I enjoyed – morbidly, of course – this discourse on how oceans and land react as the climate warms in this interview with Harvard geophysicist Jerry Mitrovica on Pocket:
Some of your research follows from the attraction of ocean water to ice sheets. That seems surprising.
[Mitrovica] This is just Newton’s law of gravitation applied to the Earth. An ice sheet, like the sun and the moon, produces a gravitational attraction on the surrounding water. There’s no doubt about that.
What happens when a big glacier like the Greenland Ice Sheet melts?
Three things happen. One is that you’re dumping all of this melt water into the ocean. So the mass of the entire ocean would definitely be going up if ice sheets were melting—as they are today. The second thing that happens is that this gravitational attraction that the ice sheet exerts on the surrounding water diminishes. As a consequence, water migrates away from the ice sheet. The third thing is, as the ice sheet melts, the land underneath the ice sheet pops up; it rebounds.
So what is the combined impact of the ice-sheet melt, water flow, and diminished gravity?
Gravity has a very strong effect. So what happens when an ice sheet melts is sea level falls in the vicinity of the melting ice sheet. That is counterintuitive. The question is, how far from the ice sheet do you have to go before the effects of diminished gravity and uplifting crust are small enough that you start to raise sea level? That’s also counterintuitive. It’s 2,000 kilometers away from the ice sheet. So if the Greenland ice sheet were to catastrophically collapse tomorrow, the sea level in Iceland, Newfoundland, Sweden, Norway—all within this 2,000 kilometer radius of the Greenland ice sheet—would fall. It might have a 30 to 50 meter drop at the shore of Greenland. But the farther you get away from Greenland, the greater the price you pay. If the Greenland ice sheet melts, sea level in most of the Southern Hemisphere will increase about 30 percent more than the global average. So this is no small effect.
The convolutions of physics can be surprising – it certainly surprised me. I tried to find a gravity map of Greenland, but the best I could do was some NASA gravity maps that are not focused on Greenland, but are fascinating nonetheless.
Credit: NASA VIsualization Explorer
This one graphically illustrates how much larger the mountains of South America are than the American Rockies, doesn’t it? The red, of course, indicates stronger gravitational attraction.
