Continuing to Lower the Sea Level Rise Contribution from Antarctica

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

The good news keeps coming in about sea level rise—or more precisely, Antarctica’s (minimal) contribution to it. Last time, we reviewed recent scientific findings indicating Antarctica was on the verge of gaining ice mass (and thus acting to draw down global sea level) as a slightly warmer Southern Ocean results in increasing snow accumulation which acts to offset ice loss from its peripheral (marine-terminating) glaciers.  Without a contribution from Antarctica, alarming visions of a large and rapid sea level rise this century—upwards of a meter  (and by some reckoning up to 6 meters)—are pretty much out the door.  Sans Antarctica, we are looking at a foot to foot-and-a-half of rise, give or take a few inches. Such an amount will undoubtedly require some adjustment and adaptation, but will not involve a wrenching transformation of society. Most of us probably wouldn’t even notice. Consider that, due to a combination of geology and oceanic warming, this same amount (or more) has been experienced in many East Coast locations in the last 100 years.

The good science news may be one reason why global warming has been so absent in the election debates. In response,  last week, the Union of Concerned Scientists helped a collection of local government officials and scientists from Florida pen an open letter to the candidates imploring them to address the issue of sea level rise during their third and final debate (held in Boca Raton).  They didn’t.

It is a good thing that they left the issue alone, for in this week’s Nature magazine comes more evidence that Antarctica is perhaps not going to be the great sea level rise contributor that other research as made it out to be (e.g. Velicogna et al., 2009; Rignot et al., 2011).

Matt King, from Newcastle University, and colleagues set out to refine the Antarctic ice mass change calculations that have been performed using data collected by the Gravity Recovery and Climate Experiment (GRACE) satellite. GRACE determines how the mass is changing underneath the satellite by measuring temporal variations in the pull of gravity.  If the strength of the local gravitational attraction increases over time, then it is inferred that the local mass must be increasing (and vice versa).  This is a handy tool for assessing trends in dynamic ice/snow mass in places like Greenland and Antarctica.

But, variations in the ice/snow burden are not the only thing that can change the gravitational pull observed by the GRACE satellite. The ground underlying the ice and snow may be changing as well. And, in fact, it is. The ground in many places around the world is still adjusting to the burdening and subsequent unburdening from the coming and going of the massive amount of snow and ice from the last ice age (and its termination). This process is known as glacial isostatic adjustment (GIA).

The problem is that while we understand that GIA is taking place, we really don’t precisely know the details, like where, when, and how fast—especially over sparsely monitored and studied places like Antarctica.

Two  years ago, a study was published that showed that the GIA model used in most GRACE-based studies was in error, and that when it was corrected, the rate of calculated ice mass loss from across Antarctica declined by some 40 percent  (from ~150 gigatons/yr to ~87 Gt/yr). Since it takes about 374 Gt of melted ice to produce 1 millimeter of global sea level rise, these findings indicated that Antarctica was contributing to sea level rise at a rate of about one-quarter of a millimeter per year (or about 1 hundredth of an inch per year). We detailed that finding, by Xiaoping Wu and colleagues, in a Cato Current Wisdom article in October of 2010.

Now along comes the new study Matt King et al. (2012) that further refines the local GIA over Antarctica. Here is how they did it:

Here we applied a new GIA model (W12a) to GRACE data to estimate the ice-mass balance for 26 independent Antarctic drainage basins from August 2002 to December 2010. The W12a model comprises a glaciologically self-consistent ice history constrained to fit data that delimit past ice extent and elevation, and an Earth viscosity model chosen such that GIA predictions from W12a best fit a suite of relative sea-level records around Antarctica. The advance of W12a on previous models applied to GRACE data is illustrated by the misfit to GPS uplift rates being halved. Our use of W12a addresses the dominant GRACE-related error in previous Antarctic analyses.

With this new model in hand, they were able to produce a new estimate of the rate of ice mass change over Antarctica from 2002 through 2010. That estimate is a loss of only 69 Gt/yr (+/- 18Gt/yr). And further, they found no statistically significant change in this rate when averaged over the whole continent—in contrast to other prominent studies (e.g. Rignot et al., 2011) which claimed a significant acceleration was taking place.

So King and colleagues’ latest refinement puts the Antarctic contribution to global sea level rise at a rate of about one-fifth of a millimeter per year (or in English units, 0.71 inches per century).

Without a significantly large acceleration—and recall the King et al. found none—this is something that we can all live with for along time to come.


References:

King, M., et al., 2012. Lower satellite-gravimetry estimates of Antarctic sea-level contribution. Nature, doi:10.1038/nature, http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11621.html

Rignot, E., et al., 2011. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters, 38, L05503, http://www.agu.org/pubs/crossref/2011/2011GL046583.shtml

Velicogna, I., 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, 36, L19503, http://www.agu.org/pubs/crossref/2009/2009GL040222.shtml