The case arises from a lawsuit filed by the Natural Resources Defense Council (NRDC) seeking to protect whales and other marine mammals from the potentially harmful effects of Navy sonar used during training missions off the California coast.
The environmental group says the sonar can cause physical injury. Beaked whales are particularly vulnerable, studies show.
In 2000, a number of beaked whales beached themselves and died in the Bahamas following a US Navy sonar training exercise nearby.
In August 2007, a federal judge ordered the Navy to cease all training using mid-frequency active sonar off southern California. The court said there was a "near certainty" of irreparable harm to the environment, with 8,000 whales or dolphins potentially experiencing temporary hearing loss and an estimated 466 cases of permanent injury to whales.
The order was later amended to allow the Navy to conduct training exercises if it halted sonar transmissions whenever a marine mammal was seen within 1.25 miles of a sonar source. In addition, the judge ordered the Navy to reduce sonar power by 75 percent during certain thermal conditions in the ocean. The chief of naval operations said the judge's order jeopardized naval training, the timely deployment of naval forces, and US national security.
Limitations on the use of sonar would "cripple" the Navy's ability to conduct realistic predeployment training and would prevent US forces from being able to detect a submarine before it was in position to attack, according to the Navy.
In urging the Supreme Court to overturn the Ninth Circuit's approval of the judge's order, government lawyers said the ruling "poses substantial harm to national security, and improperly overrides the collective judgments of the political branches and the nation's top naval officers."
They added: "The record contains 'no evidence' that marine mammals have been harmed during the 40 years of ... sonar training [off southern California]."
Lawyers for the NRDC counter in their brief that federal regulations require the Navy to prepare an environmental impact statement whenever their operations cause environmental harm. The Navy could also take steps to mitigate the anticipated harm, they said.