It melts from the top too …

By David Ferreira

The global sea level rises at about 3 mm/year. Oceans absorb nearly 90% of the heat trapped in the atmosphere by anthropogenic gases like carbon dioxide. As water warms, it expands: this effect explains about half of the observed sea level rise. The other half is due to the melting of ice stored over land, that is, glaciers, the Greenland ice sheet and the Antarctic ice sheet.

Although the latter was a relatively small contributor, recent estimates suggest an increased mass loss from Antarctica in the last decade. Up to now, Antarctica was thought to lose most of its mass at the edges.

The Antarctic ice sheet behaves a bit like a pile of dough that slowly collapses under its own weight. The ice spreads over the whole, and then over the oceans as floating ice, known as ice shelves. Ice shelves are usually found at the end of fast ice-streams channeled by mountains (there are hundreds of these around the continents). The ice shelves in contact with the “warm” ocean (~ 2-4 °C) and melt slowly. Occasionally the process is more abrupt, the ice shelves shed icebergs, some of which are many kilometres in size (an iceberg much larger than Greater London is about to break loose from the Larsen ice shelf). On long timescales, the ice loss at the edges is compensated by snow falling on top of the ice sheet. In recent decades, however, the mass loss at the edges has been slightly larger than the gain through snowfall (a transfer of water to the oceans and a contribution to the sea level rise). The leading explanation for this recent imbalance is that the rate at which warm water is brought to the ice shelves has increased, possibly because of a strengthening of the winds that drive the ocean currents.

A recent paper brings a new element into the picture: the Antarctic ice sheet does not only melt at the edges but also from the top (Kingslake et al., 2017). The surface melt process was thought to be exclusive to Greenland as Antarctica is too cold, even in summer, for temperature to rise above 0°C. So, how is this happening? Melt water in Antarctica seems to originate next to blue ice or exposed rocks. Within the white world of Antarctica, blue ice and rocks are dark. That is, they absorb more sunlight than snow and could (locally) create the conditions for melting. The melt water then gathers into elongated ponds that can grow by kilometres within weeks. Kingslake et al. have documented this process for hundreds of ice streams around Antarctica, sometimes deep into the continent, highlighting a much more widespread phenomenon than previously thought.

What are the possible consequences? These ponds can accelerate the mass loss to the ocean. For example, if they form over land, they may flush to the base of the ice sheet, “lubricate” the ice-ground interface and speed up the ice flow to the coast. If the ponds form over the ice shelves, the added pressure due to the weight of liquid water can help fracture the ice shelves and create icebergs.

Then, the natural question is whether the Antarctic ice shelf is more susceptible to rising temperatures than we think. Unlike the melting at the edge which involves indirect mechanisms through changing winds and ocean currents, surface melting could be directly influenced by increasing temperatures. How important could that be in terms of sea level rise? This remains to be quantified as modern ice sheet models do not take this effect into account, or at least underestimate it.

Reference

Kingslake et al, 2017: doi: 10.1038/nature22049

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