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Sea Level Rise Slows by 20%

January 9, 2009

Efforts to sell climate policy based on ever more scary scenarios of apocalypse cannot be sustained and are likely to work in exactly the opposite manner than desired. A good example of why this is so can be found in a recent paper (hat tip Dad) that suggests that the rate of sea level rise from 2003-2008 (2.5 mm/year) is 20% lower than that presented by the IPCC for 1993-2003 (3.1 mm/year). Whether this is "consistent with" longer-term predictions is different that whether it is "consistent with" a political strategy based on scaring people.

It seems pretty obvious that systems that exhibit a large amount of variability or are simply poorly understood on relatively short time scales are not very useful props in efforts to show the world moving inexorably towards doom.

Cazenave, A., et al., Sea level budget over 2003-2008: A reevaluation from GRACE space gravimetry, satellite altimetry and Argo, Glob. Planet. Change (2008), doi:10.1016/j.gloplacha.2008.10.004 (PDF)

From the IPCC 4th Assessment Report published in 2007, ocean thermal expansion contributed by ∼50% to the 3.1 mm/yr observed global mean sea level rise during the 1993–2003 decade, the remaining rate of rise being essentially explained by shrinking of land ice. Recently published results suggest that since about 2003, ocean thermal expansion change, based on the newly deployed Argo system, is showing a plateau while sea level is still rising, although at a reduced rate (∼2.5 mm/yr). Using space gravimetry observations from GRACE, we show that recent years sea level rise can be mostly explained by an increase of the mass of the oceans.

Estimating GRACE-based ice sheet mass balance and using published estimates for glaciers melting, we further show that ocean mass increase since 2003 results by about half from an enhanced contribution of the polar ice sheets – compared to the previous decade – and half from mountain glaciers melting. Taking also into account the small GRACE-based contribution from continental waters (b0.2 mm/yr), we find a total ocean mass contribution of ∼2 mm/yr over 2003–2008. Such a value represents ∼80% of the altimetry-based rate of sea level rise over that period.We next estimate the steric sea level (i.e., ocean thermal expansion plus salinity effects) contribution from: (1) the difference between altimetry-based sea level and ocean mass change and (2) Argo data. Inferred steric sea level rate from (1) (∼0.3 mm/yr over 2003–2008) agrees well with the Argo-based value also estimated here (0.37 mm/yr over 2004–2008). Furthermore, the sea level budget approach presented in this study allows us to constrain independent estimates of the Glacial Isostatic Adjustment (GIA) correction applied to GRACE-based ocean and ice sheet mass changes, as well as of glaciers melting. Values for the GIA correction and glacier contribution needed to close the sea level budget and explain GRACE-based mass estimates over the recent years agree well with totally independent determinations.

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