What will happen on the sea level when this huge iceberg will melt??

……………nothing!!

You can try to use a very simple analogic model to understand. Let’s take few ice cubes (they are your iceberg!) and put them in a glass (your southern ocean) and ……………wait and see!

 

Nothing happens!

This is so disappointed! And a little surprising for anyone who is not a physicist. So I asked to a physist for some help. It took him a least 5 min to understand what I did not understand. Then he told to be (with a little contempt ….): “but my dear, this is basic physics!”.

So let’s go for a short basic physics lesson!

The level of the ice will not change because the volume of the ice and the volume of the water that it will forms are quite the same. I don’t know for you but I need numbers to understand (simple numbers!). Let’s imagine that the ice in the glass is a big cube of 8 cm3 (2cmx2cmx2cm). The density of the ice is 0,92 g/cm3 so the weight of the ice will be 8×0,92 = 7,36 g.

The density of the water is 1g/cm3 so when your ice cubs melt, they will produce 7,36g of water. It’s a simple question of density. For a same weight, the ice takes more volume than the water.

You also can try to put a full bottle of water in the freezer and wait for a few hours. Then, you will see that the ice takes more place than water for the same weight (ok, don’t try this, I don’t want any trouble with your mum…).

So? Ice melting has no influence on the sea level?

Of course not!

Let’s try another simple experience:

Wait a few hours and you will see that the level of the water in your glass has increased. So, the melt of the ice which is already in water will not change the seal level (melting of the sea ice for example) but the sea level will rise if the ice was not initially in the sea. It’s the case for melting of the ice sheet.

It’s a little more complicated for ice shelves because they are the floating part of an ice sheet. A melting ice shelf will not raise sea level by themselves, but ice shelves are important indirectly to sea level, as they slow down glaciers that drain the ice sheets that are onland and above sea level. If an ice shelf melts, the glaciers they were holding back slide into the ocean quicker and this will raise sea levels…..

 

 

 

 

La glace a rendez-vous avec l’océan

The Antarctic Circumpolar Current (ACC) is the worlds largest ocean current, and the only current that flows completely around the globe. The ACC flows eastward around the Antarctic continent and connects with the southern portions of the Atlantic, Indian, and Pacific Oceans.

Nearer the continent the easterly winds cause a counter-current with a special clockwise circulation in the great indentations of the Weddell and Ross Seas and they drive to the surface the waters of the Circumpolar Deep Water (CDW) near the Ross Ice Shelf.

The CDW is a relatively salty, warm current, >3.5°C above freezing point. It is overlain by colder, fresher surface waters. The continental slope acts as a large topographic barrier for the majority of the deep water to reach and enter in the continental shelf.

Circumpolar Deep Water (CDW) is a key component of the stability of the Ross Sea Ice sheet

The West Antarctic Ice Sheet (WAIS) had known collapse events during warmer-than-present climates (Miocene and early Pliocene) and these events were probably a consequence of ocean-cryosphere interactions.

Numerical models show that the warming of ocean heat flux, which occurs when more CDW is pushed by ocean currents towards Antarctica is a key factor influencing the stability of the WAIS.

Numerical model of stages of retreats of an ice sheet (from Bassis et al., 2017)

This numerical model shows the potential for repeated WAIS retreats and advances over the past few million years.

A long-term increase of the ocean heat flux in the Ross Sea could have major consequences on the Ice sheet and on the oceanic circulation.

Some wind-driven warm waters (CDW) can interact with the ice shelf from below. In a general way, ice shelves have a major role in protective ice sheets from melting because they act as a buttress that impedes the seaward flow of ice and stabilizes marine grounding zones.

Processes of melting of the ice shelf

Changes in the frequency, duration and extent of cross-continental shelf intrusions of Circumpolar Deep Water may alter the rate at which basal melting occurs. Today, the Ross Sea feeds the Antarctic bottom water which is the densest water mass of the oceans. If the ice shelf starts to melt significantly, an important amount of freshwater will be delivered to  the Ross Sea. The fresh water produced by the melting of the ice shelf could act as a barrier preventing the warm CDW to enter in the Ross Sea. It is unknown if this process  then cause a stabilization of the ice sheet retreat, but it will affect an important change in the global circulation. Fresh Ross Sea water will more easily freeze and will possibly stop or decrease the production of salt and dense water, that feeds the ocean bottom water, but sea ice is also influenced by atmospheric circulation and temperature would could offset this.

