Models are improving, but can they catch up?
The rapid retreat and thinning of the Arctic sea ice cover over the past several decades is one of the most striking manifestations of global climate change. Previous research revealed that the observed downward trend in September ice extent exceeded simulated trends from most models participating in the World Climate Research Programme Coupled Model Intercomparison Project Phase 3 (CMIP3). We show here that as a group, simulated trends from the models contributing to CMIP5 are more consistent with observations over the satellite era (1979–2011). Trends from most ensemble members and models nevertheless remain smaller than the observed value. Pointing to strong impacts of internal climate variability, 16% of the ensemble member trends over the satellite era are statistically indistinguishable from zero. Results from the CMIP5 models do not appear to have appreciably reduced uncertainty as to when a seasonally ice-free Arctic Ocean will be realized.
It is possible that a model can capture the historical state but not the trend. We compare results of these evaluations to those based on the CMIP3 simulations. The CMIP5 models will become the main source of climate projections assessed by the International Panel on Climate Change (IPCC) in its 5th Assessment Report.
Figure 2. Time-series of modeled (colored lines) and observed (solid red line) September sea ice extent from 1900 to 2100. All 56 individual ensemble members from 20 CMIP5 models are included as dotted colored lines, with their individual model ensemble means in solid color lines. The multi-model ensemble mean is based on 38 ensemble members from 17 CMIP5 models (shown in black), with +/- 1 standard deviation shown as dotted black lines. Figure inset is based on the multi-model ensemble mean from CMIP5 and CMIP3, +/- 1 standard deviation.
Based on multi-model ensemble mean extents at the beginning of the 20th century, there is a 1.1 million km2 difference in the mean September extent between CMIP3 and CMIP5 (Figure inset, N.). During the period of satellite observations, the September CMIP5 multi-model ensemble mean tends to be slightly lower than the observed extent until 2007, after which it is higher. By contrast, the CMIP3 multi-model ensemble has a positive bias throughout the period of observations and especially during the most recent decade. Turning to the end of the 21st century, the CMIP5 multi-model ensemble mean never reaches ice-free conditions (defined here as less than 1.0 million km2), but the minus 1 standard deviation drops below the ice-free threshold around year 2045. Several CMIP5 models (CanESM2, GISS E2-R, GFDL-CM3, NCAR CESM, MIROC-ESM and ESM-CHEM) show essentially ice-free conditions by 2050, with the CanESM2 model having an ensemble member reaching nearly ice-free conditions as early as 2016 (0.54 million km2). By contrast, despite the more aggressive emission scenario (SRESA1B) driving the CMIP3 models, an overall more extensive sea ice cover is retained, with the minus 1 standard deviation reaching nearly ice-free conditions in 2075.
The lower CMIP5 March extent compared to CMIP3 results in good overall agreement with the observations, though the observed values fall below the CMIP5 multi-model ensemble means in recent years.
Turning to the modern satellite era, 1979–2011, more CMIP5 ensembles have a smaller rate of decline than observed, which at -0.84 million km2 per decade is nearly twice as large as the trend for 1953–2011. Forty-six of 56 ensemble members have trends outside of the 2 bound for the observations and 9 ensemble members have trends that are not statistically different from zero at the 90% confidence level. Although most model trends remain slower than observed, six ensemble members have rates of decline larger than observed. Overall, 64% of the ensemble member trends are statistically different from the observed trend at the 90% confidence level. In contrast, 85% of the CMIP3 ensemble members have trends that are statistically different from observed. The multi-model ensemble mean trend over the satellite period is -0.50 million km2 decade, which is 70% larger than the CMIP3 multi-model mean value of -0.35 million km2 decade.