THE ANTARCTIC ICE SHEET CONTAINS ENOUGH ICE TO RAISE GLOBAL SEA LEVEL BY ~58 M IF IT MELTED ENTIRELY. EVEN A 0.5 M SEA-LEVEL RISE CONTRIBUTION (~1% REDUCTION IN ICE-SHEET MASS) WHICH IS NOW REGARDED AS POSSIBLE ON TIME SCALES OF DECADES TO A CENTURY WOULD PRESENT A SIGNIFICANT THREAT TO COASTAL COMMUNITIES AND INFRASTRUCTURE. THIS PROPOSAL ADDRESSES ONE SIGNIFICANT FACTOR INFLUENCING THE PROJECTED RATE OF MASS LOSS FROM THE GROUNDED PORTION OF THE ANTARCTIC ICE SHEET: CLIMATE-DRIVEN CHANGES IN THE THICKNESS AND MASS OF ITS ICE SHELVES THE FLOATING PORTIONS OF THE ICE SHEET WHERE IT COMES INTO CONTACT WITH THE OCEAN. CONTACT OF ICE SHELVES WITH THE SIDEWALLS AND LOCAL HIGH SPOTS ON THE SEABED EXERTS A BACK-STRESS ON THE GROUNDED ICE STREAMS AND GLACIERS THAT CARRY ICE INTO THE OCEAN. OBSERVATIONS AND MODELS CONFIRM THAT LOWERING OF THIS BACK-STRESS CAUSED BY REDUCTION OF ICE-SHELF VOLUME INCREASES DYNAMIC LOSS OF GROUNDED-ICE MASS AND CONSEQUENT SEA-LEVEL RISE. OUR TEAM HAS RECENTLY DEVELOPED THE LONGEST AND MOST HIGHLY-RESOLVED RECORD OF SPACE- AND TIMEDEPENDENT HEIGHT CHANGE FOR THE 85% OF TOTAL ICE-SHELF AREA THAT WAS COVERED BY ESA S RADAR ALTIMETER (RA) SATELLITES UP TO 2012 [PAOLO ET AL. 2015 SCIENCE]. THIS 18-YEAR RECORD SHOWS THAT ICE SHELVES ARE CHANGING ON A BROAD RANGE OF TIMESCALES FROM ANNUAL TO DECADAL AND LIKELY LONGER. THE PROPOSED STUDY IS ORGANIZED AROUND THREE GOALS: (1) DEVELOP TECHNIQUES TO EXTEND THE ICE-SHELF HEIGHT RECORD THROUGH THE ESA S CURRENT CRYOSAT-2 RA MISSION AND THE EARLY YEARS OF NASA S ICESAT-2 LASER ALTIMETER MISSION (LAUNCH IN LATE 2017); (2) IDENTIFY CLIMATE MECHANISMS CONTRIBUTING TO INTERANNUAL VARIABILITY IN ICE-SHELF VOLUME SEEN IN THE EXTENDED TIME SERIES; AND (3) ESTIMATE THE CONTRIBUTION OF FRESHWATER FLUX TO THE SOUTHERN OCEAN FROM ICE-SHELF VOLUME CHANGES. BY COMPARING THE TIME AND SPACE VARIABILITY OF ICE-SHELF HEIGHT WITH MEASURED AND MODELED CHANGES IN THE ATMOSPHERE AND OCEAN WE WILL IDENTIFY WHICH PROCESSES HAVE THE LARGEST EFFECT ON ICE-SHELF THICKNESS AND THEREFORE ON THE BUTTRESSING EFFECT ON THE ADJACENT GROUNDED ICE. THERE ARE VERY FEW SIMULTANEOUS OBSERVATIONS OF ATMOSPHERIC AND OCEAN STATE DURING THE SAME PERIOD THAT ICE-SHELF HEIGHT DATA ARE AVAILABLE AND SO OUR CLIMATE INFORMATION WILL COME FROM EXISTING MODELS: ATMOSPHERIC REANALYSIS PRODUCTS FOR THE SAME PERIOD AS WE HAVE ICE-SHELF HEIGHT DATA (1994-PRESENT); GLOBAL OCEAN STATE MODELS SUCH AS ECCO-2; AND GLOBAL COUPLED CLIMATE MODELS DEVELOPED FOR THE IPCC AR5 REPORT. THE DAILY OBSERVATIONAL RECORD OF SEA-ICE CONCENTRATION (1979-PRESENT) PROVIDES FURTHER GUIDANCE ON VARIABILITY IN BOTH THE ATMOSPHERE AND OCEAN. THIS STUDY DIRECTLY ADDRESSES THE 2016 ROSES CALL FOR A.16: STUDIES WITH ICESAT AND CRYOSAT-2. WE WILL CARRY OUT COMPARATIVE STUDIES OF ICESAT ERS/ENVISAT OIB CRYOSAT-2 AND EARLY YEARS OF ICESAT-2 MEASUREMENTS OVER ICE SHELVES AND USE DATA FROM OTHER SATELLITE SENSORS (INCLUDING SSM/I SEA ICE CONCENTRATIONS) TO AUGMENT THESE STUDIES. INITIAL COMPARISONS BETWEEN EXISTING LASER ALTIMETER (ICESAT AND OIB) AND RA DATA WILL FACILITATE RAPID INTEGRATION OF ICESAT-2 DATA AS IT BECOMES AVAILABLE. THIS EFFORT WILL GENERATE LONGTERM CONTINUOUS HOMOGENEOUS RECORDS OF CHANGE IN THE POLAR ICE SHEETS CONTRIBUTING TO CONTINUITY OF THE ANTARCTIC ICE-SHELF CHANGE RECORD INTO THE ICESAT-2 ERA. BY COMPARING THESE RECORDS WITH CLIMATE REANALYSIS AND PREDICTIVE MODELS WE WILL ALSO QUANTIFY THE PROCESSES CAUSING CHANGES IN POLAR ICE ESPECIALLY TO IDENTIFY THE RESPONSE OF ANTARCTIC ICE SHELVES TO CLIMATE FORCING AND DEVELOP OUR PROCESS UNDERSTANDING TO IMPROVE PREDICTIVE MODELS. THE STUDY CONTRIBUTES TO NASA'S STRATEGIC PLAN OBJECTIVE GOAL 2.2 TO ADVANCE KNOWLEDGE OF EARTH AS A SYSTEM TO MEET THE CHALLENGES OF ENVIRONMENTAL CHANGE AND TO IMPROVE LIFE ON OUR PLANET