3 ms·
1) the center of the ice sheet can be much closer to the water than the center of the earth. Some water around greenland is at 600 km from the center of the ice
by wcoenen 6y ago
1) the center of the ice sheet can be much closer to the water than the center of the earth. Some water around greenland is at 600 km from the center of the ice sheet, and 6000 km from the center of the earth. This factor 10 difference is squared because gravity's strength drops off with the square of distance.
2) Both forces are orthogonal, so you take the ratio to get an idea of how much the ice sheet attraction is slanting the water surface. This might be a very small angle, but if you have a small angle sustained over hundreds of kilometers then you can arrive at a height difference of meters. E.g. if the ratio is 10^5, then you have a 1 meter height difference at 100km distance (ignoring that the ratio actually changes over that distance to simplify).
- brandmeyer 6y agoSo... why wouldn't this show up consistently in the EGM96 data? EGM96 is a definition of Earth's equipotential height constructed by measuring it with satellites like GRACE. Its as close to a definition of true sea level as you're going to get. But it doesn't have this kind of consistent uplift near tall masses throughout the model. The Southern Ocean's height is sinusoidal about Antarctica. We do see an increase in equipotential height in the Andes, and in the eastern Pacific nearby. But near the Himalayas the equipotential height is lower. https://en.wikipedia.org/wiki/File:Earth_Gravitational_Model_1996.png https://en.wikipedia.org/wiki/File:Earth_Gravitational_Model...
- wcoenen 6y agoI guess because plainly visible "tall masses" are not the only things contributing to the geoid? E.g. earth crust thickness might dominate. In the context of this discussion, we're not so much interested in the absolute shape of the geoid, as in how much it might change because of melting ice sheets