2011 - Cooperative Institute for Research in Environmental Sciences ...

John Wahr
Applications of Time-Variable
Gravity Measurements from GRACE
FUNDING: NASA, JET PROPULSION LABORATORY
The GRACE (Gravity
Recovery and Climate
Experiment) satellite mission,
launched by NASA
and the German Space
Agency in March 2002, is
providing global maps of
the Earth’s gravity field
to astonishing accuracy
every month. Because
the Earth’s gravity field
is caused by its mass distribution,
time-variations
in gravity as determined
from GRACE data can be
used to estimate monthto-month
changes in the
Earth’s mass distribution.
GRACE can recover mass
variability at scales of
about 250–300 km and larger.
We have been using these data to look at a number of
geophysical signals, particularly those that involve the
storage of water (including snow and ice) on continents
and in the polar ice sheets.
As one example, because of its large effective footprint
and its sensitivity to mass, GRACE offers the best available
method for measuring the total mass balance of the
polar ice sheets. Figure 1 shows monthly GRACE results
(black line; the orange line is a smoothed version) for the
mass variability summed over the entire Greenland ice
sheet, between April 2002 and March 2011. The trend of
the best-fitting straight line is about 220 gigatons/yr of
ice-mass loss, which corresponds to enough water each
year to cover all of Colorado to a depth of almost 1 meter.
There is a notable downward curvature to the results,
indicating that the mass-loss rate was increasing during
this time period. But hiding behind this reasonably simple
single time series is a mass-loss signal of considerable spatial
and temporal complexity. Figure 2, for example, shows
how the mass-loss rate was distributed across Greenland
for four two-year timespans, as determined from the
GRACE solutions. It shows the mass loss began in earnest
in about 2005, in southeast Greenland. That region tended
to stabilize in 2007, while at the same time significant
mass loss appeared in the northwest. During the last
couple of years, the mass-loss pattern reverted to something
closer to the 2005–2007 pattern. The main source of
these dramatic mass-loss rates is increased velocities of
outlet glaciers. The GRACE results indicate how volatile
those velocities have been and suggest that the dynamics
controlling those velocities will be difficult to sort out.
Figure 1: Monthly GRACE results (black line; the orange line is a
smoothed version) for the mass variability summed over the entire
Greenland ice sheet, between April 2002 and March 2011.
Figure 2: How the mass-loss rate was distributed across Greenland for
four two-year timespans, as determined from the GRACE solutions.
CIRES Annual Report 2011 57