Global Sea-level Rise: Past and Future

Abstract

This paper reviews observationally based estimates of past global-mean temperature change and sea-level rise and compares them with model-based estimates. The climate model used is a simple upwelling-diffusion, energy-balance model, which is coupled to a set of simple ice-melt models to give total sea-level change. For best-guess model parameter values there is reasonable agreement between observed and modelled results. The same models are used to estimate future temperature changes and sea-level rise for the standard IPCC95 set of emissions scenarios, updating earlier work. Projected warming over 1990–2100 ranges between 1.4 and 2.9^°C for the central emissions scenario (1595a), while sea-level rise ranges between 20 and 86 cm. Mid-value estimates for a climate sensitivity of 2.5^°C for a CO₂ doubling are 2.0^°C and 49 cm. Temperature and sea-level rise estimates are also given (out to 2500) for five standard (IPCC) CO₂ concentration scenarios in which CO₂ levels stabilize at 350, 450, 550, 650 and 750 ppmv. The sea-level-rise commitment after stabilization is very large: for stabilization levels of 550 ppmv or above, sea-level rise continues for many centuries at rates similar to those occurring at the stabilization point in spite of the constancy of radiative forcing. Finally, the sensitivity of these results to changes in the ocean’s thermohaline circulation is examined. The effects of a thermohaline slowdown are reduced warming rate and increased rate of sea-level rise.