\\ CU GRACE Research

The Gravity Recovery and Climate Experiment (GRACE) 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.

For 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. The top figure at right 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 December 2009. The trend of the best fitting straight line is about 260 km3/yr of ice volume lost per year, which generates enough meltwater each year to cover all of Colorado to a depth of 90 cm. There is a notable downward curvature to the results, indicating that the mass loss rate has been increasing during this time period. The lower figure shows how this mass loss rate is distributed across Greenland, as determined from the GRACE solutions. By far the largest rates occur in the southeast (note the break in the color scale), where dramatic acceleration of outlet glaciers and accompanying ice thinning has been observed over the last few years. But mass loss has also been occurring up along the western ice sheet margin, particularly within the last 2-3 years. There appears to have been a modest mass gain in the northern interior, presumably associated with increased accumulation rates there.

For other land areas, GRACE recovers changes in the total stored water: the sum of water on the surface, in the soil, and beneath the soil layer. Before GRACE, there was no practical way of measuring total water storage at regional- to global-scales.

Jacob T, Wahr J, Pfeffer WT, Swenson S (2012). Recent contributions of glaciers and ice caps to sea level rise. Nature 482. doi: 10.1038/nature10847.

Swenson S, Wahr J (2009). Monitoring the water balance of Lake Victoria, East Africa, from space. Journal of Hydrology 370(4-Jan). doi: 10.1016/j.jhydrol.2009.03.008.

Tiwari VM, Wahr J, Swenson S (2009). Dwindling groundwater resources in northern India, from satellite gravity observations. Geophysical Research Letters 36. doi: 10.1029/2009GL039401.

Swenson S, Famiglietti J, Basara J, Wahr J (2008). Estimating profile soil moisture and groundwater variations using GRACE and Oklahoma Mesonet soil moisture data. Water Resources Research 44(1). doi: 10.1029/2007WR006057.

Khan SA, Wahr J, Leuliette E, van Dam T, Larson KM, Francis O (2008). Geodetic measurements of postglacial adjustments in Greenland. Journal of Geophysical Research-Solid Earth 113(B2). doi: 10.1029/2007JB004956.

Swenson S, Chambers D, Wahr J (2008). Estimating geocenter variations from a combination of GRACE and ocean model output. Journal of Geophysical Research-Solid Earth 113(B8). doi: 10.1029/2007JB005338.

Morison J, Wahr J, Kwok R, Peralta-Ferriz C (2007). Recent trends in Arctic Ocean mass distribution revealed by GRACE. Geophysical Research Letters 34(7). doi: 10.1029/2006GL029016.

Paulson A, Zhong SJ, Wahr J (2007). Inference of mantle viscosity from GRACE and relative sea level data. Geophysical Journal International 171(2). doi: 10.1111/j.1365-246X.2007.03556.x.

Khan SA, Wahr J, Stearns LA, Hamilton GS, van Dam T, Larson KM, Francis O (2007). Elastic uplift in southeast Greenland due to rapid ice mass loss. Geophysical Research Letters 34(21). doi: 10.1029/2007GL031468.

van Dam TJ, Wahr J, Lavallee D (2007). A comparison of annual vertical crustal displacements from GPS and Gravity Recovery and Climate Experiment (GRACE) over Europe. Journal of Geophysical Research-Solid Earth 112(B3). doi: 10.1029/2006JB004335.

Zlotnicki V, Wahr J, Fukumori I, Song YT (2007). Antarctic circumpolar current transport variability during 2003-05 from GRACE. Journal of Physical Oceanography 37(2). doi: 10.1175/JPO3009.1.

Swenson S, Wahr J (2006). Post-processing removal of correlated errors in GRACE data. Geophysical Research Letters 33(8). doi: 10.1029/2005GL025285.

Swenson S, Yeh PJF, Wahr J, Famiglietti J (2006). A comparison of terrestrial water storage variations from GRACE with in situ measurements from Illinois. Geophysical Research Letters 33(16). doi: 10.1029/2006GL026962.

