Northern Hemisphere Sea Ice Extent

Abstract

Vinnikov et al. (1) strongly suggested that the observed downward trend in Northern Hemisphere sea ice extent from 1953 to 1998 relates to anthropogenic global warming. Their conclusion was based on the supposed low probability (P , 0.1%) that a trend of the observed magnitude results from natural climate variability, with P estimated by sampling the simulated ice extent in a 5000-year control experiment using the climate model of the Geophysical Fluid Dynamics Laboratory (GFDL). The natural variability of sea ice extent was characterized by its standard deviation, s: For detrended observations from 1953 to 1998, s 5 240,000 km2; for the GFDL model, s 5 250,000 km2. Vinnikov et al. took the small difference between these standard deviations as evidence that the model realistically simulates natural variability of sea ice extent