Oz Heatwave Set Off Marine Carbon Bomb; 9 Million Tons Of Carbon From Dead Seagrass
A marine heatwave in Western Australia in 2010 set off a massive “carbon bomb”, damaging the world’s largest seagrass meadow, releasing millions of tonnes of carbon that had been collected for thousands of years below the surface. Although Australia doesn’t currently count carbon released from damaged seagrass meadows in its official greenhouse gas emissions, if it did, the results mean those figures might need to be revised upwards by more than 20%.
Seagrass is a flowering grass-like plant that grows in shallow waters. It gathers carbon dissolved in the sea and buries it below the surface, often storing similar amounts of carbon in the top metre of sediment as is stored in tropical forests. But unlike forests that store carbon for about 60 years before releasing much of it, seagrass meadows often store the carbon for thousands of years until they are disturbed. That process is thought to offset up to 2% of humanity’s greenhouse gas emissions.
Recent studies have shown that when the top layer of actively growing seagrass is disturbed – either by local impacts such as boat anchors or climatic impacts like heatwaves – the carbon that has been sequestered over thousands of years can be quickly released.
Since the start of the 20th century, seagrass meadows worldwide have declined at an average rate of 0.9% per year, mostly due to direct human impacts such as coastal development and water quality degradation. Over the last century about 29% of global seagrass has been destroyed and it is estimated it is releasing carbon at a rate similar to the emissions of Australia and the UK combined.
Ed - Emphasis added.
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https://www.theguardian.com/environment/2018/mar/20/marine-heatwave-set-off-carbon-bomb-in-worlds-largest-seagrass-meadow