Submarine sonar data collected in the central Arctic Basin during middle and late summer were used to examine differences in the sea ice thickness distribution function g(h) between the periods 1958--1970 and 1993--1997. Cruises during the former period were made in July and August, whereas the 1993--1997 cruises were made in September and October. Seasonal correction was applied to adjust for the differences in thickness. While ice drafts were from only seven submarine cruises and somewhat spatially limited, results indicate that the fractional area covered by open water and first-year ice increased from 0.19 to 0.30 during the time interval. This was balanced by an 11% reduction of level-multiyear and ridged ice. Substantial losses occurred in ice thicker than 2 m, with an increase in the amount of 1--2 m ice. The volume of ice less than 4 m thick remained nearly the same and the total volume decreased about 32%. Losses in the volume of thicker ice increased with increasing thickness. Part of the change in g(h) is likely caused by increased ice area export through Fram Strait in the late 1980s and early 1990s. Because decadal variations in the North Atlantic Oscillation and Arctic Oscillation indices correlate with ice export anomalies, export-induced changes in g(h) probably tend to be cyclical in nature. However, a substantial shift in the peak of g(h) suggests that changes in thermal forcing were also a major factor in the observed thinning.