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Faster Arctic sea ice retreat in CMIP5 than in CMIP3 due to volcanoes

Impact:
CMIP5 models have been found to simulate Arctic sea ice retreat during 1979–2013 that is faster on average than in the CMIP3 models. At the same time, the CMIP5 ensemble-mean rate of global warming during 1979–2013 has been found to be larger than that in CMIP3. The difference in global warming has been previously attributed to historical volcanic forcing, which was included in all of the CMIP5 models but only about half of the CMIP3 models. However, the inclusion of volcanic forcing in the CMIP ensembles has not been considered, as far as the authors are aware, in previous analyses of the rate of simulated Arctic sea ice retreat. Here we show that a range of approaches all suggest that the change between CMIP5 and CMIP3 in the ensemble-mean 1979–2013 Arctic sea ice extent trend can also be largely attributed to the inclusion of volcanic forcing.
Specifically, major volcanic eruptions occur during the early part of this time period, and they cause temporary cooling and ice expansion. This exacerbates the model bias toward too much 1979–2013 global warming while reducing the model bias toward too little Arctic sea ice retreat. These results are consistent with the sea ice sensitivity not being substantially influenced by volcanic eruptions, which would imply that the higher level of global warming caused by volcanoes should coincide with more sea ice retreat. This suggests that the reported improvement in simulated sea ice trends was largely an artifact of comparing simulations that had volcanic forcing with simulations that did not.

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