American strategic and scientific interests at both poles will be threatened if replacements are not built soon for the Coast Guard’s two aging large icebreaking ships, said a panel convened by the National Academies, the nation’s top scientific advisory group. The country has four icebreakers now, but only two, each about 30 years old, are certified to handle the thick floating ice that cloaks the Arctic Ocean and waters around Antarctica. They have been plagued by mechanical troubles of late, the panel said. The warming Arctic climate could create shipping routes around the North Pole, but sea ice will still need to be cleared, the report said.