Most likely due to the movement of plate tectonics. They are plates of earth under the ocean that move occasionally, over the years this is probably what happened.

The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.

Explanation:

it increased 6 percent and reached that in 1980

the answer is d trust me