Quantifying the Contribution of Natural Variability to September Arctic Sea Ice Decline

The Arctic sea ice undergoes decreasing trend in recent decades. However, there is ongoing debate on the contribution of natural internal variability to recent and future Arctic sea ice changes. In this study, we contrast the trends in the forced and unforced simulations of the carefully selected global climate models with the extended observed Arctic sea ice records. Results suggest that natural variability explains no more than 42.3% of the observed September sea ice extent trend during the 35-year (1979-2013) satellite observations, which is comparable to the results as the observed sea ice record extended back to 1953 (61-year, less than 48.5%). This reinforces the evidence for a substantial role of anthropogenic forcing in the observed decline of September Arctic sea ice in recent decades. The magnitude of both positive and negative trend induced by natural variability in the unforced simulations is slightly enlarged in the context of increasing greenhouse gases in the 21st century. However, the ratio between the realizations of positive and negative trend still keeps steady, which enforces the standpoint that external forcing is still the principal determiner to the decreasing Arctic sea ice extent trend in future.