The world's oceans are under greater threat than previously believed from a "deadly trio" of global warming, declining oxygen levels and acidification, an international study said on Thursday.

 

The oceans have continued to warm, pushing many commercial fish stocks towards the poles and raising the risk of extinction for some marine species, despite a slower pace of temperature rises in the atmosphere this century, it said.

 

The oceans are warming because of heat from a build-up of greenhouse gases in the atmosphere. Fertilizers and sewage that wash into the oceans can cause blooms of algae that reduce oxygen levels in the waters. And carbon dioxide in the air can form a weak acid when it reacts with sea water.

 

"The 'deadly trio' of ... acidification, warming and deoxygenation is seriously affecting how productive and efficient the ocean is," the study said.

 

Current conditions in the oceans were similar to those 55 million years ago, known as the Paleocene-Eocene thermal maximum, that led to wide extinctions. And the current pace of change was much faster and meant greater stresses.

 

Scientists said the findings added urgency to a plan by almost 200 governments to work out a deal by the end of 2015 to limit a rise in average world temperatures to less than 2 degrees Celsius (3.6F) above pre-industrial times.

 

Temperatures have already risen by about 0.8 degree Celsius (1.4F). The report also urged tougher management of fish stocks including a ban on destructive bottom trawlers and granting more power to local communities in developing nations to set quotas.