Potential for bias in 21st century semi-empirical sea level projections

posted Jan 7, 2013, 3:02 AM by Aslak Grinsted   [ updated Feb 19, 2016, 4:04 AM ]

Abstract
We examine the limitations of a semi-empirical model characterized by a sea level projection of 73 cm with RCP4.5 scenario by 2100. Calibrating the model with data to 1990 and then simulating the period 1993-2009 produces sea level in close agreement with acceleration in sea level rise observed by satellite altimetry. Non radiative forcing contributors, such as long term adjustment of Greenland and Antarctica ice sheets since Last Glacial Maximum, abyssal ocean warming and terrestrial water storage, may bias model calibration which, if corrected for, tend to reduce median sea level projections at 2100 by 2-10 cm, though this is within the confidence interval. We apply the semi-empirical approach to simulate individual contributions from thermal expansion and small glacier melting. Steric sea level projections agree within 3 cm of output from process-based climate models. In contrast semi-empirical simulation of melting from glaciers is 26 cm, which is twice large as estimates from some process based models, however all process models lack simulation of calving, which likely accounts for 50% of small glacier mass loss worldwide. Furthermore we suggest that changes in surface mass balance and dynamics of Greenland ice sheet made contributions to the sea level rise in the early 20th century and therefore are included within the semi-empirical model calibration period, and hence are included in semi-empirical sea level projections by 2100. Antarctic response is probably absent from semi-empirical models, which will lead to a underestimate in sea level rise if, as is probable, Antarctica looses mass by 2100.

Citation
Jevrejeva, S., J. C. Moore, and A. Grinsted (2012), Potential for bias in 21st century semi-empirical sea level projections, J. Geophys. Res., doi:10.1029/2012JD017704




UPDATE
In one of the tests we calibrate against LeClercq's historical contribution from small glaciers and compare the resulting projections to Radic and Hock. Since then Marzeion et al. has projected much greater contributions, which are more in line with what we obtain with the semi-empirical model.  

Caption: Steric sea level simulated with semi-empirical model (black line) using the Gregory et al., [2006] data over period 1800-2000 for calibration (blue) and steric sea level after 2000 fro CMIP3 AOGCM experiments (blue). Colour bars are the 5-95% confidence limit for simulation by semi-empirical model (grey) and AOGCMs (blue). 
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