Ice Melt is Reaching a Tipping Point?

The oceans are heating faster than expected (slowing the changes in the atmosphere).

The southern Ice is melting in many places, possible everywhere.

And the northern Ice is rapidly disappearing altogether.  Up north, Greenland has been melting faster and faster.

The studies keep coming.

This month, an international team report a study of the “mass balance” for all the major “basins” of Antarctica [2].  Mass balance refers to the total loss due to melting and flow into the ocean balanced with the total gain of snowfall.

The picture is complicated, with Western Antarctica losing mass rapidly, generally attributed to warmer water undercutting glaciers at the coast.  Other areas have has less consistent changes, but overall most of Antarctica is losing mass (i.e. melting).

This is highly important, because the “lost” ice is melting into the ocean, raising sea levels.  If trends continue, this will raise mean sea level by several meters or more, producing major coastal flooding, at the very least.

And the news from the South is the good news.  Another team report a study of the correlation between the North Atlantic Oscillation (NAO) and the melting of the Greenland ice [1].  The NAO is a fluctuation cycle of air currents, i.e., the winds from the Western ocean onto Europe.  These winds bring more and less stormy and warm weather to Greenland as well as the continent.

“The negative phase of the NAO enhances summertime warming and insolation while reducing snowfall, especially in west Greenland, driving surface mass balance (SMB) more negative, “  ([1], p.1)

This study combines measurements of the mass of the ice inferred from the Gravity Recovery and Climate Experiment (GRACE) satellite and the Greenland GPS Network (GNET) surface measurements.  These data indicate the depth of the ice revealed in changes in gravity and the vertical movements of the rock underneath.

(When you think about it, this is really awesome science:  measuring the ongoing changes to the mass  and rise and fall of the crust.)

The results suggest that in recent decades the Greenland ice and snow has closely correlated with these winds, apparently more closely than in earlier times.  They interpret this to indicate that the mass balance in Greenland is much more responsive to the NAO cycles now, possibly indicating that the system has passed a “tipping point”.

“Greenland’s air-sea-ice system crossed one or more thresholds or tipping points, near the beginning of this millennium, triggering more rapid deglaciation” p.6

Specifically, the southwest coast of Greenland is receiving warmer winds, and melting rapidly. The patterns are not only related to the fluctuating winds, but also to the topography. Ocean terminating glaciers are melting very fast, and low lying ice is receiving less snow, leading to overall loss of mass.  Inland and high altitude ices is less affected by short term warming, and is changing relatively little.

“The decadal acceleration in mass loss in southwest Greenland arose due to the combination of sustained global warming and positive fluctuations in temperature and insolation driven by the NAO.”  (p. 6)

The “tipping point” argument is basically that the data show that “mass acceleration scales with the trend or rate of change of SMB and DMB”, i.e., the ice is responding rapidly to changes in  conditions, so all the continuing sources of warming are having immediate and larger effects every year.

As they say, “we infer that within two decades this part of the GrIS will become a major contributor to sea level rise.“

Extreme sensitivity to environmental conditions means that, in principle, human actions to mitigate atmospheric and ocean warming will reverse the trends, and increase the ice mass.  However, it isn’t clear exactly how we might do that, and it is clear that it would take decades to make much difference.  By which time, the ocean will be over our heads.

So, get used to the idea that the ice will be gone, and the oceans will be higher.  Much higher.

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1. Michael Bevis, Christopher Harig, Shfaqat A. Khan, Abel Brown, Frederik J. Simons, Michael Willis, Xavier Fettweis, Michiel R. van den Broeke, Finn Bo Madsen, Eric Kendrick, Dana J. Caccamise, Tonie van Dam, Per Knudsen, and Thomas Nylen, Accelerating changes in ice mass within Greenland, and the ice sheet’s sensitivity to atmospheric forcing. Proceedings of the National Academy of Sciences:201806562, 2019. http://www.pnas.org/content/early/2019/01/14/1806562116.abstract
2. Eric Rignot, Jérémie Mouginot, Bernd Scheuchl, Michiel van den Broeke, Melchior J. van Wessem, and Mathieu Morlighem, Four decades of Antarctic Ice Sheet mass balance from 1979–2017. Proceedings of the National Academy of Sciences, 116 (4):1095, 2019. http://www.pnas.org/content/116/4/1095.abstract