Almost two dozen marine scientists from around the world have issued a warning about an often-overlooked side effect of climate change and pollution.

In a paper published this week in Science, they say oxygen is disappearing from increasingly large areas of ocean and threatening marine life.

The research, sponsored by an international body affiliated with UNESCO, finds the problem has been growing since the 1950s. Over the last 50 years, the amount of affected ocean has expanded by 4.5 million square kilometres to 32 million square kilometres of coastal and deep-sea water.

That includes the St. Lawrence Seaway and oceans off Canada’s West Coast.

“We feel this issue has to be looked at and deserves more attention,” said Denis Gilbert, one of the 22 co-authors and a scientist with the Department of Fisheries and Oceans.

“All animals need to breathe oxygen and we know that regions of the ocean that are losing oxygen are becoming more and more common. We’re seeing the marine animals leaving those areas.”

The paper, a summary of recent research, found oxygen-deprived waters off many of the world’s coastlines, especially those adjacent to large rivers and urban areas. Low-oxygen regions also exist in the high seas.

The volume of ocean water completely devoid of oxygen has quadrupled since 1950, one study found. Marine animal populations and diversity are dropping at hundreds of coastal sites.

In the Gulf of St. Lawrence, oxygen in the lower depths has dropped by 55 per cent since 1930.

“It’s huge,” said Gilbert. “We’re already losing cod in the bottom waters.”