Bottoms up! Melting the dark side of Antarctic ice shelves

A map of Antarctica showing major glaciers and all ice shelves coloured to indicate the rate of change of thickness. Sea temperature is also indicated by a colour scale.

Antarctica’s glaciers (grey) extend beyond the shoreline into anchored but floating ice shelves (areas outlined in black using the left scale indicating change of thickness per year where red means thinning rapidly and yellow is steady).
(Source: adapted from Pritchard et al. (2012), Nature, via Antarctic Glaciers.)

Antarctic ice shelves are nature’s buttresses against sea level rise that would devastate our coastal civilisations. New research provides a clearer picture of how climate change will speed up melting of ice shelves from underneath.

Antarctica’s land area is almost completely covered by massive glaciers hundreds of kilometres long and up to 4km high. In many places around the continent, these glaciers extend well beyond the shoreline into thick floating platforms of ice called shelves.

A diagram showing that in winter, sea water freezes to the underside of the ice shelf, forming a body of cold, salty, dense water that protects the edge of the glacier from being melted by warmer ocean water, then sinks to the ocean floor, driving global ocean circulation.

In winter, sea water freezes to the underside of the ice shelf, forming a body of cold, salty, dense water that protects the edge of the glacier from being melted by warmer ocean water, then sinks to the ocean floor, driving global ocean circulation.
(Source: graphic design by Kirsten Duncan.)

Ice shelves do three important things

Winter freezing of sea water to the underside forms the cold, salty water that sinks to the bottom of the ocean and drives global ocean and air circulation.

They form a cold buffer that protects the land glaciers from being melted by the ocean.

They act like a giant doorstop preventing the glaciers from sliding off the land into the sea.

If the 25 million cubic kilometres of Antarctic glaciers slid into the ocean and melted, the sea level around the world would rise by 58 metres. That’s enough to drown more than 80 capital cities and several entire countries.

Ice science is becoming crystal clear

Computer-generated images of subsurface ocean temperature in Antarctica from three different sources. The atmosphere–ocean general circulation model shows coarse data representation. The observation data is better resolved but does not show under ice shelves. The combined model shows high resolution of temperature in ice shelf cavities.

The atmosphere–ocean general circulation model (left) produces coarse data, and observations (right) are limited by instruments. Instead, the researchers used a combination (centre) of atmosphere–ocean general circulation and circumpolar ocean models to produce a high-resolution image of subsurface ocean temperature that can ‘see’ inside ice shelf cavities.
(Source: adapted from Obase et al. (2017), Journal of Climate.)

Diagram showing that under warm climate conditions caused by excess CO2 in the atmosphere, warm air and sea water melt the ice shelf, reducing formation of the cold, salty, dense water that protects the edge of the glacier. Warm water can now intrude under the ice shelf, accelerating its disintegration.

Under warm climate conditions caused by excess CO2 in the atmosphere, warm air and sea water melt the ice shelf, reducing formation of the cold, salty, dense water that protects the edge of the glacier. Warm water can now intrude under the ice shelf, accelerating its disintegration.
(Source: graphic design by Kirsten Duncan.)

The Japanese researchers combined two major ocean–atmosphere computer models to produce a more accurate model of what happens in the cavities between the undersides of ice shelves and the sea floor below.

In cold conditions like the last ice age, the cold, salty water that sinks away from the base of an ice shelf prevents warmer ocean water from intruding under the ice shelf, thus protecting itself from basal (underside) melting.

However, warmer air and ocean temperatures caused by excess carbon dioxide slow down the formation of new ice and increase the melting of existing ice. As the seaward edge of an ice shelf shrinks, the base of the shelf creates less protective cold water. Warmer ocean water can now flow into the cavity and melt the underside, causing the shelf to shrink even faster.

Once an ice shelf disintegrates, there is nothing stopping the land glaciers from flowing into the sea, with serious implications for sea level and global weather.

The real dark side is carbon emissions

The researchers used the model to test the effects of a range of ocean, air, climate and weather factors on the rate of basal melting. They found that warming air temperature influences the conditions for melting more than any other single factor, but all the factors combine to accelerate ‘non-linear’ melting.

The scientists urge further research that combines ocean, ice and climate models to build even more accurate understanding of Antarctica’s response to future climate change. That knowledge would help us city-dwelling landlubbers plan how we protect, adapt or abandon our low-lying societies.

An ice-free Antarctica would be dire for humanity. But the ice has not all melted yet, so, we should be doing everything we can right now to limit further climate change and save the ice.

Demand that your local, state and national governments cut greenhouse gas emissions urgently.

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