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

R. Steven Nerem
Satellite Observations of
Present Day Sea-Level Change
Observations of longterm
sea-level change
can provide important
corroboration of climate
variations predicted by
models and can also
help us prepare for the
socioeconomic impacts of
sea-level change. The Topography
Experiment/
Poseidon (TOPEX/
Poseidon, 1992), Jason-1
(2001) and Jason-2 (2008)
satellites have observed
a mean rate of sea-level
rise of 3.2 mm/year since
1993 (http://sealevel
.colorado.edu; Nerem et
al., 2010). The Gravity
Recovery and Climate
Experiment (GRACE) satellite mission has precisely measured
temporal variations in the Earth’s gravitational field
since 2002. As the melting of ice in mountain glaciers and
ice sheets, in addition to other runoff, adds water mass
to the oceans, GRACE has demonstrated the ability to
directly measure this change in mass.
Recently, GRACE data have been used to assess the impact
of melting ice in Greenland and Antarctica on changes
in the Earth’s oblateness (J2) [(Nerem and Wahr, 2011).
These changes were then compared to a long time series
of J2 variations from satellite laser ranging observations
(1975–present). The results (Figure 1) show that Greenland
and Antarctica caused most of the observed change in J2
since 2002 after correcting for glacial isostatic adjustment
(GIA). Because the SLR J2 observations started to change
character in the mid-1990s, one interpretation of these
results is that ice-mass loss from Greenland and Antarctica
started to accelerate in the mid-1990s, which is consistent
with other glaciological and tide-gauge evidence.
The GRACE observations of Earth’s gravity field
changes coupled with the longer time series of J2 observations
from SLR data suggest that mass loss from Greenland
and Antarctica is accelerating, and this acceleration
Variations in the Earth’s oblateness (J2) from satellite laser ranging (SLR)
tracking data (red) and from GRACE measurements of Greenland and
Antarctica (blue) [Nerem and Wahr, 2011].
began in the mid-1990s. This is important information as
we assess the current contributions to sea-level change
and what may happen to the rate of sea-level change in
the future.
Satellite altimeter and gravity measurements are expected
to have a major role in the formulation of the fifth
Intergovernmental Panel on Climate Change (IPCC) climate
assessment in 2013, of which I am a Lead Author for
the sea-level change chapter. Satellite altimetry has shown
conclusively that sea-level rise has been greater over the
last 18 years as compared to the last century. The record of
ice-mass changes from the GRACE mission (nine years),
while too short to definitively detect climate signals, has
demonstrated the ability to measure changes in the mass
of the oceans and the mass of the polar ice sheets. Thus, as
this time series becomes longer, it is expected that satellite
gravity missions will play an equally important role to
satellite altimetry in diagnosing the magnitude of sea-level
change and its causes.
CIRES Annual Report 2011 41