  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.
