Definitions of variables for the reduced sea ice model, with their mean values and linear trends (per decade) from the 1979–2006 CICE hindcast. All variables refer to the Northern Hemisphere total. Ice areas are in 106 km2, thicknesses are in m, and volumes are in 1012 m3. Time scales are in yr and survival ratios are unitless. The values for υ and t are September averages, and the values for tf and tm are our best estimates over the CICE hindcast. The values of τs and τυ are calculated by use of (12) and (14), respectively.