COULD ESTIMATES OF THE RATE OF FUTURE SEA-LEVEL RISE BE TOO LOW?
Seven factors determine the position of sea level at a given place: 1) the volume of water in the ocean; 2) the location of atmospheric high and low pressure systems; 3) the evaporation/precipitation balance; 4) the motions of the solid Earth’s surface; 5) the gravitational attraction of ice sheets; 6) the speed of rotation of the Earth; and 7) changes in the volume of the ocean basins. Only the last two can be neglected for projections of sea-level change over the next few centuries. Three factors control the volume of water, the mass of water, its temperature, and salinity. The major masses of water outside the ocean are the ice sheets, mountain glaciers, groundwater, lakes and rivers . Melting of mountain glaciers is occurring rapidly and is currently the largest contributor to sea level change. The Greenland ice sheet is melting, and the Antarctic beginning to melt, both at increasing rates. Groundwater mining is significant and will increase as drought becomes mor widespread. In the last century the construction of dams and storage of freshwater on land acted to ameliorate se-level rise, but the trend has reversed recently. The rise of ocean temperature has a major effect roughly equal to the input of glacial meltwaters; the change in salinity has negligible effect. Regional atmospheric highs and lows cause sea-level differences of about a meter. Regional evaporation-precipitation differences produce similar effects. Ice sheets attract ocean water, up to 10% of the volume of the ice sheet itself, so that as ice sheets melt the sea level rise is about 10% larger than that of the addition of the water in the ice sheet itself.
Modern climate models project the general effects of changed boundary conditions, but do not include the many feedbacks we are just discovering. Because of these feedbacks they do not do well at projecting the rates of future climate and sea-level change.