The Thwaites Glacier, in West Antarctica, is one of the glaciers considered most at-risk for collapse over the next century, earning it the nickname “doomsday glacier.” Scientists have been studying the glacier with increasing concern over the past decade and they’ve learned a great deal about the interaction between the glacier and the bedrock it sits on as a result. Unfortunately, most of what they’ve learned has been ominous — and today’s news is no exception.
In January, for the first time, scientists bored a hole in the Thwaites Glacier more than 600 meters deep. The goal was to measure what’s happening underneath the glacier at the grounding line — the point where the glacier shifts from resting on bedrock to resting on / in the water. Ice that’s already in the water doesn’t have much impact on sea level when it melts because the only thing that’s happening is a phase change from solid to liquid. Ice that’s formerly been landlocked, on the other hand, is a net addition to the system and has a corresponding impact on sea-level rise.
The rate of glacier melt isn’t just impacted by ambient air temperature or solar radiation. The interaction between warmer seawater and colder sea ice at the grounding line has a major impact. As warmer water pools underneath the glacier, it can hollow it out from beneath. Researchers found exactly this situation at Thwaites last year, when they discovered a cavern two-thirds the size of Manhattan and nearly a thousand feet tall underneath the glacier. Now they discovered that the water making contact with the glacier at the grounding line is more than two degrees warmer than the glacier itself. You’d expect the water to be a little warmer — after all, it’s in a liquid state — but this is a larger variance than expected.
The problem with this kind of melt is that it can drastically exacerbate how much of the glacier is exposed to warm water, as the image below illustrates:
When the glacier was thicker, the grounding line sat just below the bedrock hump shown. As the ice thinned, water flowed past it, penetrating much farther inland than it had previously. Finding warm water at the junction between ground and ice implies that this kind of change could be occurring underneath the glacier.
“The fact that such warm water was just now recorded by our team along a section of Thwaites grounding zone where we have known the glacier is melting suggests that it may be undergoing an unstoppable retreat that has huge implications for global sea-level rise,” notes David Holland, director of New York University’s Environmental Fluid Dynamics Laboratory and a professor at NYU’s Courant Institute of Mathematical Sciences.
To measure this information, the team deployed Icefin, a submersible drone designed to gather information where no human could venture. The vehicle swam nearly two kilometers from the drill site, directly up to the iceberg’s grounding zone, to take its readings.
None of the gathered data indicates that the Thwaites Glacier has reached some critical melt point where the decline will suddenly begin to accelerate. But the terrain under glaciers is notoriously difficult to examine, given hundreds of meters of ice stacked on top of it. The Thwaites Glacier is already considered to have contributed about four percent of total sea-level rise since the beginning of the industrial era, but it’s not the most-vulnerable or fastest-melting glacier in Antarctica. That honor goes to the Pine Island glacier, which shows evidence of being in an even more precarious position, with the base of the glacier sitting on bedrock below sea level. This implies that there is no geological feature to prevent water from penetrating under the glacier and reaching to the depths of its base.
If you’re wondering how bedrock could be below sea level, I’d like to introduce you to a new cool thing our planet does: isostatic rebound. Ice is incredibly heavy, and the weight of carrying ice sheets can depress even a continent-sized slab of rock. As the ice sheets melt and it gets therapy, the continental craton becomes less depressed. You can see evidence for this effect at Bathurst Inlet in Nunavut:
Much of Antarctica’s rock is currently below sea level thanks to the weight of its glaciers, including apparently, Pine Island. We know that there are streams and rivers of water underneath glaciers, as well as subglacial lakes, like Lake Vostok. I doubt any of the scientists on the expedition were surprised to find such warm water underneath the glacier. But it’s an unwelcome sign of how fragile the Western Antarctic ice sheet could prove to be in the decades to come.
Top image: Cavity Camp on Thwaites Glacier. Credit: Ted Scambos
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