|Title||Faster Arctic sea ice retreat in CMIP5 than in CMIP3 due to volcanoes|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Rosenblum E, Eisenman I|
|Journal||Journal of Climate|
|Keywords||Arctic,Sea ice,Volcanoes,Climate models|
The downward trend in Arctic sea ice extent is one of the most dramatic signals of climate change during recent decades. Comprehensive climate models have struggled to reproduce this trend, typically simulating a slower rate of sea ice retreat than has been observed. However, this bias has been widely noted to have decreased in models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) compared with the previous generation of models (CMIP3). Here simulations are examined from both CMIP3 and CMIP5. It is found that simulated historical sea ice trends are influenced by volcanic forcing, which was included in all of the CMIP5 models but in only about half of the CMIP3 models. The volcanic forcing causes temporary simulated cooling in the 1980s and 1990s, which contributes to raising the simulated 1979–2013 global-mean surface temperature trends to values substantially larger than observed. It is shown that this warming bias is accompanied by an enhanced rate of Arctic sea ice retreat and hence a simulated sea ice trend that is closer to the observed value, which is consistent with previous findings of an approximately linear relationship between sea ice extent and global-mean surface temperature. Both generations of climate models are found to simulate Arctic sea ice that is substantially less sensitive to global warming than has been observed. The results imply that much of the difference in Arctic sea ice trends between CMIP3 and CMIP5 occurred because of the inclusion of volcanic forcing, rather than improved sea ice physics or model resolution.
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.