Lumpy Gravity Ball

Both computer drawings are exagerated to show the effect, but not drawn to scale.

Is the Earth's gravity constant over the entire surface? Warts and all: The Earth as seen by the GRACE satellites. The red spots represent places where the Earth's gravity is unusually strong. The blue ones are where it's weak. Image: NASA/University of Texas Center for Space Research

The short answer is Earth's gravity is constant. The red spots represent places where the Earth's gravity is unusually strong. The blue ones are where it's weak. Not that the force of gravity itself varies. Rather, it's an indication that the Earth's mass distribution isn't quite uniform. Mountain building in South America and the Himalayas produces dense, red zones; elsewhere, tectonic movements produce thin, blue, ones.

The ice stacked up on Greenland and Antarctica is piled so high it has depressed the center of these continents below sea level. If it were to melt it would form a huge lake. The Great Lakes are still "exhaling" from the ice that was piled on the surface surrounding the lakes from the last ice age. Soon the levels will stabilize and the Niagra River will flow no more.