David E. Steitz
Headquarters, Washington
(Phone: 202/358-1730)

Lynn Chandler
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-2806)

RELEASE: 02-147

Satellite data since 1998 indicates the bulge in the
Earth’s gravity field at the equator is growing, and
scientists think that the ocean may hold the answer to the
mystery of how the changes in the trend of Earth’s gravity
are occurring.

Before 1998, Earth’s equatorial bulge in the gravity field
was getting smaller because of the post-glacial rebound, or
PGR, that occurred as a result of the melting of the ice
sheets after the last Ice Age. When the ice sheets melted,
land that was underneath the ice started rising. As the
ground rebounded in this fashion, the gravity field changed.

“The Earth behaved much like putting your finger into a
sponge ball and watching it slowly bounce back,” said
Christopher Cox, a research scientist supporting the Space
Geodesy Branch at NASA’s Goddard Space Flight Center,
Greenbelt, Md.

Currently, the Earth has a significant upward bulge at the
equator, and a downward bulge at the poles. “Observations of
the Earth’s gravity field show that some phenomena are
counteracting the gravitational effects of PGR. Whereas PGR
has been decreasing the bulge in the Earth’s gravity field at
the equator, this recent phenomena is causing the bulge to
increase,” Cox said. Such changes in the gravity field can be
sensed using ultra precise laser tracking of satellites to
observe tiny changes in the orbits of those satellites and by
tracking changes in the length of day or rotation of the

Scientists believe movements of mass cause this recent change
from the high latitudes to the equator. Such large changes
may be caused by climate change, but could also be part of
normal long-period climatic variation. “The three areas that
can trigger large changes in the Earth’s gravitational field
are oceans, polar and glacial ice, and atmosphere,” Cox said.

Cox and colleague Dr. Benjamin Chao have ruled out the
atmosphere as the cause. Instead, they suggest a significant
amount of Ice or water must be moving from high latitude
regions to the equator, and oceans could be the vehicles of
this movement.

Estimates of today’s glacier and polar ice melting are too
small to explain the recent changes in the gravity field. If
melting ice were the cause of the recent changes in the
gravitational field, it would require melting a block of ice
10 km (6.2 miles) on each side by 5 km (3.1 miles) high every
year since 1997 and pouring it into the oceans.

“The recent reports of large icebergs calving in Antarctica
can’t explain this, because they were already floating in the
ocean,” Cox said. Further, radar altimeter observations of
the average sea level rise provided by the TOPEX/POSEIDON
satellite show no corresponding change in the rate of the
global sea level increase.

Consequently mass must have been redistributed within the
oceans. That’s where the ocean circulation theory comes in.
Ocean currents can redistribute mass quickly, such as the 5-
year time frame that these changes were first observed. The
TOPEX/POSEIDON observations of sea level height do show an
increase in the equatorial bulge of the oceans corresponding
to the observed gravity changes, but the data are not yet
conclusive. One critical factor is the temperature of the
world’s oceans, and its salinity, for which detailed data are
not yet available.

In 2002 NASA also launched the GRACE and JASON missions,
missions that will help to more precisely track these sorts
of changes in Earth’s geodesy, and will launch the ICESAT
mission this winter.

An article on this NASA-funded study appears in the August 2
issue of the journal Science.

For more information and images, go to:

The web site for the International Laser Ranging Service can
be found at:

For more about the TOPEX/Poseidon mission, go to: