It isn’t news that parts of the Antarctic ice cover are melting rapidly as a result of warming ocean temperatures – with associated implications not just for sea level but for atmospheric and ocean circulation.
But the Earth system is complex; it isn’t just the warming of the sea which affects the rate of melting.
The earth’s internal heat, or geothermal flux (the heat resulting from the decay of radioactive isotopes) makes a contribution to the wasting of ice sheets as well.
These fluxes, which vary according to things such as rock type and levels of volcanic activity, are key key inputs to any attempts to model rates of melting in the future.
The problem is that these inputs are difficult to assess – until now.
The Antarctic – the Great Unknown
The Antarctic continent is empty and still largely unknown. It’s big – at around 5.4 million square miles, Antarctica is almost twice the size of Australia – and it’s covered by an ice sheet which, according to the Natural History Museum’s Antarctic Fact File, “contains 90% of the world’s ice and 70% of the world’s fresh water. At its thickest point, the ice sheet is 4.7km deep.”
That’s almost three miles deep.
Configuration of the continents under all that ice means it isn’t easy for climate modellers to make an accurate assessment of the geothermal flux – and its contribution to melting. Most models have relayed on a broad assumption which takes little or no account of variation, which affects the outputs of the models.
Unfortunately, the presence of over 30 volcanoes on or around Antarctica (most notably the currently erupting Mount Erebus) clearly indicates that such broad estimates aren’t ideal – especially when studies of ice wastage show considerable variation across the content.
Given its size, it would be surprising if the Antarctic continent were structurally uniform and indeed, as the Smithsonian Museum’s Global Volcanism Program notes, “although it is generally thought to be “aseismic and immobile, is broken internally by large rift structures which have produced one of the world’s largest alkalic volcanic provinces. The 3,200-km-long West Antarctic rift system is comparable in size to the better-known East African rift.”
Which means not only that there’s a significant geothermal heat flux but also that it’s variable.
Calculating the Geothermal Flux
So, how do you work out what the level of heat is under all those thousands of metres of ice? Well, with modern techniques it’s possible to ‘see’ through the ice to the patterns of water flowing beneath the ice sheet.
The results of a study, published in the Proceedings of the National Academy of Sciences of the United States of America, show that patterns reflect the sources and quality of the melt and can be used to put a more refined value in the influence of geothermal melting.
And the ground below Antarctica is hotter than you might think. For an area of West Antarctica, which includes the rapidly-melting Thwaites Glacier, the study gives a flow of “100 milliwatts per square meter, with hotspots over 200 milliwatts. For comparison, the average heat flow of the Earth’s continents is less than 65 milliwatts per square meter.”
“Before our paper, models tended to just assume a uniform geothermal flux value beneath the glacier because it was the best you could do with the observations available (even though the presence of nearby volcanoes and other geologic evidence suggested it was probably very non-uniform),” the study’s lead author, Dustin Schroder, PhD, told Decoded Science.
“Our results provide an observation-based set of geologically realistic geothermal flux values that modelers can use to make much more realistic simulations of the future behavior (and sea level contribution of Thwaites Glacier),” he went on.
Antarctic Study: Implications for Climate Modelling
The rapid rate of wasting of parts of the WAIS, in particular the Thwaites Glacier, has serious implications for the planet – it’s estimated, for example, that the collapse of the Thwaites Glacier alone would cause an increase of global sea level of between 1 and 2 meters, with the potential for more than twice that from the entire West Antarctic Ice Sheet. And the faster the melt rate, the faster the increase in sea level.
The situation is complex.
“These two components (warm ocean water and geothermal flux) along with configuration of the subglacial water system, the shape of the ice sheet bed, and the distribution of deformable sediments and bedrock beneath the ice sheet will determine the ultimate timing, pacing, and character of the retreat of Thwaites Glacier and its eventual spread to the rest of the West Antarctic Ice Sheet,” explained Dr. Schroder.
“What is clear is that as Thwaites retreats in response to warm ocean water, its bed will be move across a much more thermally and hydrologically significant piece of geology than was previously thought. So it’s not just the ocean that’s the shaping the fate of the ice sheet, it’s also the earth beneath it.”
Melting Ice Sheets: Looking to the Future
Along with the WAIS as a whole, the Thwaites Glacier is the focal point for much study because of the rapid rate of melt. But it represents only a part of the findings and the study’s results will have wider implications for improving the quality of climate modelling.
“We can apply it to other areas where we expect that high (and spatially varying) geothermal heat flow might be playing a significant role. The most likely candidates are other areas in West Antarctica near to the West Antarctic Rift System,” said Dr. Schroder.
Knowing how the ice sheet works may not be the solution to the problems of ice sheet collapse and sea level rise; but the results of the study have the potential to increase our understanding of the complexities of the Earth system – and perhaps to get to grips with it.