The accelerating rate of mass loss of the Antarctic and Greenland ice sheets, responsible for global sea level rise, is closely linked to warming of the polar oceans.
Driven by gravity, ice streams transport inland (meteoric) ice towards the ocean. As they encounter coastal embayments, they spread out like a dough. This causes a thinning often up to the point where the ice starts to float – an ice shelf is born. Meteoric ice floats on sea water because of its density being smaller (910 kilograms per cubic meter) compared to the density of sea water (1028 kilograms per cubic meter).
An ice shelf is part of an ice sheet and floats on the ocean, but it rests on the sea floor at its fringes and on embedded islands. Contact with the sea floor slows down the seaward movement of shelf ice and thus the ice streams draining the ice sheet. Enhanced melting at the ice shelf base relaxes the braking force, e.g., if the ice detaches from embedded rumples and islands.
Only ice transported from inland to the ocean contributes to sea level rise as it replaces the volume of water of equal weight (Archimedean principle). A floating ice shelf already replaced the ocean water. Thus, melting ice shelves as well as icebergs do not contribute to sea level rise. Keep an eye on the water level while an ice cube melts in a glass of water.
Remote sensing of the Antarctic Ice Sheet revealed that ice shelf basal melting contributes the most to its mass loss. Basal melting is fueled either by the transport of warm water masses of open ocean origin or high saline shelf water at the surface freezing point into the sub-ice cavity (Fig.2).
At the 1300-m deep grounding line (more of a zone, where ice streams start to float on the ocean) of the relatively small Pine Island Ice Shelf, which fringes the Amundsen Sea, the ice is thinning at a rate of more than 100 m per year. Such high rate is caused by water masses more than 1°C warm, transported from the distant (500 kilometers) continental shelf break across the continental shelf into the ice shelf cavity (volume beneath an ice shelf filled with sea water).
So far, Antarctica’s big ice shelves with areas up to 450.000 km2 are not threatened by warm water masses of open ocean origin. Instead, cold and saline shelf water formed by intensive sea ice formation in coastal polynyas (large ice-free area near the coast) flushes the ice shelf cavity (Fig. 2), causing melt rates less than 1 meter per year.
Sea water at the surface freezing point is able to melt shelf ice because the freezing point depends on pressure and salinity. Assuming a mean ice shelf thickness of 500 m and a sea water salinity of 34 grams per kilogram, the in-situ freezing temperature amounts to -2.23 degrees Celsius. Melting at deep ice shelf bases forms a water mass, Ice Shelf water (ISW), with temperatures below the surface freezing point of -1.9 degrees Celsius. Being the coldest water mass of the global ocean, ISW can be observed as far as the Southern Ocean abyss (Fig. 2).
Results of numerical models, developed at the Alfred Wegener Institute to study the sea ice-ocean-ice shelf system under climate change, show that atmospheric conditions in the southern Weddell Sea, which borders on the Filchner-Ronne Ice Shelf (FRIS), might drastically change ocean circulation: A re-directed slope current might transport 1 degrees Celsius warm waters into the FRIS cavity (Fig. 2). This heat would fuel enhanced basal melting. Within a couple of years, the ice shelf would lose contact with the sea floor in many areas with consequences for ice stream dynamics and the mass balance of the Antarctic Ice Sheet.
Since this process can happen also at other Antarctic coast lines, ice shelf retreat might become a widespread phenomenon in a warming climate. However, bottom topography and basin structures (Fig. 3) finally determine whether such a retreat causes significant ice sheet mass loss and global sea level rise.
Fig. 2: Schematic of the hydrographic conditions in the southern Weddell Sea (Fig. 1). Today (front) cold and saline shelf water formed by intense sea ice formation flows into the ice shelf cavity. In a warming climate (back) sea ice formation is reduced, modifying the density structure on the continental shelf such that warm water of open ocean origin can flush the ice shelf cavity and enhance basal melting.
Fig. 3: Bottom topography beneath the Antarctic Ice Sheet according to Bedmap2. (Graphic: BAS).
The changes in water mass characteristics and ocean circulation projected by our models for the southern Weddell Sea strongly depend on the atmospheric forcing provided by climate models. Therefore, new simulations with improved atmospheric boundary conditions (IPCC-AR6) are needed, and the new results have to be validated by means of continuous ocean observations in the polar marginal seas and continental shelves. In addition, further improvement of the numerical models is required as coupled ocean-ice shelf-ice sheet models are just at their early stages.
Scientists at AWI are heavily engaged in this challenge. However, the significance of the model results strongly depends on accurate knowledge of cavity geometry and topography beneath the ice sheets (Fig. 3). Therefore, the subject of ocean-ice sheet interaction is a multi-disciplinary effort including colleagues from physical oceanography, geophysics, and glaciology.
Vigorous ice streams transport mass of the Greenland Ice Sheet towards the ocean. In contrast to Antarctica, large ice shelves are missing at the coast, but the calving fronts are in direct contact with the ocean. Depending on bottom topography, ice at the fronts can be several hundred meters thick, though only 1/10 is visible above sea level. At the base, warm and salty Atlantic Water, which is denser than the cold and fresh Polar Water, melts the ice.
During the last two decades, an enhanced retreat of many Greenland glaciers has been observed. Ice sheet thinning and increased mass loss can be attributed not only to higher surface melting due to warmer air temperatures but also to larger basal melt rates caused by the arrival of warmer Atlantic Water at the calving fronts. Starting in the mid-1990’s, maximum glacier retreat occured along the west and southeast coast of Greenland, where water temperatures up to 4 degrees Celsius have been observed in fjords hosting the glaciers.
Thinning of glaciers in northern Greenland has been observed first in the mid-2000’s. At the north and northeastern coast, glacier termini tend to float on the ocean like small Antarctic ice shelves. In addition to the observed thinning, increased calving of icebergs caused either a substantial retreat of the floating glacier tongue or a total collapse like in northeast Greenland.
Scientists at AWI investigate ocean-glacier interaction at Greenland’s longest floating glacier – the 79-North Glacier also called Nioghalvfjerdsbreen. More information is provided following the links to GROCE und SPP-OGreen.
Fig. 4: RV Polarstern at the calving front of 79-North Glacier. Normally, thick fast ice blocks access to the glacier, but in boreal summer 2016 the bay was ice-free for several weeks allowing RV Polarstern to conduct comprehensive oceanographic measurements. (Photo: Nat Wilson)
