The Challenge for Measuring Sea Level Rise and Regional and Global Trends

This Plenary Paper on sea level is based on several Community White Papers submitted to OceanObs09. Considerable progress has been realized during the past decade in measuring sea level change globally and regionally, and in understanding the climate-related causes of observed changes. We first review current knowledge about sea level change, globally and regionally. We summarize recent results from the 2007 IPCC 4th Assessment Report (AR4), as well as post-IPCC results relevant to sea level observations, causes and projections. New challenges are identified for the coming decade in terms of observations, modelling and impact studies. From these challenges, a number of recommendations emerge, which are listed below: a) An accurate (at the <0.3 mm/yr level uncertainty), multi-decade-long sea level record by altimeter satellites of the T/P- Jason class is essential, as is continued funding of the altimeter science team to provide leadership. To meet the goal of 0.3 mm/yr or better in sea level rate accuracy, the global geodetic infrastructure needs to be maintained on the long-term; the Terrestrial Reference Frame must be accurate and stable at the 1 mm and 0.5 mm/yr level; radiometers required for the correction of radar path delays must also be stable (or calibrated) at 0.1 mm/year. A network of tide gauges with precise positioning (GPS, or moregeneral, GNSS) should be maintained with an emphasis on long record lengths and global spatial coverage (e.g., the GLOSS Core Network plus additional stations with especially long record lengths). b) Continuity of GRACE-type space gravimetry observations is critically needed. No other data exist to measure ocean mass changes directly. To avoid an undesirable gap in data record, a GRACE Stop-Gap mission should be undertaken by space agencies to continue the geophysical time series of the current GRACE mission. Meanwhile, new concept for improving precision and resolution need to be developed.. c) Improved accuracy for the Glacial Isostatic Adjustment (GIA) forward modelling that are needed to provide corrections for GRACE, tide gauges and satellite altimetry observations over ocean, land and ice-sheets should be made available. Specifically, the GIA community should be encouraged to perform intercomparison studies of GIA modelling, similar to what has been done for coupled climate model outputs. The goal should be to produce a global, spatially varying, community wide best-estimate of GIA and its uncertainty that is appropriate for application to global sea level studies. d) Long-term maintenance of the Argo network in its optimal configuration is imperative for measuring ocean temperature and salinity; development of a shipboard CTD measurement program for absolute calibration of other in situ hydrographic data is critical to maintain the fidelity of other networks; reanalyses of historical temperature and salinity is strongly recommended; development of new methods/systems to estimate deep changes in ocean heat content and thermal expansion is needed. e) High priority should be given to the development of integrated, multidisciplinary studies of presentday and last century sea level changes (global and regional), accounting for the various factors (climate change, ocean/atmosphere forcing, land hydrology change\textemdashboth natural and anthropogenic, solid Earth processes, etc.) that act on a large variety of spatio-temporal scales. Improvement and validation of 2-dimensional past sea level reconstructions is also important, as well as development of attribution studies for global/regional sea level variations using ocean reanalyses. f) Inter comparison of sea level projections from climate models need to be developed to assess uncertainty. Projections need to include regional and decadal variability. Development and inclusion of realistic ice sheet dynamics in coupled climate models is a key issue for projecting sea level change, as the potential contribution from ice sheets like Greenland and Antarctica is much larger than any other source. Finally, as local (relative) sea level rise is among the major threats of future global warming, it is both of primary importance and urgence to: g) develop multidisciplinary studies to understand and discriminate causes of current sea level changes in some key coastal regions, integrating the various factors that are important at local scales (climate component, oceanographic processes, sediment supply, ground subsidence, anthropogenic forcing, etc.) h) implement additional in situ observing systems in vulnerable coastal areas, in particular, tide gauges co-located with GNSS stations for measuring (mainly vertical) ground motions, i) improve current altimetry-based sea level observations in coastal zones and continue to develop SWOT (Surface Water and Ocean Topography) satellite mission, a wide-swath altimeter, for accurate future monitoring of local sea level changes at the land-sea interface. SWOT is able to measure sea surface height in the presence of sea ice and is thus able to provide information on ocean circulation near ice shelves for studying the process of the breakup of ice shelves that buttress ice sheets. j) for decision support, provide reliable local sea level forecasts on time scales of decades. Improved sea level (global and regional) projections at centennial time scales are also desired. This long list of recommendations results from the interdisciplinary nature of sea level studies. Recommendations a, b and d rely on the continuity of observing systems and are directed towards space agencies and international organizations. No priority can be given as satellite altimetry, space gravimetry and Argo are all complementary and critically needed to observe and understand sea level. Recommendations c, e and f concern the sea level community itself and ask for better organisation and closer collaboration between data analysists and modellers. Finally, recommendations g to j call for better understanding of coastal impacts and call for wider collaboration between Earth science researchers.