Two new studies, published in the journal Nature, confirm the retreat of the Thwaites Glacier (192,000 km2), in West Antarctica, one of the observation points for the effects of warming that scientists follow most closely. The Thwaites is the size of Great Britain or a third of the entire Iberian Peninsula and is one of the fastest changing ice and ocean ecosystems in Antarctica. The grounding zone, where it meets the seafloor, has receded 14 kilometers since the late 1990s.
The study of the loss of ice in West Antarctica arouses scientific interest because this is one of the areas that contributes the most to the rise in sea level. Until now, thaws in the Antarctic Peninsula were known; and, more recently, western areas of the frozen continent are the subject of attention.
Thwaites Glacier sits on bedrock that slopes down towards the coast. In his case, the concern arises from the fact that in a large region the base of the ice rests on an overexcavated bed below sea level and is, therefore, susceptible to rapid and irreversible loss of ice.
This particular configuration makes this type of glacier particularly susceptible to instability in the extent of ice in contact with the sea, which could trigger the collapse of the entire glacier system and the entry of a large amount of ice into the ocean. from the interior of the Antarctic continent.
"Thwaites may already have entered a state of rapid and irreversible ice loss and its complete collapse in a few hundred years would contribute to a 65-centimeter rise in sea level," says the first article.
Furthermore, in the process it could destabilize its nearby glaciers in the Amundsen Sea, leading to a global rise of 3 meters over several thousand years.
The new observations, developed in the vicinity of the anchor line - that is, where the base of the glacier detaches itself from the bedrock and enters the ocean in the form of a floating ice shelf - show that, although the melting in large part of the floating ice shelf is weaker than expected, melting in cracks and crevices of the glacier itself is much faster.
Although the new data reduces the level of melting, the glacier continues to retreat. The new findings offer a better understanding of how this large glacial system could contribute to future sea level rise across the planet.
Antarctic glaciers with the capacity to contribute to sea level rise (if they collapse) should be imagined as something similar to the visor of a grandstand in a football stadium, in which the overhang would be equivalent to the ice shelf that it extends and floats on the sea from the anchor line or place from which the cantilever starts.
For this reason, there are three areas of the glacier system where ice melting can occur: on its surface, due to contact with a relatively "warm" atmosphere; at the base of the floating ice shelf, due to contact with seawater (which can be more or less cold); and, thirdly, inside and at its base in contact with the rocky substratum. It must be taken into account that friction with the substrate (stone) slows down the discharge of ice towards the sea and, therefore, contributes decisively to stabilizing the glacier.
In addition, depending on environmental conditions, more or less large volumes of meltwater can circulate inside and at the base of the glacier, sometimes in excess pressure. When these waters form pockets, or subglacial lakes, between the ice base and the rocky substratum, they can accelerate the rate of advance of the ice towards the sea.
One of the two studies that appeared in Nature, the one led by Peter Davis of the British Antarctic Survey (BAS), focuses on this last point. The results show that although melting has increased, the current rate is slower than many computer models had currently estimated.
“Our results are a surprise, but the glacier is still in trouble,” says Davis, an oceanographer. “If an ice shelf and a glacier are in equilibrium, the ice leaving the continent will equal the amount of ice lost through melting and calving of icebergs. What we have found is that despite the small amounts of melt, there is still rapid retreat of the glacier, so it appears that it does not take much to unbalance it,” he adds.
Peter Davis's team drilled a 600 meter deep well (using a heated water drill) about two kilometers seaward of the ice shelf anchor line, to test the waters below, between the base of the floating ice and the seabed.
To do this, they had to cross the entire thickness of the ice that forms the floating platform, until they reached the liquid water below.
These measurements were compared to melt rate observations taken at five other sites below the ice shelf. And it was concluded that the rate of melting at the base of the ice was between 2 and 5 m per year: less than the previous model indicated.
Beneath the ice was found "a column of warm and highly stable water with temperatures substantially higher than the freezing point in situ." But, despite these warm conditions, “low current velocities and strong stratification of the water column effectively restrict vertical mixing and the arrival of heat at the ice base, resulting in a strong reduction in the basal fusion,” the study says.
In this drilling, “as the depth increased, the waters were relatively warmer; but these do not mix with the cooler waters at the top of the column. They are layered. The cold waters are at the top, in contact with the base of the floating ice, and the less cold, or warmer, are at the bottom, in contact with the seabed. Therefore, since the warmer waters are not in contact with the base of the ice, its melting is much less”, says Miquel Canals, professor of Marine Geosciences at the UB (who did not participate in the study).
In a second study, Dr. Britney Schmidt, from Cornell University in the US, and a team of scientists and engineers deployed a robotic vehicle called the Icefin through the 600m-deep well. The vehicle is designed to access the anchor line, a critical element for the stability of the glacier system that was previously almost impossible to inspect.
Icefin's observations of the seafloor and ice around the anchor line provide highly relevant additional information about how melt varies below the ice shelf. The scientists discovered that stairways, or terraces, as well as cracks in the ice base are melting rapidly. Melting is especially important in cracks, as meltwater from the surface and inside the glacier channels heat through them, facilitating the transfer of salt from seawater to the ice, thus widening the cracks. .
Therefore - and by way of summary - although the vertical melt at the base of the ice shelf was less than expected, the melt along the inclined ice in these cracks and terraces is much greater and may be an important factor. in ice loss on Thwaites Glacier. And as major rifts progress along the glacier, the ice shelf may become the main trigger for ice shelf collapse, Nature says.
“These new ways of looking at the glacier allow us to understand that it's not just about how much is melting, but how and where it's happening, which matters especially in these very warm parts of Antarctica. The warm water is coming in through the cracks, helping to wear down the glacier at its weakest points," says Britney Schmidt, an associate professor at Cornell University and lead author of the second study.
