As the ice melts all over the Earth, particular attention centers on Antarctica. If and when the Southern ice cap melts, the oceans will rise by tens of meters, drowning much of where humans live. When will this happen, if ever? We need to know.
Antarctica is a continent, and that means it is big and complicated. Some places are melting fast. Others aren’t changing much, and some may actually be accumulating more ice.
One of the unknowns is what is under the ice. We can easily see the top of the snow and ice (at least some of the time.) But what is going on in the kilometers deep ice, and what is at the bottom?
The is actually a critical question for several reasons. Ice that is resting on dry, cold, rock will be solidly anchored and will generally melt only where exposed to air and ocean. But ice resting on water will melt and slide downhill, generally ending up in the ocean, where it will melt. And ice resting on warm rock will melt from below.
In Greenland, there are areas where volcanic heat under the ice is thinning and weakening the ice, and speeding the rate of melting.
This fall, researchers from Europe report on studies using airborne ice penetrating radar near the South Pole . The study identifies an area that is a geothermal hot spot under the ice, where the overlying 2+ km deep ice cap is sagging.
The saggy spot is approximately 100 x 50 km, and the sag is consistent with melting of about 6 mm per year. This seems to be much more than would be expected from the rocks, so the researchers suggest that there is geothermal flows of hot water from deeper rocks.
This finding is important for numerical models of the ice, which generally use a uniform, lower estimate of the heat of the rocks. This hot spot, and other similar anomalies could have substantial effects on the changes to the ice cap.
The study is also interesting because it uses ice penetrating radar to create a three dimensional view of the ice, from which they discover the ‘sag’ and inferred the estimated rate melting. The result gives a way to estimate the heating under the ice, even where no direct measures are available. (And direct measures of the rock under the ice cap are very difficult to obtain.)
And, as noted, in this case the estimate is much higher—more than twice as high—as previous estimates. It is noted that these results may explain the presence of numerous subglacial lakes in that area.
The details of the techniques are beyond my own paltry understanding of either ice or radar. Given the complex dynamics of the ice cap, there will need to be additional studies and cross validation to make sure these estimates are accurate.
These results will also guide ongoing plans to sample the ancient ice from this area. It is thought that this area has some of the oldest ice on the planet, and examination of embedded air bubbles would reveal a history of climate far into the past. However, the ice in this warm, sagging zone will have remelted and would not retain the ancient bubbles. So the search for records of the climate should avoid these areas .
- Jonathan Amos, South Pole: Rock ‘hotspot’ causes ice sheet to sag, in BBC News – Science & Environment. 2018. https://www.bbc.com/news/science-environment-46202255
- T. A. Jordan, C. Martin, F. Ferraccioli, K. Matsuoka, H. Corr, R. Forsberg, A. Olesen, and M. Siegert, Anomalously high geothermal flux near the South Pole. Scientific Reports, 8 (1):16785, 2018/11/14 2018. https://doi.org/10.1038/s41598-018-35182-0