1. Centre for Polar Observation and Modelling, University College London, Gower Street, London WC1E 6BT, UK
2. Met Office Hadley Centre for Climate Prediction and Research, FitzRoy Road, Exeter, Devon EX1 3PB, UK

Correspondence to: Seymour Laxon¹ Email: swl@cpom.ucl.ac.uk

Possible future changes in Arctic sea ice cover and thickness, and consequent changes in the ice-albedo feedback, represent one of the largest uncertainties in the prediction of future temperature rise^{1, 2}. Knowledge of the natural variability of sea ice thickness is therefore critical for its representation in global climate models^{3, 4}. Numerical simulations suggest that Arctic ice thickness varies primarily on decadal timescales^{3, 5, 6} owing to changes in wind and ocean stresses on the ice^{7, 8, 9, 10}, but observations have been unable to provide a synoptic view of sea ice thickness, which is required to validate the model results^{3, 6, 9}. Here we use an eight-year time-series of Arctic ice thickness, derived from satellite altimeter measurements of ice freeboard, to determine the mean thickness field and its variability from 65° N to 81.5° N. Our data reveal a high-frequency interannual variability in mean Arctic ice thickness that is dominated by changes in the amount of summer melt¹¹, rather than by changes in circulation. Our results suggest that a continued increase in melt season length would lead to further thinning of Arctic sea ice.