A Reconciled Estimate of Ice-Sheet Mass Balance

Shepherd et al.: Science 30 November 2012: Vol. 338 no. 6111 pp. 1183-1189 DOI: 10.1126/science.1228102:

We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth’s polar ice sheets. We find that there is good agreement between different satellite methods—especially in Greenland and West Antarctica—and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year−1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year−1 to the rate of global sea-level rise.

Aunty says Sea-level rise from polar ice melt finally quantified but then immeadiately backs off to the most definitive assessment so far, which seems more accurate. In case you can't read the abstract, they're saying that only E Ant is gaining mass, and that at a low rate, so overall Ant is losing, and Greenland is losing even more. Still - that adds up to 0.6 mm/yr. So it will have to grow if its to become interesting by 2100. And undoubtedly it will, but that means predicting it remains interesting, since (linear) extrapolation is obviously pointless.

Oh go on then, I'll say a bit more. Quoting Aunty:

The lead author of the research, Prof Andrew Shepherd of Leeds University, said the study brought to an end 20 years of disagreement between different teams.

"We can now say for sure that Antarctica is losing ice and we can see how the rate of loss from Greenland is going up over the same period as well," he added.

"Prior to now there'd been 30 to 40 different estimates of how the ice sheets are changing, and what we realised was that most people just wanted one number to tell them what the real change was.

"So we've brought everybody together to produce a single estimate and it turns out that estimate is two to three times more reliable than the last one."

I'm not sure that "most people just wanted one number so we gave them what we wanted" is a good argument. Not having read the paper, I don't know how they converted multiple numbers into one (by James's barometer-dropping perhaps? By actually trying to work out why they differed? Either of those would make sense).

More like this

From NASA: PASADENA, Calif. - An international team of experts supported by NASA and the European Space Agency (ESA) has combined data from multiple satellites and aircraft to produce the most comprehensive and accurate assessment to date of ice sheet losses in Greenland and Antarctica and their…
[RealClimate has a post with more useful detail.] I last commented about Antarctica and SLR when I "reviewed" the AR5 cryosphere chapter. As I noted there, things have come on quite a way since I were a lad (2005, 2004) and the major advance looks to be GRACE, even if they sometimes recalibrate…
These masses of ice are now contributing more new meltwater to the world's seas than all other melting ice combined. The Greenland and Antarctic ice sheets are losing mass at an accelerating pace, according to a new NASA-funded satellite study. The findings of the study -- the longest to date of…
A new look at twenty years worth of research shows that polar ice is in fact melting, and raising sea levels, faster than anticipated. Greg Laden writes "Greenland is losing ice about 500% faster now than it was in the early 1990s, while Antarctica is losing ice at about the same rate." Altogether…

Well, the authors all now have one more paper in their c.v.

More seriously, policy wonks want that number, not 30--40 of them.

By David B. Benson (not verified) on 29 Nov 2012 #permalink

One more paper for Judith Curry to ignore.

"...19 years of satellite
radar altimeter (RA) data, 5 years of satellite laser
altimeter (LA) data, 19 years of satellite radar
interferometer data, 8 years of satellite gravime-
try data, 32 years of surface mass balance (SMB)
model simulations, and estimates from several
glacial isostatic adjustment models, to produce a
reconciled estimate of ice-sheet mass balance
To enable a direct comparison, we reprocessed the geodetic
data sets with use of common time intervals and
common definitions of the East Antarctic, West
Antarctic, Antarctic Peninsula, and Greenland ice-

Ugh. I was googling something, and one of my hits was to a climatewiki dot org run by Heartland. Its quality, is, of course, atrocious - for the fun of it I looked at the Arctic sea ice section and I think the most recent data point it talked about was in 2001, and there was lots of, "such and such a paper analyzed trends from 1990 to 1999, but the decline was all concentrated in one year, and so it might come back". Kind of funny/tragic/wrong, really.

Ok, after bleaching my eyes, here's the kicker quote: "Furthermore, it could be argued from their data that from 1990/91 onward, sea ice area in the Arctic may have actually increased. "

I think this wiki page was written in 2011. How could they be so very very very... I can't quite come up with an adjective sufficient to describe their stupidity here.

Well, MMM, that's entirely routine work from professional liars, so the words you are looking for are 'predictable' and 'intentional'.

Radi made a simple calculation using above data about average yearly mass changes of the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula (–142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year−1, resp.)

Given the rough estimate of overall ice sheet volume of 33 million cubic kilometers of Antarctica and Greenland together, Radi has calculated that it takes approx. 150'000 years until all of the ice of the ice sheets of Antarctica and Greenland has disappeared into the oceans, based on the average ice mass balance deficit of 220 gigatonnes of the 1992 to 2011 period.

