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The Antarctic ice sheet is melting. And that’s not good news for mankind


It happens, as it does This yearthat the ice of the Rideau Canalin Ottawa, is too thin to allow the rink to open.

On a larger scale, thousands of miles further south in Antarctica, sea ice is also struggling to form. Indeed, Antarctic sea ice extent has reached a minimum record on February 13th.

While there has been a sharp decrease in the extent of Antarctic sea ice since 2016, the mass of the Antarctic ice sheet has been declining for a long time.

Expert in meteorology and polar climatology, I propose to shed light on the effect of global warming on Antarctica.

Antarctic sea ice concentration on February 13. The orange line represents the corresponding median sea ice extent for the period 1981-2010. Image courtesy of the National Snow and Ice Data Center, University of Colorado, Boulder (NSIDC).

Antarctica, the white continent

I’Antarctican ice-covered continent surrounded by ocean, holds 90% of the world’s ice. This ice cover, called the “ice cap” (ice sheet), is a mass of ice of terrestrial origin that is the result of the accumulation and compaction of snow over thousands of years. The seaward extension of the ice cap forms a floating ice shelf (ice shelf).

The Antarctic Ice Sheet is made up of the East Antarctic Ice Sheet and the West Antarctic Ice Sheet. Most of the latter is anchored below sea level. Around Antarctica, the extent of sea ice, which forms from ocean water, increases in winter and decreases in summer.

Antarctica is warming faster

Antarctica is not spared from global warming. On the contrary. In a context of global warming, the increase in temperature at high latitudes is greater compared to the increase in the global average temperature. This phenomenon is known as “polar amplification”.

Ice-albedo feedback is one of the processes that explains this phenomenon. Indeed, the increase in temperature near the surface contributes to the melting of the ice, which contributes to the increase in temperature. For what ? Because albedo (i.e. the fraction of solar energy that is reflected from a surface) of the ocean and underlying ground is lower than that of ice.

Over the past four decades, global warming has caused the decrease in sea ice extent average in the Arctic, but not in the Antarctic. If we do not find in the recent past a significant trend of reduction in the extent of the Antarctic mean sea ice, it is because the regional trends, positive and negative, compensate each other, and that there is a strong internal variability.

Scientists explore Antarctic icebergs by boat
Antarctica is not spared from global warming. On the contrary.

However, the extent of the Antarctic sea ice has shrunk sharply since 2016. The reduction in the area of ​​sea ice cover contributes to the increase in temperature (ice-albedo feedback), but it does not contribute to the rise in sea level. antarctic ice sheetits mass has been declining since at least the year 1990, with the greatest rate of loss occurring in the last decade.

In his sixth assessment reportthe Intergovernmental Panel on Climate Change (IPCC) indicates that in Antarctica, the temperature will continue to increase and the mass of the ice sheet will continue to decrease. Note that the growth of the ice cap is much slower than its retreat, which implies that if it continues to melt during this century, this melting will not be reversible on the human time scale.

With regard to Antarctic sea ice, the degree of confidence in its climate projections is low. For what ? Among other things, because simulations with climate models do not capture the observed evolution of the latter well enough. Therefore, no conclusions can be drawn from this.

Collapse of the ice cap: what consequences?

There sustained melting of ice from the West Antarctic Ice Sheet could indicate that an unstable (self-reinforcing) pullback from it has begun or is imminent. But there is a lot of uncertainty about this phenomenon.

The mechanism that would explain this unstable retreat is known as “marine ice sheet instability”. If the bedrock on which the marine ice sheet sits slopes inward, the position of the grounding line (area from which the ice, which sits on the bedrock, begins to float) is unstable . Indeed, the thinning of the ice shelf causes the grounding line to retreat, which leads to an increase in the flow of ice from the ice sheet to the sea, and, subsequently, to the thinning of the ice shelf. And so on.

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Diagram showing the process of marine ice sheet instability (MSI). Blue arrows indicate grounding line retreat, and white arrows indicate ice flow. Source: Pattyn, F. The paradigm shift in Antarctic ice sheet modelling. Common Nat 9, 2728 (2018). CC BY 4.0 license.

There complete melting of the West Antarctic ice sheet would cause a 3.3 m rise in global sea level. Currently, the world is heading towards a 2.8°C warming by the end of the century. Sustained warming between 2°C and 3°C would be sufficient to make this ice cap almost disappear. But this phenomenon would take millennia.

The bottom line is that the melting of the Antarctic ice cap is and will be contributing for a long time to sea level rise, which will test humanity’s ability to adapt. .

The rise in sea level between now and 2100 will particularly affect countries located in the tropics.

Like what, “what happens in Antarctica does not stay in Antarctica”.

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