date: Tue Aug 8 09:18:21 2006 from: Tim Osborn subject: summary for soap sea level report to: Alex Wright , Orson van de Plassche , simon.tett@metoffice.gov.uk, jason.lowe@metoffice.gov.uk, j.m.gregory@reading.ac.uk, k.briffa@uea.ac.uk Dear all, below is the summary I've drafted for the sea level report. The report is now finished except for any comments you may have on this summary plus any final comments on the report from Orson. Thanks for all your help. Please send any comments ASAP. Cheers, Tim --------------------- Simulations with the HadCM3 climate model indicate that global-mean sea level is closely controlled by the climate response to external radiative forcings, with only a small contribution from internally-generated climate variability. At the global-scale, simulated sea-level rise accelerates in response to anthropogenic forcings during the twentieth century, but natural forcings increase the rate of rise in the early decades of this century and decrease the rate in more recent decades (due to more active volcanism), so that in response to the combined natural and anthropogenic forcings, sea level rises at almost a constant rate through the twentieth century (56 mm/century). This is significantly less than the increase inferred from tide gauges (100-200 mm/century). Regionally and locally the simulated sea level is more strongly influenced by variations in ocean circulation even on multi-decadal time scales, particularly outside the tropics. In particular, the sea level in parts of the North Atlantic Ocean is strongly influenced by variations in the Atlantic Meridional Overturning Circulation (MOC); along the east coast of North America changes of up to 200 mm appear to be associated with variations in the HadCM3 Atlantic MOC of 2 Sv. The model simulations were compared with estimates of past sea level developed from salt-marsh foraminifera. The uncertainties associated with both the climate model experiment and the sea level reconstructions are so large relative to the simulated changes that the model-data comparison is unable to provide confirmation that key aspects of the simulated sea level (e.g., the fall from 1500 to 1820 or the change to rising levels that occurred around 1820) match changes in the real world. For the twentieth century, the mean rate of rise from the sea level reconstructions in the north-east Atlantic exceeds that in the HadCM3 simulation by a factor of two to three (in agreement with the local and global tide-gauge data). ---------------------