Oceans are facing multiple threats -- heated by climate change, becoming more acid by absorbing CO2, suffering from overfishing and pollution -- a review by the International Program on the State of the Ocean found.
Dead zones formed by fertilizer run-off are also a problem, the group said, and conditions could lead to mass extinction events of the type experienced in the distant past.
"We have been taking the ocean for granted. It has been shielding us from the worst effects of accelerating climate change by absorbing excess CO2 from the atmosphere," the group reported in its review.
"Whilst terrestrial temperature increases may be experiencing a pause, the ocean continues to warm regardless. For the most part, however, the public and policymakers are failing to recognize -- or choosing to ignore -- the severity of the situation," it said.
The group is publishing a set of five papers based on workshops in 2011 and 2012 in partnership with the International Union for Conservation of Nature's World Commission on Protected Areas, the BBC reported Thursday.
"What these latest reports make absolutely clear is that deferring action will increase costs in the future and lead to even greater, perhaps irreversible, losses," the IUCN's Dan Laffoley said.
"These findings give us more cause for alarm -- but also a road map for action," he said. "We must use it."