Mean Sea Level science issues: questions in discussion

Sea level budget estimated from altimetry, Grace and Argo

The Intergovernmental Panel on Climate Change (IPCC) estimated, for the decade 1993-2003, a rise in the mean level of the oceans, half of which is explained by the warming of the water and the remainder being essentially due to shrinking glaciers and polar ice caps. However, since 2003 thermal expansion seems to have stopped growing, while the mean sea level continues to rise, albeit at a slower rate. Comparing different measurement sources may improve our understanding of this phenomenon.

Gravimetric measurements from the GRACE satellite (2003?2008), temperature and salinity measurements obtained from the Argo network (2004?2008), and altimetric sea height measurements obtained for the same period have therefore been compared.

The GRACE data enable variations in mass to be measured, and hence the addition of water to the ocean. We can therefore deduce the amount of water being added around Greenland and the Antarctic (melting indlandsis or ice sheets), as well as that coming from inland waters. In addition, studies [Meier et al, 2007] have estimated the contribution of the glaciers and ice sheets which is more widely spread over the oceans. The sum of these estimates, and the total obtained using GRACE measurements, plus an estimate of the isostatic adjustment are consistent (around 2 mm/year). This contribution of water being added to the ocean during the period 2003-2008 corresponds to around 80% of the total rise in mean sea level as observed by satellite altimetry.

: Upper curve (crossed line): altimetry-based sea level curve; Middle curve (open circles): total land ice contribution using the GRACE-based ice sheet mass balance [Cazenave et al., 2008] and Meier et al. [2007] glaciers contribution; Lower curve (black triangles): GRACE-based ocean mass change (GIA correction applied).(Crédits Legos)

: Steric sea level. Upper curve (black triangles): estimated from the difference between altimetry and GRACE-based ocean mass. Middle curve (open circles): estimated from the difference between satellite altimetry and total land ice plus land waters contribution; Lower curve: ARGO-based estimate [Cazenave et al., 2008]. (Crédits Legos)

Strictly speaking, the difference should result from ocean warming (steric level). In fact it is of the same magnitude as the rise calculated using the temperature and salinity measurements from the Argo network. Both series of independent measurements indicate a relative stagnation of this steric level for the period 2003-2008.

Source	Rate (mm/an)
Sea level (altimetry; 2003-2008)	2.5 +/-0.4
Ocean mass (Grace; 2003-2008)	1.9 +/-0.1
Ice sheet (Grace; 2003-2008)	1+/-0.15
Glaciers and ice caps (2003-2008; [Meier et al., 2007])	1.1+/-0.24
Terrestrial waters (2003-2008)	0.17+/-0.1
Sum of ice and waters	2.2 +/-0.28
Steric sea level (altimetry minus Grace; 2003-2008)	0.31+/-0.15
Steric sea level (Argo; 2004?2008)	0.37+/-0.1

[text and table from Cazenave et al., 2008]

Cazenave, A., K. Dominh, S. Guinehut, E. Berthier, W. Llovel, G. Ramillien, M. Ablain, G. Larnicol, 2008: Sea level budget over 2003-2008: a reevaluation from Grace space gravimetry, satellite altimetry and Argo. Glob. Planet. Change, doi:10.1016/j.gloplacha.2008.10.004
Meier, M.F., M.B. Dyurgerov, U.K. Rick, S. O'Neel, W.T. Pfeffer, R.S. Anderson, S.P. Anderson, A.F. Glazovsky, 2007: Glaciers dominate eustatic sea-level rise in the 21st century. Science 317 (5841), 1064?1067.