Recent Patterns of Antarctic Surface Air Temperature Trends in the Context of Natural Variability, as Simulated by the CMIP5 Models

One of the hallmarks of climate change in recent decades has been the dramatic warming of the Western Antarctic Ice Sheet (WAIS), whereas the Eastern AIS (EAIS) has experienced a slight, and insignificant cooling trend. Here we examine the extent to which this spatial asymmetry in Antarctic surface air temperature (SAT) trends might be a response to anthropogenic climate forcings, such as greenhouse gas emissions and ozone depletion, or rather the manifestation of large natural climate variability. We compare the observed annual mean Antarctic SAT over the 1960-2005 and 1979-2005 time periods to SAT data from 40 Coupled Model Intercomparison Project Phase 5 (CMIP5) historical integrations. We find that the CMIP5 multi-model mean shows near-homogeneous warming throughout Antarctica, i.e. it lacks the observed spatial asymmetry between WAIS and EAIS. As the multi-model mean is, by construction, the forced response, we conclude that the observed spatial asymmetry in Antarctic SAT results from natural variability, which acts to mask the anthropogenic warming in the EAIS region, while amplifying the WAIS warming. In addition, contrasting historical (i.e., forced) and pre-industrial (i.e., control) integrations from these same 40 CMIP5 models, we show that the observed total Antarctic and EAIS SAT trends fall well within the distribution of trends arising naturally in the system, and that the forced response in the models is small compared to the natural variability. In contrast, we do find that the WAIS SAT trend is more likely driven by anthropogenic forcings.