When great ice fields melt, will any resulting rise in sea level be equal at all latitudes, or will the Earth鈥檚 rotation make it greater at the equator? If there is a difference, has this been factored in when discussing the effect of melting ice sheets?
The pattern of sea level change is far from uniform, although the effect of the Earth鈥檚 rotation is rather small.
If the Earth was covered in ocean, and an extra layer of water was added, then this layer would be about 0.5 per cent thicker at the equator than at the poles. This is because the effect of the Earth鈥檚 rotation makes gravity (the sum of the gravitational attraction of the Earth and the opposing force caused by the Earth鈥檚 rotation) about 0.5 per cent weaker at the equator than at the poles. For a 7-metre sea level rise (about what would be expected from the melting of either the Greenland ice sheet or the West Antarctic ice sheet), that would mean a rise at the equator of about 3.5 centimetres more than at the poles.
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A second effect of rotation comes from the fact that the mass of the melting ice is moving from near the poles to a broader distribution around the whole globe, making it move away from the Earth鈥檚 rotation axis. This increases the Earth鈥檚 moment of inertia and so, to conserve angular momentum, the Earth鈥檚 rate of spin will slow (like a spinning ice dancer slowing down when they stretch out their arms).
Spreading 7 metres鈥� worth of water from the pole right across the whole globe would slow down the Earth by about 1 part in a million, making the day about 0.1 seconds longer and reducing the force that creates the Earth鈥檚 equatorial bulge. This force would then relax slightly, partially cancelling the above effect. Again, we are left with sea level differences of a few centimetres between the equator and pole.
However, by far the largest effect would be caused by what is known as self-attraction and loading. The Greenland ice produces a gravitational attraction that pulls the ocean towards it. As the ice melts, this attraction decreases, and the ocean relaxes away from Greenland. In addition, the removal of the heavy load of ice from Greenland allows the Earth鈥檚 surface to bounce back up. This causes mass redistributions in the Earth that offset some of the change in the gravity field caused by the loss of ice mass. The net effect is that the sea level within about 1000 kilometres of Greenland actually goes down, but the sea level rises further away by a little extra.
鈥淭he Greenland ice produces a gravitational attraction which pulls the ocean towards it鈥�
Researchers are now working on using the pattern of sea level change as a 鈥渇ingerprint鈥� to help determine whether changes are a result of melting in Greenland or in Antarctica or elsewhere. Measurements of changes in the gravitational field by the satellite mission provide our current best estimates of the mass balance of these ice sheets.
Finally, things are complicated by the fact that sea level is not actually level. The permanent currents that make up the ocean circulation lead to slopes in the sea surface, making it depart from a level surface by up to 1 metre. Adding such a large amount of fresh water to the ocean is likely to lead to changes in this circulation and to more complicated patterns of sea level change.
Climate simulations so far have only considered the global mean sea level change, together with regional patterns associated with ocean currents. Adding in the effects of self-attraction and loading, which can be calculated separately, is quite a new departure.
Chris Hughes, Proudman Oceanographic Laboratory, Liverpool, UK