|Title||Sea Ice Trends in Climate Models Only Accurate in Runs with Biased Global Warming|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Rosenblum E., Eisenman I|
|Journal||Journal of Climate|
|Type of Article||Article|
|Keywords||CMIP5 models; expansion; extent; Meteorology & Atmospheric Sciences; surface climate; variability|
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 sea ice changes during 1979-2013 are examined 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. All of the simulations with 1979-2013 Arctic sea ice retreat as fast as observations are found to 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, it is found that simulated Arctic sea ice retreat as fast as observations would occur less than 1% of the time. This implies that the models are not consistent with the observations. In the Antarctic, simulated sea ice expansion as fast as observations is found to typically correspond with too little global warming, although these results are more equivocal. As a result, 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.
In each hemisphere, the observed 1979–2013 trend in sea ice extent falls at least marginally within the distribution of the CMIP5 simulations. Consistent with this, a number of previous studies have suggested that internal climate variability could explain the difference between the observed sea ice trend and the ensemble-mean simulated trend in each hemisphere.
The results presented here suggest that this viewpoint breaks down when we account for biases in simulated 1979–2013 global-mean surface temperature trends. We find that simulated Arctic sea ice retreat is accurate only in runs that have far too much global warming. This suggests that the models may be getting the right Arctic sea ice retreat for the wrong reasons. Similarly, simulated periods with accurate Antarctic sea ice trends tend to have too little global warming, although these results are more equivocal. Relatedly, the simulations do not capture the observed asymmetry between Arctic and Antarctic sea ice trends.
We quantify how this bias influences the level of agreement between models and observations by estimating what the simulated sea ice trend in each hemisphere would be in runs that matched the observed level of global warming. This analysis relies on the approximately linear relationship between sea ice extent and global-mean surface temperature in the simulations , which allows us to scale the results from simulations with varied levels of global warming or use simulations from different time periods. These results suggest that the difference between observed and modeled sea ice trends in each hemisphere cannot be attributed to simulated internal climate variability alone. This implies systematic errors in the Arctic and Antarctic sea ice changes simulated with current climate models, or possibly errors in the observations.