A severe heatwave off north-western Western Australia hammered the world's largest region of seagrass, triggering the release of as much as nine million tonnes of carbon dioxide, a paper by international researchers has found.

Two months of temperatures 2-4 degrees above average in the summer of 2010-11 resulted in the loss of about 1000 square-kilometres of seagrass in Shark Bay by 2014, or about a fifth of its extent, according to the paper which was published on Tuesday in Nature Climate Change.

Shark Bay, one of the world's largest intact seagrass regions was hammered in a marine heatwave that lasted two months in the summer of 2010-11. Credit:Paul Lavery

It was “an unprecedented increase of water temperatures over a long period that resulted in a stress for the plants, and they died”, Oscar Serrano, a researcher at Edith Cowan University and one of the paper's lead authors, said.

Shark Bay is a global hotspot for seagrass, accounting for about 2.4 per cent of the world's total area, with 12 species. It is also an important habitat for turtles and dugongs and many small fish species.