Sunday, January 8, 2017

Sea level rise: Antarctica doubles the trouble

On the trail in Marie Byrd Land.
On the trail in Marie Byrd Land. 
Photo © Bruce Luyendyk 

Sea level has been rising for decades, now at an increasing rate. Mostly we don’t notice this except in superstorms and hurricanes, or if we have visited the same beach for decades and wonder why it looks different (less sand, and cliff retreat)

If you're reading this, you aren’t someone who lives in a bubble. You know that scientists predict even more sea level rise during the next few human generations and beyond. If you live in Miami or New York or New Orleans or Orange County (and elsewhere of course) you and your offspring will for sure deal with significant sea level rise. Up to now, contributions to future sea level rise from Antarctic ice sheets this century is predicted to be small. New research shows this is not the case. 

The Intergovernmental Panel on Climate Change (IPCCis the go-to institution where we can find best-estimates of what will likely happen to sea level during the 21st century. In earlier blogs I gave some of the IPCC predictions (5/20/14; 6/9/14; 9/17/14; 9/6/15). I also repeated the statements by the IPCC in 2013 ( AR51 ) that sea level rise models for the next century have not included the more dramatic possibility of Antarctic Ice Sheet shrinkage and its contribution to sea level rise. The reason given by the IPCC is that the process of how the ice sheet might shrink was very murky – research was underway and not available to include in the predictions.

A problem identified with modeling the growth and shrinkage of Antarctic ice has been that models did accurately reproduce the growth of the ice sheet but not the shrinkage of it. The exercise is to construct a computer model that reproduces known geologic constraints on the ice sheet history—when it was larger and when it was smaller. It proved nearly impossible to model the shrinkage of the ice sheet. Specifically, shrinkage of the vast East Antarctic Ice Sheet proved the most difficult. The marine-based West Antarctic sheet proved easier—but most of Antarctica’s ice is on the East. Cleary, this is what we want to know—shrinkage means sea level goes up. What makes it shrink? How will the ice sheet shrink under the different scenarios for global warming that the IPCC has told us to expect in this century?

In the past few months, climate modelers at Penn State and University of Massachusetts, Amherst have added new physical parameters to those that affect the melting rate of Antarctic ice, and this has made the  difference2. The new computer models published in Nature can reproduce both the growth and the shrinkage of the Antarctic Ice sheet that we know happened in the geologic past. Now a model can be computed for what Antarctic ice shrinkage will contribute to sea level rise this century.

The early models keyed on removal of the floating ice shelves (9/6/15) by melting from underneath from a warming ocean. The new approach added two more impacts. First, melting of ice shelves from above by a warmer atmosphere. Second, after disappearance (retreat) of ice shelves, the collapse of remaining exposed ice cliffs over 80 meters high. The broken pieces fall into the sea as icebergs. This is a runaway process. As the ice breaks off in cliffs, the ice sheet moves downhill to the continent edge. Taller cliffs get exposed and these too break off, etc.

The researchers tested this idea against two periods in the geologic past when data show the Antarctic sheets retreated. In fact, during the last time the ice sheets shrank (130,000 – 116,000 years ago; smaller, but did not disappear) sea level was 6 to 9 meter higher than today, as reported in AR5. The researchers input atmospheric warming and ice sheet cliffing and reproduced that observation and another further back in geologic time.

These new ideas proved to be the missing “dynamic processes” needed to model Antarctic Ice Sheet shrinkage.

Armed with the new model technique the researchers asked the important question: “what is the future?” They ran three predictions starting back from 1950 and up to year 2100. Each prediction assumed different warming scenarios selected by the IPPC, the RCP2.6, 4.5, and 8,5; —translation; no change to the current rate of increasing greenhouse gas emissions (8.5), abrupt reduction of the rate (2.6) and somewhere in between (4.5). The details are in the Nature publication but the important conclusion is:

“When applied to future scenarios with high greenhouse gas emissions, our palaeo-filtered model ensembles show the potential for Antarctica to contribute >1 m of GMSL [global mean sea level] rise by the end of this century, and >15 metres of GMSL rise in the next 500 years.” (R. M. DeConto, D. Pollard, Nature 531, 591 (2016).)

Sounds like a lot and it is—but wait, this is for Antarctica alone. The IPCC GMSL estimates don’t include “dynamic” contributions for the continent ice shrinking. Dynamic means processes like described above. The IPCC AR53 estimates that at the end of this century GMSL rise due to ocean warming, mountain glacier melting, Greenland ice melting, ground water runoff and more is in the range 0.26 to 0.82 m. To these estimates Antarctic ice loss contributions need to be added. Including Antarctica, GMSL is then projected to rise up to 2 meters by 2100.

The 2013 IPCC report stated that Antarctic ice would mostly maintain itself with a balance of shrinking in the West and growing in the East. Now we must think about these dynamic models that account for what is in the geologic record. If upheld, these new estimates show that Antarctica will be the main contributor to sea level rise in the lifetimes of our children and grandchildren—and the current estimates circulating in the literature and media are too low by a factor of two.

1. IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

2. R. M. DeConto, D. Pollard, Nature 531, 591 (2016)

3. IPCC: Table SPM-2, in: Summary for Policymakers. IPCC AR5 WG1 2013, p. 21