Both Greenland and Antarctic Ice Sheets Melting from Below
/Amidst all the hype over melting from above of the Antarctic and Greenland ice sheets due to global warming, little attention has been paid to melting from below due to the earth’s volcanic activity. But the two major ice sheets are in fact melting on both top and bottom, meaning that the contribution of global warming isn’t as large as climate activists proclaim.
In central Greenland, Japanese researchers recently discovered a flow of molten rocks, known as a mantle plume, rising up beneath the island. The previously unknown plume emanates from the boundary between the earth’s core and mantle (labeled CMB in the following figure) at a depth of 2,889 km (1,795 miles), and melts Greenland’s ice from below.
As the figure shows, the Greenland plume has two branches. One of the branches feeds into the similar Iceland plume that arises underneath Iceland and supplies heat to an active volcano there. The Greenland plume provides heat to an active volcano on the island of Jan Mayen in the Arctic Ocean, as well as a geothermal area in the Svalbard archipelago in the same ocean.
To study the plume, the research team used seismic topography – a technique, similar to a CT scan of the human body, that constructs a three-dimensional image of subterranean structures from differences in the speed of earthquake sound waves traveling through the earth. Sound waves pass more slowly through rocks that are hotter, less dense or hydrated, but more quickly through rocks that are colder, denser or drier. The researchers took advantage of seismographs forming part of the Greenland Ice Sheet Monitoring Network, set up in 2009, to analyze data from 16,257 earthquakes recorded around the world.
The existence of a mantle plume underneath Antarctica, originating at a depth of approximately 2,300 km (1,400 miles), was confirmed by a Caltech (California Institute of Technology) study in 2017. Located under West Antarctica (labeled WA in the next figure), the plume generates as much as 150 milliwatts of heat per square meter – heat that feeds several active volcanoes and also melts the overlying ice sheet from below. For comparison, the earth’s geothermal heat is 40-60 milliwatts per square meter on average, but reaches about 200 milliwatts per square meter beneath geothermally active Yellowstone National Park in the U.S.
A team of U.S. and UK researchers found in 2018 that one of the active volcanoes drawing heat from the mantle plume in West Antarctica is making a major contribution to the melting of the Pine Island Glacier. The Pine Island Glacier, situated adjacent to the Thwaites Glacier in the figure above, is the fastest melting glacier in Antarctica, responsible for about 25% of the continent’s ice loss.
The researchers’ discovery was serendipitous. Originally part of an expedition to study ice melting patterns in seawater close to West Antarctica, the team was surprised to find high concentrations of the gaseous helium isotope 3He near the Pine Island Glacier. Because 3He is found almost exclusively in the earth’s mantle, where it’s given off by hot magma, the gas is a telltale sign of volcanism.
The study authors calculated that the volcano buried underneath the Pine Island Glacier released at least 2,500 megawatts of heat to the glacier in 2014, which is about 60% of the heat released annually by Iceland’s most active volcano and roughly 25 times greater than the annual heating caused by any one of over 100 dormant Antarctic volcanoes.
A more recent study by the British Antarctic Survey found evidence for a hidden source of heat beneath the ice sheet in East Antarctica (labeled EA in the figure above). From ice-penetrating radar data, the scientists concluded that the heat source is a combination of unusually radioactive rocks and hot water coming from deep underground. The heat melts the base of the ice sheet, producing meltwater which drains away under the ice to fill subglacial lakes. The estimated geothermal heat flux is 120 milliwatts per square meter, comparable to the 150 milliwatts per square meter from the mantle plume underneath West Antarctica that was discussed above.
All these hitherto unknown subterranean heat sources in Antarctica and Greenland, just like global warming, melt ice and contribute to sea level rise. However, as I’ve discussed in previous posts (see here and here), the giant Antarctic ice sheet may not be melting at all overall, and the Greenland ice sheet is only losing ice slowly.
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