More on Antarctic Ice Melt

I see that the news item about a recent paper measuring Antarctic ice melt continues getting a lot of traction.
See my post of yesterday for more context.

Quick summary: new paper uses data sets that end in 2008 to assert that Antarctica is, overall, gaining mass, and not contributing to sea level rise. Yet, very accurate satellite measures of sea level show an accelerating rise (not to mention the water up around people’s ankles in Miami and elsewhere..)  So what’s up?

Here is an explainer from Richard Alley of Penn State:

The study is interesting, and Jay Zwally has done good work over many years on this topic. But, the consensus Shepherd et al paper, on which Jay is a coauthor, came up with an answer that I think differs from the new one by more than the updates in the isostatic corrections (note that if you need background on any of this, I can send more).  So, this new paper is at least somewhat at odds with multiple other lines of evidence.  The new work could be right; all of this is difficult and the error bars are sometimes large and difficult to quantify exactly, but as noted below, you probably are better starting with the multi-sensor/multi-parameter/multi-group answer (actually, you’re better going to the IPCC report).  

Let me give an analogy, and then go back to comments on Jay’s study: In the Shepherd et al IMBIE paper, the average rate of mass loss from the ice sheets over the 20 years covered was 0.6 mm/yr of sea-level rise.  The total size of the ice sheets is a bit over 60 m of potential sea-level rise, so at that rate, the ice sheets would take over 100,000 years to disappear, which is a loss of 0.001% per year.  The equivalent for a professor on a diet would be losing 1/3 of one typical potato chip per year.  That this is measurable is fantastic; that there might be some uncertainty and a different way to interpret that signal is not impossible.

You can see the difficulty in the measurement Jay is making.  There is a history of satellite degradation (the laser in the satellite was burning out, and that affected how easily the reflection from the ice sheet could be seen), and that raises concern about changes in the measurement arising from causes other than changes in surface elevation.  The data disagree with published results on change in surface elevation of Lake Vostok from GPS on the surface.  And, there are other technical details that could be discussed and that could involve errors. Zwally and company believe they solved these problems, the reviewers and editors approved this, but it is one paper, and there is still a large body of literature including the IMBIE paper that points in a somewhat different direction. Almost always, the best is to start with the assessed science (IPCC, or the whole IMBIE team) and work from there, so this new paper really shouldn’t change your starting point. 

Perhaps more importantly, suppose for a moment that this is correct, and ask what it means. The IPCC has worked hard to “close” the sea-level budget, matching the observed change in sea level (with its uncertainties) with the observed causes (thermal expansion of the ocean, mountain glaciers melting, ice sheets shrinking, and any mining of groundwater in excess of water being stored from reservoir building.  If the new estimate is correct, then some other estimate(s) is/are probably off at least a little.  But, there may be enough room in what we know about the others to accommodate this (Jonathan Gregory would be the best source on this).

My MSc was among the early studies on the very long time-scale response of the central regions of the ice sheets (Alley and Whillans, 1984, JGR).  And yes, the response time to accumulation-rate increase is many millennia, so a long-ago accumulation-rate increase can still be affecting the central regions of the East Antarctic ice sheet a little.  But, try this as a possible interpretation.  (Stefan Rahmstorf is the expert on this, and you should ask him about it.)  The sea level has been rising, in response to warming caused primarily by our CO2, from some combination of the warming causing ocean water to expand, the warming causing mountain glaciers to melt, the warming causing ice-sheet shrinkage in Greenland and Antarctica, and perhaps us pumping water out of the ground (which eventually reaches the ocean) more rapidly than we raise groundwater from dams we built.  We can relate how much sea level has risen to how much warming has occurred, and people have built simple models to do so.  Then, one can use such a simple, semi-empirical model to project how future warming will impact future sea-level change.  There are other ways to estimate future sea-level change, but this is one.  IF there is a long-term trend towards sea-level fall because of ice-sheet growth in East Antarctica, then our warming has caused more sea-level rise than we previously thought, to explain the rise we have observed and the extra mass that went onto Antarctica, with some other place changing a bit more than we had previously estimated.  The long-term trend in Antarctica would continue, but if our warming causes more rise than we previously thought, and we warm more, then we would end up with more rise than we originally thought.  So, there is an interpretation that would take this new work and raise the future sea-level rise we expect from our warming.

So, overall, interesting new study, but wise to avoid the one-paper syndrome and stick with the IPCC, which uses a lot from the Shepherd et al, and considered the full range of estimates from GRACE gravity, input-output and altimetry while trying to close the sea-level budget by also considering thermal expansion and mountain-glacier melting versus the observed sea-level rise.  This is one study, other studies will come, and there will eventually be another authoritative assessment, in all likelihood.  But, even if this one proves to be right, the meaning is not obvious.  What is absolutely obvious is that this does NOT in any way mean that we don’t need to worry about sea-level rise from our warming. 

A number of experts are disputing the conclusion of a recent NASA study that says more ice is accumulating in Antarctica than is being lost due to climate change. They argue that the study contradicts more than a decade of other scientific measurements — including previous NASA studies.

The NASA report issued last week, “Mass Gains of Antarctic Ice Sheet Greater than Losses,” argued that snow accumulation in East Antarctica has added enough ice to the continent to outweigh the losses from the continent’s thinning glaciers, especially those in West Antarctica. These ice gains, the report noted, would likely not last more 20 to 30 years due to the speed with which ice is melting due to climate change.

But previous NASA studies, including data released last year, have warned that melting in West Antarctica is “unstoppable.” Researchers have also said melting ice could add as much as four feet to long-term sea level rise predictions, which warn of a three-foot rise by 2100.

Last week’s study — which challenged a 2013 report by the United Nations Intergovernmental Panel on Climate Change (IPCC) saying Antarctica was losing ice overall — has triggered heated debate.

“Please don’t publicize this study,” said Theodore A. Scambos, a senior research scientist at the National Snow & Ice Data Center, a polar research center at the University of Colorado at Boulder.

Other critics said the study contradicts 13 years of satellite measurements of Antarctica’s ice by NASA’s GRACE mission.

“There is no quality data to support the claims made by the authors of [ice] growth in East Antarctica,” said Eric Rignot, principle scientist for the Radar Science and Engineering Section at NASA’s Jet Propulsion Laboratory.