A key part of this WP is the analysis of new and updated sea ice data for the early 20th century in collaboration with Nick Rayner at the Met Office Hadley Centre (see Rayner et al., 2003). This new dataset enables for the first time an analysis of the sea ice retreat during the early 20th century, and of its contribution by atmospheric dynamics, external forcing, and by the seasonal and regional characteristic of the early 20th century warming. This longer time horizon bases present understanding of sea ice dynamics and of sea ice response to warming on a much more solid footing. The long-term sea ice change over the entire 20th, and, where possible, into the 19th century, is also analyzed and quantitatively compared to that in coupled model simulations (limited to those with a state-of-the-art sea ice model), capturing both periods of retreat and increase.

The output is a much improved understanding of the response of sea ice to warming, and address question iv).