GRACE measures the gravity of the planet, and it provides information that the size of the Greenland ice sheet is declining. Eschenbach's half a point, quoting an article on Grist by Seth Shulman:
So the next time you read something that breathlessly says …OK, so scientists aren't "just guessing", let's leave that aside. And it really isn't clear what the prior expectation was.
“If this activity in northwest Greenland continues and really accelerates some of the major glaciers in the area — like the Humboldt Glacier and the Peterman Glacier — Greenland’s total ice loss could easily be increased by an additional 50 to 100 cubic kilometers (12 to 24 cubic miles) within a few years”
… you can say “Well, if it does increase by the larger estimate of 100 cubic km per year, and that’s a big if since the scientists are just guessing, that would increase the loss from 0.007% per year to around 0.010% per year, meaning that the Greenland Ice Cap would only last until May 23rd, 12010.”
In fact the story is not the rate of ice loss, nor the ice sheet as a whole, but the confirmation of estimates of accelerating mass loss at the fringes:
The team found that uplift rates near the Thule Air Base on Greenland's northwest coast rose by roughly 1.5 inches, or about 4 centimeters, from October 2005 to August 2009. Although the low resolution of GRACE -- a swath of about 155 miles, or 250 kilometers across -- is not precise enough to pinpoint the source of the ice loss, the fact that the ice sheet is losing mass nearer to the ice sheet margins suggests the flows of Greenland outlet glaciers there are increasing in velocity, said the study authors.and
"When we look at the monthly values from GRACE, the ice mass loss has been very dramatic along the northwest coast of Greenland," said CU-Boulder physics Professor and study co-author John Wahr, also a fellow at CU-Boulder's Cooperative Institute for Research in Environmental Sciences.As you see, this isn't "just guessing", and GRACE has higher resolution than the subcontinental scale that Eschenbach is going on about. The press release doesn't provide maps. Here's a nice little piece at geoinformatics.com that does.
"This is a phenomenon that was undocumented before this study," said Wahr. "Our speculation is that some of the big glaciers in this region are sliding downhill faster and dumping more ice in the ocean."
Here's what Grace sees:
Here's some independent evidence about the melt season, which you can see maps nicely onto the GRACE data and gives you a good idea of the resolution of GRACE.
What we see is Greenland softening at the edges. Ice is sort of a glassy substance, that flows much faster as it warms, so we see the beginnings of a possible failure mechanism for the whole ice cap structure. That is what we should be worrying about, and it means that Eschenbach is, willingly or unwillingly, performing some sleight of hand here.
But ultimately, he is objecting not to the press release, but to Grist's take on it. And here Eschenbach's point stands on its own.
Finally, the original article that got my blood boiling finishes as follows:Yuppers. There is something to what he says.
The good news for Luthcke is that a separate team using an entirely different method has come up with measurements of Greenland’s melting ice that, he says, are almost identical to his GRACE data. The bad news, of course, is that both sets of measurements make it all the more certain that Greenland’s ice is melting faster than anyone expected.
Oh, please, spare me. As the article points out, we’ve only been measuring Greenland ice using the GRACE satellites for six years now. How could anyone have “expected” anything? What, were they expecting a loss of 0.003% or something? And how is a Greenland ice loss of seven thousandths of one percent per year “bad news”? Grrrr …
I’ll stop here, as I can feel my blood pressure rising again. And as this is a family blog, I don’t want to revert to being the un-reformed cowboy I was in my youth, because if I did I’d start needlessly but imaginatively and loudly speculating on the ancestry, personal habits, and sexual malpractices of the author of said article … instead, I’m going to go drink a Corona beer and reflect on the strange vagaries of human beings, who always seem to want to read “bad news”.
But it's only half true. Pretty much exactly half true.
I'm not fond of false symmetries, it makes for such an easy target for an essay. In this case, I'll make an exception; I see a very close to perfect symmetry here.
Lots of other people cherry pick information to support their point of view. The site Eschenbach writes for is a fine example. If only he weren't guilty of cherry-picking in the opposite direction, if only other people didn't only want to read "good news", we might be able to make some progress.
Greenland is melting detectably and contributing detectably to sea level rise. The quantity is now reasonably well constrained. That's good news, scientifically. It's slightly bad news as far as sustainability is concerned (the change might still have been undetectable, but it isn't.) It's too early in the record to detect any acceleration (*). If and when it accelerates, we'll be in a position to detect that, too. Grist does not have a real basis for "faster than anyone expected", but Eschenbach does not have a basis for being sanguine about it either.
By the way, you will note the increase in mass in the Greenland interior. That is increased snowfall. While this mitigates the net melt and the sea level rise a bit, it's consistent with expectations from global warming: increased winter temperature =>increased winter column moisture => increased snowfall. It is a negligible term in the force balance so far, but in the long run it would also increase the pressure gradient and tend to further accelerate the glacial flow.
Update: (*) It's clear that Greenland once must have been in mass balance just from basic mathematical principles, so arguably any loss at all must be an acceleration. In fact, early measurements in the mid 20th c. did seem to show mass balance, though they were very crude.
My point is that the GRACE record itself shows no acceleration of mass loss over a decadal time scale. I'm basing that on this figure, from Velicogna and Wahr, which I believe is a mass total, not a mass flux. Sorry if this was unclear.
I have little doubt that ice sheet mass is retreating and accelerating. The retreat is not yet rapid, and this is the closest thing to a legitimate point of Eschenbach's. The GRACE record is just too short to provide a convincing demonstration of acceleration in itself, though it certainly is suggestive.
Some of these guys are just down the hall from me. I guess I should just ask them!
Update: It's looking a little stronger on "acceleration"; thanks to a correspondent. Here's the latest Velicogna. I don't call this a slam dunk, myself.
Update: Gavin has a really nice piece on RC that addresses some of the issues raised here.