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On the trail in Marie Byrd Land.
Photo © Bruce Luyendyk
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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 (IPCC) is 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
