Gravity & Sea Level

Changes in global sea level arising from man's activity is today a subject of wide interest and concern. A number of satellite missions, in Europe and the United States, are aimed at improving our knowledge and understanding of the ongoing changes in sea level. The ERS, Jason, CryoSat, Champ, GRACE and GOCE satellites are all aimed at contributing to aspects of the sea level problem. However, changes in sea level arise from a range of processes at the Earth's surface and within the 'solid' Earth that act on timescales that range from 1 to 10 7 years. The pattern of sea level change that will emerge from the ongoing sequence of satellite altimeter missions will be the net contribution of all these processes. An understanding of the likely behaviour of the components of the problem is essential to gaining a comprehensive capability to predict sea level rise. Sea level rise is often accounted for by summarising mass budgets. In fact, however, the problem of sea level rise is complicated because a number of contributions interact with one another. For example, at the centre of concerns about sea level are the ice sheets. Changes in their mass not only effect eustatic sea level directly; they effect it too through viscous flows in the Earth's mantle that change the shape and gravity field of the Earth, and they effect the ocean dynamical circulation because freshwater fluxes affect the buoyancy of the ocean water masses. Thus a comprehensive understanding of sea level needs not only to identify causes of sea level changes, but also ways in which they may interact. This study is concerned with a comprehensive survey of the magnitude and spatial distribution of the evolution of sea level in the 20 th and 21 st centuries. Its particular contribution is to examine not only the (now fairly well known) individual contributions to sea level change, but in addition to examine in addition the interactions that may occur between changes the ice sheets, the solid Earth, and the ocean dynamical circulation. Sea Level & Gravity 6 In the first section of the report, 'The Status of Understanding', we survey what is known concerning the various contributions to sea level. The section is split into three sections, which describe respectively the contributions of the 'solid' Earth, the ice sheets and the oceans. 'Solid' is here placed in inverted commas, because of course variations …