The lake before June 2019 and after, as a doline. Image: Warner et al, Geophysical Research Letters, 2021
A massive lake, filled with billions of cubic feet of water, vanished within the span of a few days in Antarctica, leaving a sinkhole that is visible in images captured from outer space, reports a new study.The bizarre disappearance of the lake reveals unprecedented new details about the dynamics occurring at the surface of Antarctic ice shelves, and could shed light on whether instabilities in the region are linked to human-driven climate change.
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Roland Warner, a glaciologist with the Australian Antarctic Program Partnership at the University of Tasmania, first noticed signs of the missing lake while examining satellite imagery captured in January 2020, which “serendipitously revealed a striking collapsed surface structure” at Amery Ice Shelf, according to the study, which was published last Wednesday in Geophysical Research Letters.At this time, Australia was gripped by an intense bushfire season, and Warner had been regularly checking satellite images of the damage. “Looking down to Antarctica, for a break from watching the destruction, I noticed a spell of several clear days on Amery Ice Shelf and decided to see how the summer surface melt season was progressing,” he said in an email. “The collapsed surface feature caught my eye.”Warner thought the structure might be a doline, or sinkhole, that could point to a dramatic and recent drainage event. With the help of sophisticated satellites such as NASA’S WorldView and the European Space Agency’s Sentinel constellation, he was able to look back in time and pinpoint when the vast lake emptied out through channels that burst open in the ice below it. Warner checked his findings with researchers who study surface meltwater patterns on Amery Ice Shelf, and who became co-authors of the study.
“It was bringing together the expertise of the team that made this more than just an ‘Oh! Look what I found’ paper,” he said. “Without the satellite data we pulled together we would not have achieved such a quantitative assessment of the scale and size of the disruption caused by the lake draining.” Indeed, data from NASA’s ICESat-2 captured during the southern winter revealed that there was a huge intact lake consisting of meltwater at the site of the doline on June 9, 2019. It held an estimated 21 to 26 billion cubic feet of water, equivalent to twice the volume of San Diego Bay. On June 11, the lake was gone. Warner and his colleagues think that the weight of the lakewater exceeded what the ice could bear, sparking a process called “hydrofracture” in which burst fissures allow water to flow through the ice shelf and out into the ocean. As the water levels rapidly fell, an ice lid that perennially covered the lake collapsed some 260 feet and then flexed upwards about half that distance, creating the distinctive doline structure that first tipped Warner off to the lake’s disappearance.While hydrofractures have been implicated in the collapse of more fragile ice sheets, a drainage event of this size and speed has never been observed in detail before. The loss of the lake is especially unusual considering that it was supported by a relatively sturdy part of Amery Ice Shelf, which is about 4,590 feet thick.
Since the main lake basin is visible in satellite observations dating back to 1973, the June 2019 collapse appears to be its first significant drainage event for nearly half a century, and provides an “unprecedented opportunity to observe doline formation and details of the associated surface meltwater features,” according to the study.“I think the depth of the lake, the large volume of water, and the implication that it took decades to accumulate it is a new insight,” Warner said. The doline appears to be seasonally collecting a small amount of meltwater again, which will make it an interesting place to watch in order to better understand the surface hydrology of ice shelves in Antarctica. This topic is especially important as the study notes that the consequences of surface melt on the future stability of Antarctic ice shelves are “inadequately understood.” For instance, little is known about how much this meltwater is contributing to rising sea levels, or if individual drainage events are linked to human-driven climate change. Climate change models do predict an increase in surface meltwater on Antarctic ice shelves as a result of higher global temperatures, but Warner points out that more field data is needed “to improve the skill of the climate models in predicting surface melting.”“With only limited observational data, the main thing we know about this part of Amery Ice Shelf is that the summer melt seasons are highly variable year to year,” he said. “There is not enough data to see a trend in the data from the recent past.”The sudden vanishing act of the lake offers a window into the complex processes of this remote icescape in an era of rapid climate change. To that end, Warner said that he and his colleagues plan to continue monitoring the doline in the coming years, while also reconstructing the evolution of the lost lake using the wealth of space imagery captured since the 1970s. “How did the lake evolve to develop its thick insulating ice lid and eventually store such a large volume of water?” Warner said. “We can look back at the surface history of the system in several decades of satellite images.”