Wahr J, Swenson S, Velicogna I (2006). Accuracy of GRACE mass estimates. Geophysical Research Letters 33(6). doi: 10.1029/2005GL025305.

Swenson S, Wahr J (2006). Estimating large-scale precipitation minus evapotranspiration from GRACE satellite gravity measurements. Journal of Hydrometeorology 7(2). doi: 10.1175/JHM478.1.

Velicogna I, Wahr J (2006). Measurements of time-variable gravity show mass loss in Antarctica. Science 311(5768). doi: 10.1126/science.1123785.

Velicogna I, Wahr J (2005). Greenland mass balance from GRACE. Geophysical Research Letters 32(18). doi: 10.1029/2005GL023955.

Chambers DP, Wahr J, Nerem RS (2004). Preliminary observations of global ocean mass variations with GRACE. Geophysical Research Letters 31(13). doi: 10.1029/2004GL020461.

Wahr J, Swenson S, Zlotnicki V, Velicogna I (2004). Time-variable gravity from GRACE: First results. Geophysical Research Letters 31(11). doi: 10.1029/2004GL019779.

Jayne SR, Wahr JM, Bryan FO (2003). Observing ocean heat content using satellite gravity and altimetry. Journal of Geophysical Research-Oceans 108(C2). doi: 10.1029/2002JC001619.

Swenson S, Wahr J, Milly PCD (2003). Estimated accuracies of regional water storage variations inferred from the Gravity Recovery and Climate Experiment (GRACE). Water Resources Research 39(8). doi: 10.1029/2002WR001808.

Swenson S, Wahr J (2002). Estimated effects of the vertical structure of atmospheric mass on the time-variable geoid. Journal of Geophysical Research-Solid Earth 107(B9). doi: 10.1029/2000JB000024.

Swenson S, Wahr J (2002). Methods for inferring regional surface-mass anomalies from Gravity Recovery and Climate Experiment (GRACE) measurements of time-variable gravity. Journal of Geophysical Research-Solid Earth 107(B9). doi: 10.1029/2001JB000576.

Velicogna I, Wahr J (2002). A method for separating antarctic postglacial rebound and ice mass balance using future ICESat Geoscience Laser Altimeter System, Gravity Recovery and Climate Experiment, and GPS satellite data. Journal of Geophysical Research-Solid Earth 107(B10). doi: 10.1029/2001JB000708.

Velicogna I, Wahr J (2002). Postglacial rebound and Earth's viscosity structure from GRACE. Journal of Geophysical Research-Solid Earth 107(B12). doi: 10.1029/2001JB001735.

Wu XP, Watkins MM, Ivins ER, Kwok R, Wang P, Wahr JM (2002). Toward global inverse solutions for current and past ice mass variations: Contribution of secular satellite gravity and topography change measurements. Journal of Geophysical Research-Solid Earth 107(B11). doi: 10.1029/2001JB000543.

van Dam T, Wahr J, Milly PCD, Shmakin AB, Blewitt G, Lavallee D, Larson KM (2001). Crustal displacements due to continental water loading. Geophysical Research Letters 28(4). doi: 10.1029/2000GL012120.

Velicogna I, Wahr J, Van den Dool H (2001). Can surface pressure be used to remove atmospheric contributions from GRACE data with sufficient accuracy to recover hydrological signals?. Journal of Geophysical Research-Solid Earth 106(B8). doi: 10.1029/2001JB000228.

Wahr J, Wingham D, Bentley C (2000). A method of combining ICESat and GRACE satellite data to constrain Antarctic mass balance. Journal of Geophysical Research-Solid Earth 105(B7). doi: 10.1029/2000JB900113.

Wahr J, Molenaar M, Bryan F (1998). Time variability of the Earth's gravity field: Hydrological and oceanic effects and their possible detection using GRACE. Journal of Geophysical Research-Solid Earth 103(B12). doi: 10.1029/98JB02844.