Newton And Sea Level Rise

  • Date: 26/06/10

By how much would sea-level rise if the Greenland ice sheet disappears ? Probably quite a lot, but not in Germany, or in North Western Europe for that matter. There, sea level would virtually unaffected. To formulate it a bit provocatively, Greenland is for Western Europeans irrelevant. They should be rather observing Antarctica more closely.

Global sea-level rise is caused about by several factors, among which the most important the expansion of the water column due to rising ocean water temperatures and the melting of the polar ice-sheets. Both effects are obvious and do not require further explanation. However, the shrinking of the polar land-ice masses does not lead to a sea-level rise uniformly distributed over the globe. Quite the contrary, its fingerprint is substantially heterogeneous. If the Greenland ice sheet melts, most of the sea-level rise would occur in the southern Hemisphere. If, on the other hand, it is the West-Antarctic Ice sheet that collapses, Nature’s wisdom would produce a targeted maximum of sea-level rise right in front of the White House. This surprising effect is caused by very well-known physics – gravitational attraction – but it is seldom found in the public discussion of global sea-level rise.

The mechanisms by which this spatial distribution of sea-level response to the collapse of polar ice sheets is not difficult to understand either, albeit its magnitude may be surprising for many of us. Basically, sea-level in the Arctic, North Atlantic and North Pacific due to the additional gravitational pull of the large ice mass locked on top of Greenland, is a bit higher than it ‘should be’. If this ice mass melts, the volume of the global ocean will increase accordingly and thus sea-level would tend to rise on average. But at the same time the gravitational pull that maintained the sea-leve in the Nordic seas will also disappear, and sea-level will tend to drop in those areas close to the present position of the ice-sheets.

The calculation of the final spatial distribution of this gravitational effect is somewhat complex, but it is possible. Other effects come into play as well, but their magnitude is just able to slightly modulate the overall fingerprint of the gravitational pull. For instance, melting of the polar ice sheets and the subsequent distribution of water masses over the the whole ocean changes the rotational speed of the Earth – in a similar way as an ice skater turns more slowly when he extends his arms away from his body. This in turn slightly affects sea-level as well

It turns out that for the Arctic Ocean, the gravitational effect overwhelms the increase in ocean volume; so that melting of Greenland ice causes a drop of sea-level in this ocean (see Figure). For Northern Europe, both effects roughly cancel (see the zero isoline separating the dark blue and light blue colors). Sea-level rises unabated in the Southern Hemisphere. In the case of Antarctic ice melting, we roughly find a mirror image, with sea-level dropping in the Southern Ocean and rising in the Northern Hemisphere. For the case of melting of ice sitting on the West-Antarctic peninsula, the maximum sea-level rise occurs in the Western North Atlantic.

Top: sea-level rise (in mm) caused by melting of an amount of Antarctic land-ice equivalent to 1 mm of globally average sea-level rise. Bottom: analogous calculation, but for Greenland land-ice. From Mitrovica et al, Nature 409, 1026 (2001). See also, Bamber et al. Science 324, 901 (2009).

Greenland glaciers and glaciers on the Antarctica Peninsula- the area in the Antarctic continent at greatest risk of melting, may react in different ways to overall warming. West Antarctic glaciers terminate below sea-level and thus are exposed to a much greater degree to ocean heat flux and warmer water temperatures. It is therefore possible that the West Antarctic Ice sheet may turn to be less stable to higher temperatures than Greenland. On the other hand, temperatures are projected to rise more in the Arctic region than over Antarctica, so that in this end it is not quite certain which one of the polar ice sheets will be the major contributor to the ocean mass. This introduces further uncertainties to sea-level projections at regional scales.

Klimazwiebel, 17 June 2010

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