Why is Earth’s Girth Bulging?

John Roach
For National Geographic News

This is heavy.

Something massive is moving on or within the Earth and causing the planet’s gravity field to get wider around the equator and flatter at the poles, according to a pair of scientists studying the field with sensitive satellite instruments.

The scientists are uncertain as to the reasons for this phenomenon, which was just the opposite for several decades prior to 1997, but think the answer possibly lies within long-term variation in the oceans.

“Starting after 1997, the world that was getting rounder started getting more oblate [flattened at the poles],” said Christopher Cox, a research scientist at the Space Geodesy Branch of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

At first, the scientists had any number of explanations for this observed phenomenon—from changes in the atmosphere to the amount of water in the oceans to ocean tidal effects. But the Earth kept getting fatter at the equator and flatter at the poles.

“It has finally gotten so big that we can’t explain it with any known mechanism,” said Cox, who co-authored a paper in the August 2 issue of the journal Science on this change in shape of the Earth’s gravity field.

The change in shape since 1997 is very subtle: an increase in equatorial radius of about one 25th of an inch (one millimeter) per year, according to measurements.

“What is interesting is that it tells us that some mass redistribution occurred inside the Earth system, probably the climate system,” said Anny Cazenave, an Earth scientist at the National Center for Space Studies in Toulouse, France. “This is an interesting constraint to climate models.”

Post-Glacial Rebound

Prior to 1997 the Earth was getting rounder as it recovered from thousands of years of being squished at the poles by the weight of Ice Age glaciers, an effect scientists refer to as post-glacial rebound (PGR).

As the ice sheets melted, the land that was beneath the ice started rising. As the ground rebounded in this fashion, the gravity field changed. The effect is likened to putting a finger on a rubber ball and watching it slowly bounce back.

“These extremely heavy objects basically squished the Earth, caused the land to sink and move away from the edges,” said Cox. “When they melted, they can’t instantly bounce back up.”

Observations of the Earth’s gravity field sensed with laser tracking instruments on ten satellites in orbit around Earth indicate that some phenomenon is counteracting the PGR effect and causing the Earth to get wider at the equator.

Changing Shape

“Now that we have seen the signal, what is it?” asked Cox, who said that he and his colleague Benjamin Chao at Goddard Space Flight Center were perplexed by the phenomenon.

Scientists agree on three areas that could cause large changes in the Earth’s gravity field: the oceans, polar and glacial ice, and the atmosphere.

Cox and Chao have ruled out the atmosphere as the cause and suggest that it is the result of either a large amount of water moving into the oceans as a result of global warming, movement within the Earth’s core, or some other phenomenon within the oceans.

Measurements of the amount of ice melting and flowing into the oceans are too small to account for the observed changes in the gravity field. To be the cause, it would require melting a block of ice 6.2 miles (10 kilometers) on each side by 3.1 miles (5 kilometers) high every year since 1997.

“The recent reports of large icebergs calving into the ocean in Antarctica can’t explain this because they are already floating in the ocean,” said Cox in a statement. As well, satellite observations of average sea level rise show no corresponding change in the rate of global sea level increase.

The researchers also considered movements in the Earth’s core. Scientists had assumed that such changes of mass could not cause any observable changes in the gravity field, but recent modeling efforts suggest that in some cases the changes could be detected.

“However, even in those cases, it is several times too small to cause the recent changes,” said Cox, although he added that the scenario could not be completely ruled out.

The process of elimination has led the scientists to consider that the mass must be related to a long-term variation in the oceans. An example of such a variation is the Pacific Decadal Oscillation, said Cox.

Cazenave, who co-authored a perspective on this research also in the August 2 issue of Science, agreed that the observed change in the Earth’s gravity field is most likely related to variation within the oceans.

“We believe that the most plausible source is in the ocean, in particular change in the circulation of the mid- to high-latitude Pacific Ocean,” she said. “But this has to be confirmed by additional observations and modeling.”

In Sync

And while the scientists are as equally uncertain as to the effects of a gravity field that is more oblate, the change in shape curiously coincides with a span of years in which the world’s timekeepers have not had to add any leap seconds.

“It used to be that these things (leap seconds) were inserted into our time system once or twice a year. For the last several years we have not had to do that,” said Cox.

Is there a link between what is causing the change in gravity field and the fact that clocks are in sync with Earth’s rate of rotation?

Donald Sullivan, Chief of the Time and Frequency Division at the National Institute of Standards and Technology in Boulder, Colorado, does not rule out the possibility, but cautions that any number of changes in gravity can influence the Earth’s rate of rotation.

“The moon and planets, along with geophysical and climate variations can affect the rotation rate of the Earth,” he said. “If you think about this for a moment, you’ll realize that all it takes is a small change in the average distribution of mass to change this rate.”