Environmental phenomenon: how are the lakes collapsing and releasing meltwater during the winter

In the heart of winter 2018, a lake of water thaw almost 50 years ago disappeared from the surface of the ice in West Greenland. The lake was covered with snow and ice when it collapsed, but stored liquid water inside. The water disappeared into the newly formed cracks and slid through the approximately 2 km thick ice cap. He water it struck the bedrock under the ice and flowed from below the ice sheet into the sea.

This meltwater acted as a lubricant between the bedrock and the thick ice on top. As a result, the large mass of ice could slide faster towards the coast, accelerating an unusually large region of inland ice. The drainage of this lake caused the collapse of several other lakes in the adjacent area. In total, the collapsed lakes released approximately 180 million tons of meltwater that ended up in the world’s oceans.

This phenomenon has aroused the interest of a team of international researchers who have just shown for the first time how 18 lakes Meltwater in Greenland collapses during the winter, causing the edges of the ice to flow faster. This new discovery is essential for understanding how climate change influences the flow of ice masses in the Arctic.

This activity is illustrated by new international research based on satellite data and carried out by the University of Grenoble Alpes in France with contributions from DTU Space of the Technical University of Denmark (DTU) which has just been published in Geophysical Research Letters.

Meltwater lakes on the surface of the ice form in the summer when the ice melts. melt. It is well known that these lakes can collapse and empty during the summer. But surprisingly, this also happens in winter. This is the first time that these specific lake drainages have been shown to cause large ice accelerations in winter, when temperatures are very low,” explained post-doctoral researcher and researcher Nathan Maier, lead author of the study. ‘article.

As a researcher of Grenoble Alpes University in France, he led the broad international research collaboration behind the discovery. Today, he is a researcher at the Los Alamos National Laboratory in the United States. “In total, the lakes dried up as part of this incident resulted in 180 million tonnes of water spilling into the sea. This is roughly equivalent to the content of 80,000 Olympic swimming pools which are 50 by 25 by 2 meters,” said Nathan Maier.

The 50-year-old lake, which was the first to be drained, was located about 160 km inland at the top of the ice cap, and had meltwater and a frozen ice cap in reason for the low temperatures of winter. When it collapsed and water flowed under the ice on the west coast of Greenland, it set off a cascade of events that also dried up other lakes. Among other things, the pressure of water flowing under the 50-year-old lake ice likely contributed to more cracks forming in the ice above, causing these lakes to also leaks.

A total of 18 lakes collapsed, accelerating an area of 5200 square kilometers of the ice cap. The researchers note that this happened during a month in the winter of 2018 when the air temperature was below freezing. “We have only investigated a limited area, but we have good reason to suspect that similar events are occurring in many other places around the world. Greenland. If this applies to larger parts of the ice sheet, it could be quite large amounts of meltwater disappearing in this way and causing the ice sheet to slide out to sea more quickly,” Jonas said. Kvist Andersen, postdoctoral researcher at DTU Space in Denmark and co-author of the article.

The area studied mainly includes the large Jakobshavn Isbræ, which flows into the sea in West Greenland and is the fastest-moving glacier in the world, as well as a smaller glacier to the south which ends on land. It seems obvious to conclude that the lakes began to collapse in winter due to the global warming. Especially when a nearly 50-year-old lake suddenly empties in the middle of winter and meltwater finds its way into the sea and contributes to sea level rise. But that’s not a given, according to Researchers.

“It is not yet known if drains like these will be more frequent in a warmer future and then they will further contribute to the mass loss of the ice sheet. Further research is needed to better understand the mechanisms, or triggers, that cause lakes to dry up. At present, our understanding of how surface melting will affect Greenland’s future mass loss rests entirely on the assumption that melting only affects the speed of ice flow during the summer. . Our discovery, by which we detected that the large accelerations of ice flow caused by the drainage of stored meltwater during the winter, dramatically changes the way we understand ice sheet hydrology on annual time scales,” said Nathan Maier.

The researchers arrived at the new results by analyzing large amounts of radar data e satellite optical images. For this reason, they considered that winter meltwater drainage should be included in new climate models. It’s not just the oldest lakes that are collapsing after having existed for decades. There are several types of cycles, according to scientists. Some lakes form and collapse within a year; for others, it happens every few years. Collapsed lakes affect the ice cap, or glacier, melting in Greenland in several ways. On the one hand, the water from the lakes ends up in the sea.

It also lubricates the ice sheets from below, causing them to slide more quickly towards the coast, exposing them to further melting. Moreover, it changes the structure of the huge masses of ice. Specialists speculate that there could also be other mechanisms at play. “It is essential to describe what happens when the melting process takes place in winter so that this knowledge can be included in future models of climate change. “said Jonas Kvist Andersen. Researchers used synthetic aperture radar (SAR) interferometry based on data from the European Space Agency’s (ESA) Sentinel-1 satellites to map how the 18 lakes of the West Greenland Ice Sheet are drained and how the water went away afterwards. at the sea.

Visual and optical photos were recovered from other European and American satellites. They were used to identify the lakes and their evolution for a few months in the winter of 2018. The radar images were supplemented by older satellite photos. In this way, the scientists were able to follow the development of lakes over several decadeseven fixed when they have been emptied.

The Sentinel-1 satellites, which cover the Arctic from an orbit just under 700 km above Earth, have a SAR unit, which sends radar signals obliquely towards the surface of the ice sheet, from where they are sent back to the satellite. By analyzing the differences and trips in the phase of the radar signal, it is possible to measure the movement of the ice surface relative to the satellite.

When various measurements are compared, a distinction can be made between horizontal motion (when ice flow is accelerated) and vertical motion (when meltwater pushes the overlying ice upward). In this way, information was obtained about the movement of meltwater and ice after the water drained from the bottom of the lake. Jérémie Mouginot, Florent Gimbert and Olivier Gagliardini also took part in the survey.

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