Impact of climate drift on twenty-first-century projection in a coupled atmospheric-ocean general circulation model

Reducing climate drift in coupled atmosphere-ocean general circulation models(AOGCMs) usually requires 1000-2000 years of spinup, which has not been practicalfor every modeling group to do. For the purpose of evaluating the impact of climatedrift, the authors have performed a multimillennium-long control run of the Goddard Institute for Space Studies model (GISS-EH)AOGCMand produced different twentiethcentury historical simulations and subsequent twenty-first-century projections by branching offthe control run at various stages of equilibration. The control run for this model is considered at quasi equilibration after a 1200-yr spinup froma cold start. The simulations that branched offdifferent points after 1200 years arerobust, in the sense that their ensemble means all produce the same future projection of warming, both in the global mean and in spatial detail. These robust projections differ from the one that was originally submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), which branched offanot-yet-equilibrated control run. The authors test various common postprocessing schemes in removing climate drift caused by a not-yet-equilibrated ocean initial state and find them to be ineffective, judging by the fact that they differ from each other and from the robust results that branched offan equilibrated control. Theauthors' results suggest that robust twenty-first-century projections of the forced response can be achieved by running climate simulations from an equilibrated ocean state, because memory of the different initial ocean state is lost in about 40 years if the forced run is started from a quasi-equilibrated state.