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

TL;DR

The study shows that the faster Arctic sea ice retreat in CMIP5 models compared to CMIP3 is mainly due to volcanic forcing inclusion, which caused temporary cooling and influenced sea ice trend simulations.

Contribution

This paper demonstrates that volcanic forcing in climate models explains the differences in Arctic sea ice retreat rates between CMIP3 and CMIP5, not improvements in sea ice physics.

Findings

Volcanic forcing was included in all CMIP5 models but only half of CMIP3.

Volcanic forcing caused temporary cooling in the 1980s and 1990s.

Models simulate less sensitive Arctic sea ice response than observed.

Abstract

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, 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 the most recent phase of the Coupled Model Intercomparison Project (CMIP5) compared with the previous generation of models (CMIP3). Here we examine simulations from both CMIP3 and CMIP5. We find 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. We show 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. We find that both generations of climate models simulate Arctic sea ice that is substantially less sensitive to global warming than has been observed. The results imply that the much of the difference in Arctic sea ice trends between CMIP3 and CMIP5 occurred due to the inclusion of volcanic forcing, rather than improved sea ice physics or model resolution.