Radi thinks that weather and climate alarmists can therfore calm down and resume normal daily activities outside climate alarmims. There is no danger of significant sea flooding in the next few thousand years.

[As pointed out below, you've made the classic error, of doing a "correction" calculation that is nonetheless meaningless.

What you've calculated is the rate at which the ice sheet would disappear, given the current rate of loss. But this isn't a very interesting number. What we're actually interested in is the likely rate of SLR *in the future*, which requires something more that simple extrapolation to produce a useful result -W]

By Radical Denier (not verified) on 02 Dec 2012 #permalink

Richard Kerr wrote in his commentary on the paper 'In the end, reconciling the diverse ice-loss estimates proved to be more straightforward than had been feared. It turned out that gains and losses of ice can vary greatly from season to season and from place to place. So surveys made over different, albeit overlapping, time periods and regions yielded rather different loss rates. Once the data were adjusted to uniform regions and periods and a few other modifications were made, “there's no reason to believe the data sets are saying different things at all,” Shepherd says. “They're showing the same thing.” '

Judging by the number of authors of the paper it seems that all the ice experts must have signed it and agree now !

[That may well be part of it. But for Antarctica, which was the bit I knew, seasonality effects like that weren't important. Perhaps the GPS isostatic-rebound has helped there -W]

By Alastair McDonald (not verified) on 02 Dec 2012 #permalink

Radi made a simple calculation but included several simple mistakes, to whit....

1/ As the graph above shows the average rate is not representative of the current rate. The contribution from ice sheet melting is increasing with time, and in a warming world this trend will continue. Radi's calculation is already an underestimate of the rate of melting and will get worse year on year..
2/ Serious flooding will occur way before all the ice sheets are gone.
3/ Radi fails to include the effects of thermal expansion into his calculations.

By Gilbert Smith (not verified) on 02 Dec 2012 #permalink

In Anarctica the difference is between the East and West ice sheets. The West is melting faster than the East is growing. Their different trajectories has been one of the confusing factors.

By Alastair McDonald (not verified) on 02 Dec 2012 #permalink

...Perhaps the GPS isostatic-rebound has helped there -W]

I've been loosely following this issue for a few months and there has been plenty going on.

I think one of the key recent developments is a new GIA model for Antarctica (Whitehouse 2012) which produces better fits to available observational data than the previously-used Ice-5G model. Pippa Whitehouse gave a talk concerning the new model at an ISMASS workshop in July this year. It was also mentioned in a recent post on RC, co-authored by Whitehouse, which was specifically referring to their own Antarctic ice mass loss estimate, pubslished in print just last week in Nature (online in October).

One of the changes in the new GIA model is significantly less present-day movement, which has the consequence of producing lower mass loss results from GRACE data, more in-line with altimeter-based results.

Jay Zwally was also at that workshop presenting a talk entitled 'Mass Balance of Antarctic Ice Sheet 1992 to 2008 from ERS and ICESat: Gains Exceed
Losses', stating that his tally found overall mass gain in Antarctica using altimeter data. Predictably, this was picked up by Watts and co. Interesting though that Zwally is here listed as coauthor on a paper which seems to state quite definitvely that mass change is almost certainly negative overall in Antarctica.

Looks like I missed out the RC link: http://www.realclimate.org/index.php/archives/2012/11/weighing-change-in-antarctica/

Regarding GPS stations, Pippa Whitehouse says in the ISMASS talk that they are very sparsely-situated in Antarctica, especially in really key areas for understanding GIA (also a point made in the RC post), so they are only really useful for model validation.

[Yes, but having a few points to pin the rest should help -W]

I also read an article that made this same claim, that the ice sheets were all getting thinner and that this was "certain".

But then that article also had one sentence tossed in that said well maybe they are actually getting thicker and not actually melting. Seems the people who tell us how "certain" they are, are not so certain!

[Its a shame you didn't bother provide a link to the article you claim said all this, because without that link what you're saying is meaningless -W]

And about this scientific consensus. If all these scientist agree that global climate change is happening, why do they still except grants each year to study it? No one is still doing research to prove the earth is round........

[Just like no-one is still doing research into Quantum Mechanics. Or General Relativity. Or evolution. Oh, wait -W]

Let's take all that money we spend on "studying" global warming and apply that to any solution the politicans want to try!

But then we would have a lot of climate change scientist out of work. They may have to find some other way to make money, maybe go on the talk shows...... wonder if they will all talk the same talk when their incomes are not based on pleasing the politicans!

Heather, pick up the phone or email any scientist at your local university. It seems you haven't got the foggiest idea about how research is done and how research funding is allocated.

The first part of this AGU video session (actually just the slideshow and audio, but close enough) is by Andrew Shephard talking about this paper. Pretty brief though and I don't think it says anything more than has already been surmised.