One of the most important questions is what is happening in Antarctica, which has vast amounts of ice, and is subject to complex forces that may be thinning and ultimately melting the ice. If and when Antarctica fully melts, sea levels will rise tens or meters, which is pretty much the end of the story for human life as we know it.
This spring a group or European researchers report a new analysis of satellite observations that estimates the changes in the grounding lines of glaciers in Antarctica . The grounding line is the border where a glacier first extends out over the ocean. This line may migrate when ocean water seeps under the glacier, generally due to the bottom of the glacier melting. Furthermore, glaciers melt faster over water, so grounding lines migrating inland probably indicates the glaciers will be melting faster.
It is difficult to observe conditions deep under the ice, and there are a lot of glaciers in Antarctica. Some might be melting faster and others not.
The new study used radar altimetry from CryoSat-2, which measures the altitude of the surface of the ice. Using the instrument to measure the bedrock, ocean, and elevation of the ice, the study developed estimates of the movement of the grounding lines for 34% of Antarctica. To date, there have been only a few such estimates for limited areas, so this work greatly expands the view of the overall activity.
The BBC produced a nice diagram of how the measurement works.
The study concludes that most of the glaciers are stable, 10.7% of the Antarctic grounding line retreated and 1.9% advanced faster than 25m/yr. (25 m/yr is the “typical” rate of retreat since the last glaciation.) This was not the same everywhere, some areas are retreating and some advancing. But overall, “grounding-line retreat […]coincided with sectors in which ice streams are known to be thinning”. In fact, the study shows that on average, the grounding-line retreats 110m for every meter the ice thins.
This study is important because it gives a broad view across most of the continent. This view is particularly important given the variations locally and even in different years.
While these findings are certainly consistent with other data, they also show how important local conditions are. For example, some areas where the grounding-line is not changing have bedrock topography that holds the water out. Other areas show large changes in the grounding line in some periods and not others, indicating that local conditions play a strong, if unknown, role.
(I’m not sure that the BBC headline “Antarctica ‘gives ground to the ocean‘ accurately describes the findings. And who is that supposed to be a quote from?)
This is a neat study, and it shows a great strength of satellite observations to obtain continental scale datasets, even where it is very difficult for humans to visit.
Clearly, there should be corroborating studies from other instruments and, ideally, ground truth. Not that it will be easy to measure the underside or thickness of these glaciers.
- Jonathan Amos, Antarctica ‘gives ground to the ocean’, in BBC News – Science & Environment. 2018. http://www.bbc.com/news/science-environment-43627673
- Hannes Konrad, Andrew Shepherd, Lin Gilbert, Anna E. Hogg, Malcolm McMillan, Alan Muir, and Thomas Slater, Net retreat of Antarctic glacier grounding lines. Nature Geoscience, 11 (4):258-262, 2018/04/01 2018. https://doi.org/10.1038/s41561-018-0082-z