Australasian Science Magazine

Credit: Alessandro Silvano

By Alessandro Silvano & Steve Rintoul

A scientific voyage has revealed startlingly warm waters around a glacier in East Antarctica that could raise global sea levels by 4 metres.

Antarctica is remote, but small changes here can have global consequences. However, direct measurements in this harsh and remote place are hard to get. As a consequence, how changes in Antarctica affect the rest of the world is still not well understood. This is especially true in East Antarctica, where the great distance from inhabited areas makes scientific campaigns even harder. Large sectors of the East Antarctic coast remains unexplored.

In December 2014 scientists on board the Australian icebreaker Aurora Australis managed for the first time to reach the front of the largest glacier in East Antarctica, the Totten Glacier. Heavy sea-ice had blocked previous attempts to reach the glacier, but this time things went differently. A change in winds opened a window of open water just in front of the glacier.

The captain and the crew of the Aurora Australis immediately took advantage of this rare chance and drove the ship through a narrow pathway to the glacier (Fig. 1). The scientists onboard had 48 hours to collect samples of water in front of the Totten Glacier before the winds were forecast to change and force the ship to leave the area.

Figure 1. After waiting weeks in the open water of the Dalton Polyna, a path opened through the sea-ice and the Aurora Australis reached the front of the Totten Glacier (its track is denoted by the red line). Satelite image courtesy NSIDC

We were all excited after years of waiting, and managed to collect thousands of samples. These revealed something never seen before in this sector of Antarctica: warm ocean waters from the Southern Ocean reaching the Totten Glacier.

The measurements showed that water about 2°C above freezing was able to reach the Totten Glacier and melt it from below. Considering that typically waters around Antarctica are roughly at freezing point, 2°C above freezing is a lot. Indeed, the ocean heat transported to the glacier was high enough to cause the most rapid melting among all major glaciers in East Antarctica. This discovery explained what satellite data were already suggesting: ice loss of the Totten Glacier caused by warm ocean waters.

Scientists were aware of similar findings on the other side of the continent, in West Antarctica. Previous surveys had shown that warm ocean waters were able to drive rapid melting of some glaciers in West Antarctica, causing sea level to rise globally by about 0.3 mm every year. On the other hand, East Antarctica was thought to be isolated from warm waters, and thus less important for present and future sea level rise. The Australian voyage showed that this assumption was wrong, at least for the Totten Glacier.

But why is the Totten Glacier so important? Simply because it contains ice for the equivalent of 4 metres of global sea level rise. If this glacier melts, many cities would be flooded by the ocean waters.

Luckily the complete collapse of the glacier can take several centuries or more. Currently the contribution of the Totten Glacier to sea level rise is about 0.02 mm every year. However, future changes needs to be assessed because even a few centimeters of sea level rise can have dramatic implications for coastal regions and small islands.

The scientific expedition to the Totten Glacier was already telling something that scientists didn’t know, but a deeper analysis of data collected during that voyage revealed something even more unexpected in the Dalton Polynya, an area of open water just east of the Totten Glacier.

Strong cooling and salinification of ocean waters occurs in polynyas due to intense freezing of the ocean surface in winter. During sea-ice formation, some salt is left behind in the ocean, a process called brine rejection. Thus sea-ice formation causes the underlying waters to become both cooler and saltier. The waters in these polynyas are therefore the most dense on the planet, and they sink into the abyss. This process allows heat and carbon dioxide to be sucked from the atmosphere and stored in the ocean depths for centuries, thus limiting global warming.

Dense waters formed in polynyas are not only important for trapping heat and carbon dioxide in the ocean, but they also limit the melting of nearby glaciers since they are very cold. Scientists assumed that these dense waters were present in the Dalton Polynya because lots of sea-ice forms there. Instead, data collected during the Aurora Australis expedition showed that these cold and dense waters were absent.

The team involved in the expedition had to combine the samples they collected with computer simulations to explain this discovery. They concluded that freshwater released by melting of the nearby Moscow University Glacier by warm ocean waters was preventing the formation of cold and salty waters in the Dalton Polynya.

Freshwater from melting ice makes the ocean less salty near the surface. This fresh and light layer isolates warm and salty waters at depth from the cold Antarctic atmosphere. This process is similar to oil floating on top of water because it is less dense. The same happens in the Dalton Polynya with freshwater that sits above the warmer and saltier ocean waters. Thus the cap of freshwater stops the formation of cold and dense water, and allows warm waters at depth to retain their heat.

The process is summarised in Figure 2.

1. Warm water drives rapid melting of the Moscow University Glacier.
2. Freshwater from melting of the Moscow University Glacier spreads into the ocean and is transported to the Dalton Polynya by Antarctic currents.
3. The freshwater stops the formation of cold and dense waters in the Dalton Polynya, and thus warm ocean waters are not cooled in the polynya.
4. Warm waters transported by Antarctic currents can then reach the Totten Glacier and drive rapid melting.

Figure 2. Melting of the Moscow University Glacier by warm ocean waters prevent the formation of cold, salty waters in the Dalton Polynya, exacerbating melting of the Totten Glacier.

The Aurora Australis expedition helped scientists to better understand how susceptible glaciers in East Antarctica are to erosion by warm ocean waters, and that glaciers can “talk” to each other. When the same analysis was repeated in some parts of West Antarctica, the scientists again observed that melting of one glacier can trigger rapid melting of a nearby glacier by changing the surrounding ocean.

A storm in the Dalton Polynya observed from the deck of the Aurora Australis.

This is not always the case. In other areas of Antarctica, freshwater from melting of a glacier is too small to stop the formation of cold and dense waters. Consequently, nearby glaciers are melting only slowly.

These discoveries have global implications. Glacier melting causes sea levels to rise and reduces the formation of dense waters that help the ocean to trap heat and carbon dioxide. Global warming will presumably increase the melting of Antarctic glaciers, both driven by the ocean below and by the atmosphere above.

This second aspect is currently not very relevant in Antarctica, where air temperatures remain below zero most of the time. But this can change in the future, especially if fossil fuels continue to be used at current rates.

In the case of increased ice melting by global warming, both freshening and warming is expected to happen in Antarctic waters through the new processes observed by this voyage. Warming means more melting, and freshening means less formation of dense Antarctic waters.

These processes feed off each other to accelerate climate change in a case of “positive feedback”. This positive feedback is thought to be the primary cause of rapid sea level rise at the end of the last glacial age, around 15,000 years ago, when the sea level rose 1–5 metres per century. The Aurora Australis voyage revealed that this feedback is already underway.

If anthropogenic emissions of carbon dioxide are sustained in the coming decades, some computer simulations predict that melting of Antarctic ice will produce up to 1 metre of sea level rise by the end of the 21st century. This is in addition to contributions from other sources of sea level rise, like melting of glaciers in Greenland and ocean warming that causes waters to expand and rise.

However, these projections are still highly uncertain. This is partly due to the fact that we still have much to learn about present changes in Antarctica. If the present is not well-known, it is hard to predict the future.

New technologies like underwater robots and drones are developing rapidly and becoming available to the scientists. Computer capabilities are becoming stronger and stronger, helping to better simulate the present and future of Antarctica.

Recent studies have shown that feedback between ice and ocean can accelerate changes in Antarctica. These new technologies will help us understand how fast these changes will be in the future.