Consistent multi-technique geodetic estimates of present-day contributions to regional sea level change

This proposal concerns the present-day contributors to global and regional sea level change. Changes in sea level will not be the same world-wide, because the ocean must respond gravitationally to the changes in water mass situated on the continents, as ice sheets thicken or melt and other stores of water accumulate or drain into the oceans (the total water mass must remain fixed). We can use these patterns of change in sea level to identify the contributing factors to sea level rise: in principle, each locality of varying continental water storage will produce a different 'fingerprint' of sea level variation, although in practice we can only resolve changes over areas spanning several hundreds of kilometres. At decadal timescales, globally-averaged sea level will change either (1) because there is a change in the mass of water in the ocean, or (2) because the ocean has expanded in volume due to changes in its temperature and/or salinity. Here, we seek to evaluate the extent of these two effects. In dealing with sea level, we must recognise that there are two ways of measuring it: geocentric (also known as 'global' or 'absolute') sea level, measured with respect to the centre of mass of the whole Earth-ocean-atmosphere system, and relative sea level, measured with respect to the nearby land. Relative sea level is what is important to coastal communities, but it varies from place to place because the land itself may be moving vertically due to plate tectonics, glacio-isostatic adjustment (or 'post-glacial rebound'), present-day changes in the pattern of mass weighing on the Earth's surface, and local effects. These variations make it important to calibrate relative sea level observations, to avoid bias in their global average; conversely, the impact of geocentric sea level change can only be understood if we know the expected vertical movement of the land. In this project, we will bring together a wide range of geodetic measurements. Direct observations of the changing gravity field of the Earth, from the GRACE twin-satellite mission and from laser and microwave ranging to other satellites, will constrain the changes in oceanic and continental water mass distribution. Measurements of the changes in Earth's shape, again from laser and microwave ranging to satellites, will also constrain the loading effects of this mass redistribution on the solid Earth. In contrast, satellite radar altimetry will measure the sea surface height (including both mass change and thermal expansion) with respect to the Earth system's centre of mass; tide gauges will measure sea surface height with respect to the local land surface, but many of them can have their vertical land movements calibrated with GPS. The differences between these various sea surface and ocean mass estimates will constrain the oceanic heat and salinity content, which can be compared with the sparse data and models that exist. We will then be able to identify the recent and likely future contributions of different locations of continental water storage, and oceanic heat/salinity content, to sea level rise. Many of these geodetic datasets have only just reached maturity, or are benefiting from recent step changes in the accuracy of their analysis. This study is therefore well-timed. In addition to helping climate modellers and communities world-wide to understand sea level change, our results will be of particular interest to oceanographers trying to analyse the changing patterns of the oceans' physical properties and circulation.