Greenland Melting?

"Melting Greenland fuels sea level rise says" Greenpeace. "A new report sheds light on Greenland's quickening meltdown — and why that's distressing" says Time. "Greenland's Glaciers Moving Faster, Losing Mass" says Kansas City Infozine. You get the idea (all of that via google news). Though the best one seems to be "Greenland ice melting faster than thought" by physorg.com. So what can they mean? Can it be "Recent Ice-Sheet Growth in the Interior of Greenland" in Science? Oops no, wrong sign, and anyway that was sooooo 2005 :-) Although to be fair even that mentions thinning below 1,500m.

Nope, it must be "Changes in the Velocity Structure of the Greenland Ice Sheet" Science, 17 February 2006:

Using satellite radar interferometry observations of Greenland, we detected widespread glacier acceleration below 66° north between 1996 and 2000, which rapidly expanded to 70° north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year. As more glaciers accelerate farther north, the contribution of Greenland to sea-level rise will continue to increase.

Which is similar to "Rapid and synchronous ice-dynamic changes in East Greenland" by Adrian Luckman et al in the less visible GRL, though New Scientist found it, as did the BBC.

90 to 220 km^3/y is an increase of about 0.4 mm/y in global sea level according to a quick calc (and I'm sure the arithmetic fiends will be quick to jump on me if I'm wrong...). This is 20% of the current 2 mm/y obs (or 13% of the 3 mm/y obs, if you take the more recent satellite obs). Or if you think it *caused* the increase from 2 to 3, its about half of that... The TAR estimates put Greenland into context (as they were then; oh, and here).

So... what does it all mean? I don't know. I wrote this post to find out... originally it was going to be about Luckman, then I realised there was the Rigot thing too. How confusing. Maybe RC will do it properly :-)


coby said...

I get it that 200km^3 is a huge amount of ice, yet less than 1mm is not much sea level rise, but I think the point is the change we are seeing.

Anyway, I am just using this as an excuse to ask a related question: what are the implications of sea level rise for Antarctic ice sheets? Given that [some|lots|a bit] of that ice sits on bedrock below current sea levels, is there a possibility of some lifting as that level rises? At the very least I would expect the lowering of pressure at the bottom of the ice, no?

Any thoughts on how that might effect the current dynamics?

William M. Connolley said...

Hmmm, not my field guv, but its slightly hard to see a sea level rise of less than 1m changing the balance much. Especially (he suddenly thinks) because the tides do that anyway. Certainly the under-ice topography isn't known to that accuracy.

stephan harrison said...

I suppose that increased instability of Greenland and WAIS could increase the likelihood of rapid purge events where large quantities of icebergs are discharged into the sea. These have happened in the past (Heinrich events) and could raise sea levels very quickly by displacement.

William M. Connolley said...

Heinrich events, AFAIK, are from the Laurentide ice sheet (though I think there is some slight dispute about this). As it gets warmer I guess the mechanism could xfer to Greenland, though thats rather speculative. Evidence against would be the lack of them during the last interglacial, which is supposed to have been somewhat warmer than present.

Eachran said...

William, I see that your favourite journal, The Economist, last week featured the Greenland ice sheet instability quoting Science I believe. I recall already saying that once on-side The Economist will be good at reporting in the future.

Doesnt look terribly good for low lying land dwellers or even the London Olympics.

Can RC do a review please?

William M. Connolley said...

That will be http://www.economist.com/displaystory.cfm?story_id=5518916, if you have a subscription:

"THE biggest unknown factor in making predictions of rising sea levels in response to global warming is the role played by the massive ice sheets that cover Antarctica and Greenland."

probably wrong... I would guess that over the next century at least, the largest uncertainty is in the magnitude of the thermal expansion term, just because its the biggest term.

"...That is worrying enough. But Dr Rignot and Dr Kanagaratnam also found that the Greenland ice sheet experienced a greater area of surface melting in 2002 and 2005 than at any previous time since records began in 1979. Most of this has been in the south of the island, which is where the accelerating glaciers lie. Water flowing from the surface could ease the passage of the glaciers into the sea. Taking both factors into account, the contribution made by the Greenland ice sheet to the rise in global sea levels has increased from 0.23mm a year in 1996 to 0.57mm in 2005."

Yup. Seems fair enough. But note how they choose to run this as a pure-science piece, and very carefully avoid any policy implications.

And... they can't tell the difference between the gulf stream and the north atlantic drift. But almost no-one else can (wiki can, now, cos I fixed it) so I guess they can be forgiven :-)

RC... hmmm...