Anticipating environmental and biogeochemical changes in the Southern Ocean using Earth System Models: the importance of evaluation

Earth System Models (ESM) are global climate simulation models that are widely used around the world to project future climate and oceanographic status to inform management and conservation. For the Ross Sea region (RSr), we summarise the results of a New Zealand study just published where the physical and biogeochemical performance of 16 CMIP-5 and 16 CMIP-6 (Coupled Model Intercomparison) ESM were examined relative to present day (1976–2005) observational data sets. The 2023 study updated and extended previous work carried out in 2016. A relative ranking scheme using error metrics and published ESM properties was used to rank the models from “best” to “worst” performing in the RSr. This ranking then provides us with a measure of confidence about future RSr predictions of environmental condition, the argument being that models that do well for our present ocean will do better into the future. An ensemble of the better-performing (“inner”) models was then used to develop an envelope of future changes expected over the next 50 – 100 years for the Ross Sea region using high emission scenarios (“business as usual” scenarios, RCP8.5/SSP585). We recommend that CCAMLR supports the evaluation of ESMs for the Southern Ocean and encourages members to use the better-performing (“inner”) models for setting a plausible envelope of future conditions in the Southern Ocean.