Principal Investigator (PI): Josh Willis, NASA's Jet Propulsion Laboratory

The trend in global mean sea level is one of the most important observations of human-caused climate change. It is simultaneously an indicator and an impact of anthropogenic global warming. And, it is poorly predicted. Estimates of global mean sea level rise range from 20 cm to 2 meters by the end of the 21st Century (Willis and Church, 2012). The stakes are high. The cost of adapting to a 2 m rise over the next century would be an order of magnitude greater than adapting to a 50 cm rise (Nicholls et al., 2011). With such high uncertainty in future projections, the need for continuous monitoring, and the establishment of a highly accurate Earth Science Data Record (ESDR) of global sea level is all the more essential. If the rate of sea level rise were to suddenly accelerate, satellite altimeters would be the first to detect it. In this way, they can function as an early warning system for sea level rise, provided the data are sufficiently accurate.

We propose to develop an ESDR of sea level change based on observations from satellite altimeter missions. Sea level has been measured from space by precise radar altimeter missions since 1992. However, corrections to such observations undergo frequent improvements, and our team is at the forefront of those efforts. These include using of precision orbit determination (POD) and terrestrial reference frames, radiometer calibrations for the wet path delay correction, the dry path delay correction, sea state bias, tides (which also affect the POD and are not a correction but a well understood signal that is removed), instrument corrections, etc. All of these corrections will be improved and made consistent across the missions. Both alongtrack, gridded (in space and time), and globally-averaged data products will be generated in widely used formats with metadata conforming to widely used standards. We will apply these improvements to existing and future altimetric satellites (e.g., the upcoming Satellite with Argos and Altika (SARAL) data). We will also provide globally-averaged estimates of the steric and mass components of sea level rise derived from Argo floats and the Gravity Recovery and Climate Experiment (GRACE) satellites from 2005 to date. The sea level data and their uncertainty estimates will be carefully validated against a network of tide gauges and a dedicated calibration site (the Harvest Platform).

Satellite altimeters provide the most accurate estimates of global sea level rise, and well over 150 scientific publications per year make use of these data. Global sea level change, however, is not uniform and multi-decadal trends in sea level change reveal complex spatial patterns that are likely to be a mixture of natural and anthropogenic variability (Han et al., 2010; Gille, 2008). A highly accurate climate data record of sea level change will be critically important if such changes are to be explained and attributed to either anthropogenic or natural drivers. Indeed, such an understanding is urgently needed in order to make improved projections of regional sea level change, which are still poorly constrained by state-of-the-art climate model projections.

An ESDR of sea level change will also help to fulfill NASA's strategic mission of advancing Earth system science to meet the challenges of climate and environmental change, and to serve the strategic goals of the US Global Climate Change Program to advance science and inform decisions. Due to the nature of sea level rise as both indicator and impact of climate change, the scientific community, policy makers and the public at large will all benefit from improved knowledge of the record of sea level change, both locally and globally.