  Observations indicate that the Arctic sea ice cover is rapidly retreating
while the Antarctic sea ice cover is steadily expanding. State-of-the-art
climate models, by contrast, typically simulate a moderate decrease in both the
Arctic and Antarctic sea ice covers. However, in each hemisphere there is a
small subset of model simulations that have sea ice trends similar to the
observations. Based on this, a number of recent studies have suggested that the
models are consistent with the observations in each hemisphere when simulated
internal climate variability is taken into account. Here we examine sea ice
changes during 1979-2013 in simulations from the most recent Coupled Model
Intercomparison Project (CMIP5) as well as the Community Earth System Model
Large Ensemble (CESM-LE), drawing on previous work that found a close
relationship in climate models between global-mean surface temperature and sea
ice extent. We find that all of the simulations with 1979-2013 Arctic sea ice
retreat as fast as observed have considerably more global warming than
observations during this time period. Using two separate methods to estimate
the sea ice retreat that would occur under the observed level of global warming
in each simulation in both ensembles, we find that simulated Arctic sea ice
retreat as fast as observed would occur less than 1% of the time. This implies
that the models are not consistent with the observations. In the Antarctic, we
find that simulated sea ice expansion as fast as observed typically corresponds
with too little global warming, although these results are more equivocal. We
show that because of this, the simulations do not capture the observed
asymmetry between Arctic and Antarctic sea ice trends. This suggests that the
models may be getting the right sea ice trends for the wrong reasons in both
polar regions.
