G - I
ICE MASS BALANCE BY SATELLITE G
The main objective of the proposed work is the study space and time mass variations of ice mad snow over polar regions, the estimation of the water mass balance using monthly GRACE geoids for the recent period, as well as its connection with global climate change, the global hydrological cycle, and in particular the investigation on the possible causes of the sea level rise from ice melting. GRACE (Gravity Recovery and Climate Experiment) is the first geodetic mission dedicated to the measurement of the time-variations of the Earth’s gravity field, it enables the detection of water mass transfers. The on-going GRACE mission (launched in 03/2002 for a nominal lifetime of 5 years; quasi-polar orbit) provides monthly maps of tiny spatio-temporal variations of gravity due to the redistributions of mass inside the surface fluid envelops of the Earth. These satellite measurements represent vertically-integrated gravity effects of water mass reservoirs (oceans, atmosphere, continental waters and ice sheets) and of the solid Earth that need to be unravelled. Monthly solutions of the continental water storage (2002 to present) have been recently extracted from the raw GRACE geoids (at the resolution of 500 km), using an iterative generalized least-squares approach developed in our laboratory (LEGOS; Schmidt et al., 2005; Ramillien et al., 2005). After correction of important geophysical effects like mass changes induced by vertical s to study the ice sheets for the following objectives: (1) Mass balance of the ice sheets: mapping for the first time the time variations of loss/gain of ice mass observed by GRACE, in response to climate change. This will provide monthly time series and geographical maps of the integrated variations of mass over large remote regions like Antarctica and Greenland. (2) Contribution to the sea level rise: Estimating the mass balance of these polar regions, which can be further expressed in terms of sea level contribution. Recent results indicate that over the last 50 years and last 12 years, the contribution of thermal expansion of the oceans to sea level rise is 25% and 60% respectively (Cazenave and Nerem, 2004; Lombard et al., 2005a, b). For both time spans, the water mass addition to the oceans is estimated to about 1.4 mm/yr (Miller and Douglas, 2004; Lombard et al., 2005b). Recent direct estimates of mountain glaciers and ice sheets melting only explain 0.9 mm/yr (IPCC, 2005). In addition for the ice sheets (Antarctica and Greenland), these estimates based on airborne laser altimetry and other approaches are still uncertain (e.g., Thomas et al., 2004; Krabill et al., 2005). Measuring the mass balance of the ice sheets on inter-annual time scale using GRACE will represent a major improvement. (3) Snow pack changes: Another objective of this project is to monitor the snow pack changes in the Arctic regions using GRACE. Changes of the snow pack are currently monitored from space using passive microwave radiometry. However, interpretation of these observations in term of snow mass change is very difficult because of problems related to surface conditions and snow state. Here again, GRACE can provide important constraints on snow mass change at a global scale. Snow mass solutions have been recently computed from the monthly GRACE geoids made available bythe GRACE project (GFZ and CSR) using a robust inverse method of separation of hydrological signals (Ramillien et al., 2004; Ramillien et al., 2005) (4) Monitoring of the permafrost: GRACE is also able to provide information on melt of the permafrost in the boreal regions (e.g., Siberia) through associate water mass redistribution. Used in combination of water level (and discharge) measurements from satellite altimetry over Arctic rivers, GRACE data will allow us to detect and extract the permafrost signals. Cazenave A and R S Nerem, Present-Day sea level change: observations and causes, Review of Geophysics, 42, RG3001, doi : 10.1029/2003RG000139, 2004. Church, J., J.M. Gregory, P. Huybrechts, M. Kuhn, K. Lambeck, M.T. Nhuan, D. Qin, and P.L. Woodworth, Changes in sea level, in Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by J.T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson, pp. 881, Cambridge Univ. Press, Cambridge, 2001. Church J. A., N.J. White, R. Coleman, K. Lambeck and J. Mitrovica, Estimates of the regional distribution of sea level rise over the 1950-2000 period, J; climate, 17, 2609-2625, 2004. Krabill W., E. Hanna, P. Huybrechts, W. Abdalati, J. Cappelen, B. Csatho, E. Frederick, S. Manizade, C. Martin, J. Sonntag, R. Swift, R. Thomas and J. Yungel, Greenland Ice Sheet : increased coastal thinning, Geophys. Res. Lett., in press, 2005. Lambeck, K., and P. Johnston, The viscosity of the mantle: Evidence from the analysis of glacial rebound phenomena, in The Earth's mantle, composition, structure and evolution, edited by I. Jackson, pp. 461-502, Cambridge Univ. Press, Cambridge, UK, 1998. Lombard A., Cazenave A., Le Traon P.Y. and Ishii M., Contribution of thermal expansion to present-day sea level rise revisited, in press Global and Planetary Change, 2005a. Lombard A., Cazenave A., Cabanes C. and R.S. Nerem, 20th century sea level rise: new estimates of thermal and water mass contributions, in press, Global and Planetary Change, 2005b. Miller, L., and B.C. Douglas, Mass and Volume Contributions to 20th Century Global Sea Level Rise, Nature, 428, 406-408, 2004. Ramillien G. and Cazenave A., First results on land hydrology fluctuations (2002-2004) from the GRACE space mission, EPSL, in press, 2005. Ramillien G, A. Cazenave, O. Brunau, Global time variations of hydrological signals from GRACE satellite gravimetry, Geophys. J. Int., 158, 813-826, 2004. Rignot, E., A. Rivera, and G. Casassa, Contribution of the Patagonia Icefields of South America to Sea Level Rise, Science, 302, 434-437, 2003. Schmidt R., Flechtner F., Reigber Ch., Schwintzer P., Gunter A., Doll P., Ramillien G., Cazenave A., Petrovic S., Jochman H. and Wunsch J., GRACE observations of changes in continental water storage, in press, Global and Planetary Change, 2005. Thomas R., E. Rignot,G. Casassa, P. Kanagaratnam, C. Acuna, T. Akins, H. Brecher, E. Frederick, P. Gogineni, W. Krabill, S. Manizade, H. Ramamoorthy, A. Rivera, R. Russell, J. Sonntag, R. Swift, J. Yungel and J. Zwally, Accelerated Sea Level Rise from West Antarctica, Science, 306,255-258, 2004. 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