Tag Archives: greenland ice sheet

Gravity

You might reasonably expect a satellite to orbit the earth in a trajectory smooth as a ball bearing in its race. Reasonable but wrong. It’s more like an Old Ford on a country road, bouncing and rattling over gravitational potholes.

The gravitational topography of the planet is less like a cue ball, more like a golf ball with all its bumps and dimples. You’ll remember your high school physics lessons on gravitation. Large objects exert gravitational force at a distance. The more dense the object, the greater the force. Massive mountain chains like the Rockies, Himalaya, and Andes create positive gravitational anomalies—areas of increased gravitational force. Negative anomalies are associated with declivities like the Mariana Trench, a rift in the sea floor of the Pacific Ocean 6.8 miles deep.

As a satellite approaches the Andes it’s pulled subtly closer to the earth. When it approaches the Mariana Islands and the adjacent sea floor trench, it bobs slightly higher. The effect is local and canceled when the satellite reenters the normal gravitational field. It’s rather like bouncing down the washboard surface of a dirt road.

The gravitational force exerted by the Andes isn’t limited to circling satellites. The roots of the Andes Mountains are washed by the Pacific Ocean. The Pacific itself is pulled toward the mountains like a massive standing wave.

It’s not only massive piles of rock that creates gravitational anomalies. The Greenland ice sheet is almost 1,500 miles long and 680 miles wide. It covers roughly 80% of the surface of Greenland—660,235 square miles. It’s typically over a mile thick and almost 2 miles at its deepest—683,751 cubic miles of ice.  And it’s the second largest body of ice on the planet.

That much ice warps the surrounding ocean, pulling it like taffy. The impact of melting Greenland ice on sea level has been recognized for some time but the effect of its reduced gravitational field has only recently been acknowledged. It further complicates a complex picture.

If all the Greenland ice sheet were to melt it’s estimated the global sea level might rise as much 23 feet but it would have little impact on sea level in the Arctic ocean. That’s counter-intuitive. The reason? Rise in sea level expected from melting ice would be countered by the fall in seal level resulting from reduced gravity. Northern Europe might be spared. New York would not.

The gravitational influence of the Greenland ice is limited to the Arctic Ocean. Melting of the northern ice would contribute to the volume of the oceans globally, increasing sea level worldwide, but that rise in the Arctic would be offset by the declining sea level resulting from reduced gravity. Areas beyond the northern ice’s gravitational influence such as the Eastern Seaboard of the US would suffer the unmitigated rise in sea level.

Western Europe isn’t without risk. If the Antarctic ice sheet melts, the sea level will likely fall in the Southern Ocean but rise dramatically in the North Atlantic.

And, of course, the change in the gravitational field would affect the Earth’s rotational momentum, but that’s a nightmare for another day.

earth orbit photo