discovery of a key process responsible for the decline of ice shelves

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A team of Australian researchers has discovered the existence of a key process driving the basal melting of some Antarctic ice shelves. A breakthrough that will ultimately make it possible to better anticipate future changes in sea level. And for good reason, the process in question is not present in current climate models. The results were published in the journal PNAS this February 7.

Over the past 20 years, the Antarctic ice sheet has experienced an estimated ice loss of 2.5 trillion tonnes. This trend is the concrete expression of the changes that have occurred in the western part of the gigantic ice cap. In fact, despite the existence of vulnerable areas, the eastern part is generally less affected.

Cumulative ice loss over the Antarctic cap between 2002 and 2020. The average loss over the period is 149 billion tonnes of ice per year. Finally, the lack of data around 2017 and 2018 is linked to the switch to new satellites. Credits: NASA.

The southern continent experiences extremely low atmospheric temperatures throughout the year. Also, despite the average warming, most of the ice loss is not due to direct surface melting. In this very remote region, it is above all the ocean that comes into play.

Ocean heat and melting of the Antarctic ice cap

The latter presents a complex pattern of evolution marked by a slight cooling on the surface, but by a rapid warming of water masses at depth. Thus, the ice shelves that form the marine extensions of the ice sheet are gradually eaten away from below in contact with abnormally hot water. However, these natural barriers act in such a way as to slow down the ice flow upstream by a flying buttress effect. In fact, their thinning or collapse causes an increase in this flow towards the ocean. An increasing amount of ice is therefore lost as a result of this self-sustaining process.

icebergs ice shelf
Schematic representation of an ice shelf. Hot water circulates in depth and causes melting from below. Note the direction of ice flow from the mainland to the ocean. Credits: screenshot / AGU.

In a new study, researchers have discovered a new mechanism by which submarine melting is articulated. The results show that it is not necessarily the fluctuations of the sea currents under the platforms that play the most. “Using computer modeling, we can now see thata process known as double diffusion convection occurs, due to the unique ocean conditions under the ice shelves. Where cold, fresh water sits above a warmer, saltier ocean ”, says Bishakhdatta Gayen, co-author of the paper.

Towards a detailed understanding of the processesartwork

The phenomenon of double diffusion acts on a small scale. It therefore requires the use of very high resolution modeling to be represented in the simulations. However, as there are very few direct observations, digital models are currently the only possibility of highlighting the presence of this phenomenon under the Antarctic ice shelves. “Supercomputer simulations allow us to virtually study these remote environments”, explains Benjamin Keith Galton-Fenzi, also co-author of the study. The video below illustrates the melting mechanism linked to the double diffusion convection.

“This process is currently absent from ocean-climate models, which only take into account the melting due to ocean currents”, notes the paper in its summary. However, this is however the main mechanism dictating the rate of melting in areas of weak currents. The work carried out by the researchers will thus call for a review of our understanding of how oceanic processes affect the Antarctic ice cap. There is little doubt that many assumptions made in various previous works will have to be abandoned. “This is actually a very important piece of the puzzle for climate models because ultimately it helps us understand the basic forces that drive sea level rise,” tell Bishakhdatta Gayen.

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