Earth observing satellites have created a new age of science, giving us a continuous record of our planet over decades. Crucially, satellites measure everywhere, including hard to reach areas in the middle of the ocean and the ice covered poles. This has given us new insights into the behavior of the ice in the north and in Antarctica.
But one thing satellites can’t do very well is see what’s going on under the ice. And this matters a lot, because the rock and water under the ice is critical for how the ice melts or accumulates.
To be fair, it isn’t easy to observe under the ice at Antarctica or anywhere. Aircraft only see the surface. Instruments on the ice don’t see the underside or the water. Ships can only visit the edges.
This summer, researchers report on an expedition in 2019 to the Thwaites glacier in West Antarctica which deployed an autonomous robot submarine to measure the water under Thwaites [3]. The submarine collected sonar maps of the bathymetry, as well as temperature and chemistry of the water.
The Thwaites glacier is particular interesting and important because it is large (Gigatons of ice, enough to raise sea level a half meter and more). And it has been changing, moving faster and slower, and shrinking towards the land.
The new research has found a complex undersea geography, with channels of cold and warm water. Many of the details were not known from satellite data or from theoretical models.
Among other things, a large southward (i.e., in towards shore) flow of warm water, major outflows of melt water, and areas where cold and warm water are mixing.
The same expedition explored a “knob” on the sea floor that has an intriguing pattern of grooves [1]. These formations are interpreted as formed in the past when the glacier retreated over that area. Basically, the ice rises a bit at high tide and drops at low tide. When the ice moves, it drops a little one way or another from where it rose. When the glacier is moving rapidly, this leaves a fingerprint of these tidal motions.
And the fingerprint can be used to estimate the speed of the glacier.
Cool!
The data indicates that sometime in the past, the glacier retreated very fast, in a burst that is several times what has been observed in the satellite era. It isn’t clear when this burst of activity occurred, it was probably 70 years ago or even farther in the past. (This is now considered “the pre-satellite era”.)
This finding shows that glaciers have retreated suddenly and rapidly in the past, and presumably can do so in the future. These bursts have not been observed by ships or satellites, so current theories do not account for them in predictions.
Overall, this finding suggests that at Thwaites and elsewhere changes are not necessarily following a steady or constantly accelerating trajectory.
And, of course, these surprises show why we need to actually collect data in situ.
- Alastair G. C. Graham, Anna Wåhlin, Kelly A. Hogan, Frank O. Nitsche, Karen J. Heywood, Rebecca L. Totten, James A. Smith, Claus-Dieter Hillenbrand, Lauren M. Simkins, John B. Anderson, Julia S. Wellner, and Robert D. Larter, Rapid retreat of Thwaites Glacier in the pre-satellite era. Nature Geoscience, 2022/09/05 2022. https://doi.org/10.1038/s41561-022-01019-9
- Elizabeth Rush, What Antarctica’s Disintegration Asks of Us, in New York Times. 2022: New York. https://www.nytimes.com/2022/09/08/opinion/environment/antarctica-ice-sheet-climate-change.html
- A. K. Wåhlin, A. G. C. Graham, K. A. Hogan, B. Y. Queste, L. Boehme, R. D. Larter, E. C. Pettit, J. Wellner, and K. J. Heywood, Pathways and modification of warm water flowing beneath Thwaites Ice Shelf, West Antarctica. Science Advances, 7 (15):eabd7254, https://doi.org/10.1126/sciadv.abd7254
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