The largest glacier on New Zealand, the Tasman Glacier, was 18 miles long in 1990. New measurements last week showed the glacier was 14 miles long. Despite global warming since the 1850s, the glacier had been protected from the sun's heat by its cover of rock debris. The ice cliff at the glacier's front edge is receding at a rate of 590 feet a year. Other glaciers on New Zealand's central alps also are retreating rapidly, melted by warmer temperatures and lower snowfall levels.