Seagrass stores carbon 35 times faster than rainforests, preventing billions of tonnes of greenhouse gases escaping every year, but its crucial role in slowing climate change has been largely overlooked, experts said today.
While rainforests lock in carbon for a century at most, seagrass does so for thousands of years. When exposed to air, the the carbon-rich sediment below the seagrass meadows begins to oxidise, releasing greenhouses gases into the atmosphere and warming the globe.
About 50% of Australia's seagrasses have been destroyed by dredging and pollution, said seagrass expert Dr Peter Macreadie, ARC DECRA Fellow at the University of Technology, Sydney.
“The world seems to be primarily focused on reducing greenhouse gas emissions from industry, transport and food but there's little attention being paid to carbon that is leaking out from underneath the ground, on land and in the ocean.”
Dr Macreadie, who is currently analysing carbon storage in seagrass sediment from Jervis Bay, estimated that seagrasses offset approximately 2% of Australia's annual greenhouse gas emissions.
“It's a huge amount that's being offset and it's a huge amount that's being stored – and you can also put dollar figures on that,” said Dr Macreadie.
“If you say that carbon is worth $23 a tonne, then Australia's seagrasses, both the plants and the carbon in the sediment, are worth about $45 billion,“ he said.
Losing the seagrass from around Australia, would be equivalent to releasing up to three times Australia's annual greenhouse gas emissions, Dr Macreadie said.
“When someone is cutting down a forest, it's quite noticeable, and you can chain yourself to a tree,” he said.
“But most people don't care or notice when seagrasses are being destroyed; they are out of sight and out of mind – a tragedy of the commons.”
Despite its role in locking in greenhouse gases, seagrass is currently not on the National Greenhouse Gas Inventory, a compilation of Australia's emissions data.
A spokesperson for the Department of Climate Change and Energy Efficiency said emissions from non-terrestrial sources are not currently included in national greenhouse accounts due to accounting rules set by the Intergovernmental Panel on Climate Change (IPCC) and the UN Framework Convention on Climate Change (UNFCC).
“Currently, there are provisions for inclusion of emissions from wetlands, however these are of the freshwater, inland type,” the spokesperson said in an email.
“The IPCC and UNFCCC periodically review the international accounting rules. The IPCC is currently drafting a wetlands supplement to the 2006 guidelines that may include coastal wetlands and seagrasses in the future.”
Professor Carlos Duarte, Director of the Oceans Institute at the University of Western Australia, said a CSIRO-funded project called the Coastal Carbon Cluster will deliver estimates on the carbon sequestration capacity of seagrasses.
“The IPCC is taking a serious look at this option,” he said.
Professor Duarte said seagrass meadows rank among the most effective carbon sinks in the biosphere.
“The key to their role relies on two key factors: their high productivity and capacity for carbon capture and their capacity to preserve carbon in their sediment for millennia,” he said.
Unlike forests, which release greenhouse gases when they burn, the carbon in the seagrass soils is protected from fire, said Professor Duarte.
“One hectare of Posidonia meadow has as much as 10 times the CO2 sequestration capacity than a pristine hectare of Amazonian forest does,” he said.
“These ecosystems also rank amongst the most threatened in the biosphere,” due to fertiliser and herbicide run-off, dredging and port development, Professor Duarte said.
Chris Riedy, Associate Professor at the Institute for Sustainable Futures, said seagrass carbon storage is an “overlooked way of reducing emissions”.