Important seaways and waterways in the Northern Hemisphere are blocked or menaced by ice during many months of the year. Depending upon the climatic conditions, the period of difficulty may increase to the detriment of shipping movements upon which vital economic activities may depend.
Sea ice prevents effective exploitation of mineral and oil deposits along the northern shores of Canada, Alaska and the USSR. Recent climatic changes have resulted in the Icelandic fishing industry being paralyzed by drift ice which had been absent since the 1920s. Such ice causes damage to harbours and ships. Ice in the Great Lakes and St. Lawrence Seaway is hazardous to ships and requires aerial surveillance.