Ocean freshening, sea level rising, sea ice melting

Estimates of 20th Century sea level rise are typically 1.5 to 2 mm/y, with a steric contribution of (0.5 +/- 0.2) mm/y. Estimates of the eustatic contribution vary widely between -1.1 and + 1.3 mm/y. We attempt an independent estimate of eustatic sea level rise based on the measured freshening of the global ocean, and with attention to the contribution from melting of sea ice (which affects freshening but not sea level). Our estimate is based on a secular decrease in global average salinity estimated by Antonov et al. [2002] which, if assumed due entirely to run-off, would produce a eustatic rise of (1.8 +/- 0.7) mm/y, and would correspond to a run-off volume of 650 cu km/y. Measurements with upward looking sonars mounted on submarines have suggested a historical thinning of the arctic ice sheet equivalent to 525 +/- 105 cu km/y. Allowing for some growth in Antarctic sea ice, a reduced figure of (430 +/- 130) cu km/y is obtained, allowing about 220 cu km/y of run-off from land sources such as glaciers. This would produce a eustatic rise of only 0.6 mm/y, for a total of 1.1 mm/y, somewhat less than IPCC estimates. This also has implications for our understanding of glacial retreat for a total of 1.1 mm/y.