It was all over the news, all over the world: 97% of the surface of the Greenland Ice Sheet melting! Quite a lot of sources, including,  de Volkskrant and het NOS journal got carried away, and announced, either on twitter or on national TV, that 97% of all Greenland ice was gone. They should have spent a second to ponder this: for instance, the offices of de Volkskrant would be flooded if that had indeed been the case. But 97% of the surface experiencing melt is spectacular enough in itself. Generally, no more than 50% undergoes melting in summer. 97% is really rare.

Some twitter sources also mentioned this event had been predicted. In an article, that attracted quite some attention, Jason Box of Ohio State University, and co-workers, stated they expected melt over 100% of the surface to occur in the near future. So what did they base that on, and were they really that precise?

The map showing decreased reflectivity over almost the whole of the ice cap, which gets the bulk of the attention.

Box et al. studied the reflectivity of the Greenland ice. Reflectivity, or albedo, is one of these things that stabilises ice sheets; it reflects sunlight back so effectively that the radiation can hardly make a start at melting any ice before it finds itself reflected back into space. But if high temperatures manage to get the melting process going, this lowers the reflectivity, and then your ice and snow are in peril. This self-reinforcing process, also known as positive feedback, might well herald your ice cap’s decline. What’s even worse is dirt blown on top of the snow; this may start melt at lower temperatures.
So what did Box and his fellow scholars do? They basically measured reflectivity and melt from a satellite, calibrated these results with observations from weather stations on the surface, and ran a climate model in order to get an idea of the sensitivity of the reflectivity to temperature. And what is so new about this research? Satellites have been measuring the albedo of Greenland for many years, and ground-truthing with weather stations has been done since early days too. But the results of Box et al. go all the way to the year 2011, bringing this research up to date. And their combination of observations and modelling could potentially give new insights in how the process works.

Observations of reflectivity
So what did Box and colleagues find? The reflectivity of the Greenland Ice Sheet is at a low point; 8% lower in 2011 than it was in 2000. And this is not an incident; they have observed a significant trend, though admittedly a short-term one. They further found a 26% increase in melt between 2000 and 2011. And to give you an idea of how much that is: if that rate would remain constant at 2011 level, the ice cap would be lost in roughly 6000 years. And the sensitivity of the reflectivity to temperature? That’s where it gets confusing. Over large areas of the ice sheet, reflectivity only goes up with higher temperatures. This can be explained by warm air bringing in more snowfall. But strangely enough, snowfall doesn’t always correlate with higher reflectivity in their data. And when you look at the sensitivity of the reflectivity to temperature, or in other words; by how much the albedo goes up or down with every degree temperature change, it becomes clear that their data is only statistically robust in the regions that are melting already.
The authors warn that they think summer melting will occur over the entire ice sheet in another decade, if the coming years will be like 2010 and 2011. But that is a big “if”. Box emphasizes only the decreased reflectivity in his own blog post, without being too specific about the lack of straightforward relation with actual melting. The big take home message of this paper might be that the processes governing ice melt are not yet sufficiently understood. And we want to understand it, if only to get an idea what we should do with our coastal defences. The amount of melt in 2011 measured already translates to more than a millimetre of globally averaged sea level rise. And that does not sound like much in itself, but it does when you realise it was only 1.7 mm/year on average for the 20th Century in total; that includes for instance Antarctica, mountain glaciers, and thermal expansion.

So did they predict the ~100% melt?
Well. In a way they did. But what they really predicted was a shift to net melt over the area that nowadays experiences net snow accumulation, averaged over the whole summer. They did not mention short periods of 100% surface melt. However, you can’t get to net summer melt without, well, melting large areas of the surface once in a while. So people who say “they predicted this!” are exaggerating. But Box and colleagues are right in saying that this event greatly supports their conclusions. Given the uncertainties in their data, this was more a lucky guess than rock-hard data, yet I hope it will attract attention to the danger of Greenland melt. It’s not as if we who are alive today will ever see an ice-free Greenland, but we may well see a Greenland Ice Cap that raises average sea level by 2mm per year or more, and that is something we need to prepare for. Those who love Amsterdam, London, New York or one of these other iconic cities near sea level might wish Box luck in keeping up the good work…

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