Reconstructing Arctic Sea Ice Over the Common Era Using Data Assimilation

Arctic sea-ice decline in recent decades has been dramatic, however few long-term records of Arctic sea ice exist to put such a decline in context. Here we employ an ensemble Kalman filter data assimilation approach to reconstruct Arctic sea-ice concentration over the last two millennia by assimilating temperature-sensitive proxy records with ensembles drawn from last millennium climate model simulations. We first test the efficacy of this method using pseudo-proxy experiments. Results showgood agreement between the target and reconstructed total Arctic sea-ice extent (R2 value and coefficient of efficiency values of 0.51 and 0.47 for perfect model experiments, and 0.43 and 0.43 for imperfect-model experiments). Imperfect-model experiments indicate that the reconstructions inherit some bias from the model prior. We assimilate 487 temperature-sensitive proxy records with two climate model simulations to produce two gridded reconstructions of Arctic sea ice over the last two millennia. These reconstructions show good agreement with satellite observations between 1979–1999 CE for total Arctic sea-ice extent with an R2 and coefficient of efficiency of about 0.60 and 0.50, respectively, for both models. Regional quantities derived from these reconstructions show encouraging similarities with independent reconstructions and sea-ice sensitive proxy records from the Barents, Baffin Bay and East Greenland seas. The reconstructions show a positive trend in Arctic sea-ice extent between around 750–1820 CE, and increases during years with large volcanic eruptions that persist about 5 years. Trend analysis of total Arctic sea-ice extent reveals that for time periods longer than 30 years, the satellite era decline in total Arctic sea-ice extent is unprecedented over the last millennium.