Regardless of this uncertainty, an intensification or decrease of sea-ice could then lead to an alternative mode of deep ocean ventilation during the next century and have consequences about the whole oceanic circulation and heat exchange between low and high latitudes.

Expedition 374 will test the hypothesis that changes in ocean heat flux across the Ross Sea continental shelf and slope drives the WAIS retreats and sea ice variability.

Previous scientific expeditions in the Ross Sea (from DSP to Expedition 374)

In 2006-2007, the multinational ANtarctic geological DRILLing (ANDRILL) brought together a team of over 200 scientists, drillers, and technicians. They spent a combined six months on the Ross Ice Shelf over two seasons. They drilled 2 holes through several hundred feet of ice and ocean water to reach the marine sediments. These sediments are the memory of past climate. They contain the ice sheet history of the Ross Ice Shelf.

Localization of the Andrill station on the Ross Sea ice shelf

Scientists wanted to learn more about how this ice sheet has behaved during warmer conditions than today, especially during MMCO (Mid Miocene Climate Optimum) and during the Pliocene warm period. They discovered that west Antarctica ice sheet was dynamic during these two periods and there was an interval of time, particularly warm, when the ice sheet was very much reduced, compared to today.

Expedition 374 will not be the first expedition to drill in the Ross Sea. The first drilling expedition in the Ross Sea was DSDP (Deep Sea Drilling Program) leg 28, with the Glomar Challenger in 1973. At this occasion, scientists discovered for the first time that Antarctica had a much longer history of glaciation than the northern hemisphere.

The Glomar Challenger during the DSP 28

Since 1973, the technologies of drilling have improved and many seismic data have been gathered. Today, scientists know much better where to drill in order to recover the missing information about the ice sheet history from the Ross Sea.
Andrill sites record the ice sheet history in an area that is close to the mountains and to the Ross Ice Shelf. The cores that we will collect during Expedition 374 will be further off shore and they will provide information from near the edge of the ice sheet, during the maximum advance over the continental shelf, and where these ice sheets were more directly connected to the Southern Ocean. Having all these data to compare will help numerical models to improve in their accuracy. Their results can be put in perspective with the real records and if they are enough accurate to predict the past, they will be reliable to predict how the ice sheet will evolve in the future, in warmer conditions than today.

By having a better knowledge of the past, we could have a better understand of what the future might be.

Un paysage de glace

« So much water, so much water….” is a famous quote by our former president Mac Mahon (French president from 1873 to 1879). He would have said these words in front of dramatic flooding in the city of Toulouse. If this quote was applied to Antarctica today, I would like to switch it to “So much ice, so much ice….”

Antarctica is the biggest freshwater reservoir on the planet, mainly in the form of ice. About 98% of Antarctica is covered by ice that averages 1.9 km (6,200 ft) in thickness.

Cross section of Antarctica ice sheets (source: Andrill)

Ice sheet (= les calottes glaciaires)

The Antarctic ice sheet covers about 98% of the Antarctic continent and is the biggest mass of ice on Earth. It covers an area of almost 14 million square kilometers and contains approximately 61% of all fresh water on Earth. In East Antarctica, the ice sheet rests on a major land mass and has an average thickness of 3 km while in West Antarctica the thickness is around 2 km.

Simplified cartoon of an ice sheet feeding into an ice shelf, showing the grounding line (where the glacier begins to float). Source: antarcticglaciers.org

Ice shelves (les plate-formes glaciaires)

The ice shelves are the floating extension of land-based ice sheets. They surrounded the entire continent and cover an area of the size of Greenland. We are now sailing towards the Ross Ice Shelf which is one of the most important ice shelves in Antarctica with an area of roughly 487,000 square kilometers and about 800 kilometres (500 mi) across (about the size of France!!). It is several hundred meters thick.

Sea ice (la banquise)

Sea ice forms from sea water and is free floating. Its formation follows the seasons: the sea surface freezes during winter and begins to melt in the spring to summer. During the transformation of sea water into ice, most of the salt stays in the remaining sea water so that the sea becomes more salty and the ice is mostly fresh water.

There are different types of sea ice, thin sea ice, and thicker multi-year sea ice (frozen sea water that has survived several summer melt seasons, getting thicker as more ice is added each winter).