The rapid pace of global warming and ice melting at the poles have underscored the "critical" need for the United States to build four new polar icebreaker ships, US officials said Tuesday. Congress called for the report amid concerns about the United States' lack of a fleet -- with just three aging icebreakers, one of which is entirely broken down, and another designed mainly for science research. "For more than 30 years, studies have underscored the need for US icebreakers to maintain presence, sovereignty, leadership, and research capacity, but the nation has failed to make the recommended investments," said an accompanying statement by Richard West, retired rear admiral of the US Department of the Navy and chair of the committee that authored the report.