Present-day sea level change: Observations and causes

The determination of the present-day rate of sea level change is important for a variety of scientific and socioeconomic reasons. With over a decade of precision sea level measurements from satellite altimetry in hand and with the recent launch of new satellite missions addressing different aspects of sea level change, observationally, we have more information on sea level change than ever before. In fact, the geocentric rate of global mean sea level rise over the last decade (1993-2003) is now known to be very accurate, +2.8 +/- 0.4 mm/yr, as determined from TOPEX/Poseidon and Jason altimeter measurements, 3.1 mm/yr if the effects of postglacial rebound are removed. This rate is significantly larger than the historical rate of sea level change measured by tide gauges during the past decades (in the range of 1-2 mm/yr). However, the altimetric rate could still be influenced by decadal variations of sea level unrelated to long-term climate change, such as the Pacific Decadal Oscillation, and thus a longer time series is needed to rule this out. There is evidence that the sea level rise observed over the last decade is largely due to thermal expansion, as opposed to the influx of freshwater mass from the continents. However, estimates of thermal expansion are still sufficiently uncertain to exclude some contribution of other sources, such as the melting of mountain glaciers and polar ice. Moreover, independent measurements of total ice melting during the 1990s suggest up to 0.8 mm/yr sea level rise, an amount that could eventually be canceled by change in land water storage caused by anthropogenic activities. Another important result of satellite altimetry concerns the nonuniform geographical distribution of sea level change, with some regions exhibiting trends about 10 times the global mean. Thermal expansion appears responsible for the observed regional variability. For the past 50 years, sea level trends caused by change in ocean heat storage also show high regional variability. The latter observation has led to questions about whether the rate of 20th century sea level rise, based on poorly distributed historical tide gauges, is really representative of the true global mean. Such a possibility has been the object of an active debate, and the discussion is far from being closed.