Modeling Secular Crustal Motion and GRACE Trends for Separation of Surface Glacial Hydrology and Mantle Isostatic Flow

Ivins, Erik R.
Climate, Oceans and Solid Earth Section, JPL/Caltech, Pasadena, CA; Erik.R.Ivins@jpl.nasa.gov; http://science.jpl.nasa.gov/people/Ivins/

The ice sheets of Greenland and Antarctica and the smaller glacier systems of Patagonia, Alaska, Svalbard Is., Norway, Baffin Is., the Canadian Rockies, etc., are all cryopsheric systems affected, one way or another, by warming global temperatures. Changes in these hydrological systems may have solid Earth responses associated with surface mass redistribution of the present and past that are not elastic. The GRACE satellite system has recorded glacial hydrological trends now for more than 8 years. Land glacier extent and volume in the Antarctic Peninsula have been in a state of dramatic demise since the early 1990?s. Using JPL global mascons, and other harmonic releases by the various analysis centers, GRACE gravity trend observations have recently been combined with Global Positioning System (GPS) bedrock uplift data in order to simultaneously solve for ice loss and for viscoelastic Earth glacial isostatic adjustment (GIA). The bedrock uplift owes to Holocene and Little Ice Age (LIA) cryospheric loading-unloading sequences. Methods are discussed that solve for the present-day ice loss rate at -41 ñ 6 Gt/yr in northern Peninsula, as well as regions south of 67^oS. The estimates are shown to be consistent with changes in thickness and out-flux using InSAR and speckle-tracking in the 5 years that preceded the GRACE observing period; 2003-2010. Bounds are recovered for elastic lithosphere thicknesses of 20 ≤ h ≤ 50 km and on upper mantle viscosities of 3 x 1019 ≤ ? ≤ 1020 Pa s. Antarctic Peninsula ice models with a prolonged LIA, extending to AD 1930 are favored in all ?2 fits to the GPS uplift data. This result is largely decoupled from Earth structure assumptions for a range of reasonable load-unloading histories that we sample in the study. The GIA corrections are important, as they account for a large fraction of the space-determined secular gravity change. Collectively, when combined with observations of 1998-2002, the on-land ice losses correspond to mass increases of the oceans equivalent to 0.12 ñ 0.025 mm/yr of sea-level rise for the last 15 years. For several target areas near the Trinity Peninsula and northern Graham Land, the GIA-corrected mass losses are directly compared with ICESat-determined ice height changes spanning 2002-2007.