Change of Arctic sea-ice volume and its relationship with sea-ice extent in CMIP5 simulations

The future change of September Arctic sea-ice volume, simulated by 30 state-of-the-art climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), is examined, which depends on both ice extent and ice thickness. In comparison with the September sea-ice extent, the September sea-ice volume has larger spread in the historical simulation but faster convergence in the projection simulation, especially in the context of increasing greenhouse gas emissions. This indicates that the ice volume might be more sensitive to external forcings than the ice extent. Using the averaged projection of those climate models from the 30 CMIP5 models that can better reflect the 'observed' sea-ice volume climatology and variability, it is shown that the September sea ice volume will decrease to similar to 3000 km(3) in the early 2060s, and then level off under a medium-mitigation scenario. However, it will drop to similar to 3000 km(3) in the early 2040s and reach a near-zero ice volume in the mid-2070s under a high-emission scenario. With respect to the historical condition, the reduction of the ice volume, associated with increasing greenhouse gas emissions, is more rapid than that of the ice extent during the twenty-first century.