The polar ice cap has thinned by 40 percent in the last 30 years, according to preliminary findings presented this week at a conference of the Arctic Research Consortium of the United States. Other research presented at the conference documents how dramatic climate changes are making it difficult for Alaska natives to maintain subsistence lifestyles. Changing weather conditions and thinning sea ice seem to be causing declines in the populations of walruses, seals, seabirds, and other animals. And because summers have been wetter, natives are having a harder time drying fish and caribou meat for winter.