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 March 26, 2018
Threatened blue carbon ecosystems store carbon 40 times faster than forests

 They can stink like fish and rotten eggs, breed swarms of mosquitos and lack the glamour of coral reefs. But mangroves, with other coastal habitats, are vitally important to our climate --- and they're under threat.

In just the past decade, scientists have discovered that some of our underappreciated coastal habitats --- called "blue carbon ecosystems" --- play a huge role in tackling CO2 emissions.

But human activities such as burning fossil fuels and coastal development have already caused half of them to disappear.

What is blue carbon?

Blue carbon coastal ecosystems --- such as mangroves, seagrass meadows and tidal wetlands --- are named for their place at the boundary between land and sea, and their unmatched ability to suck CO2 out of the atmosphere and store it in the ground below.

This process is called carbon sequestration.

It was intense carbon sequestration by ancient forests and algae millions of years ago that helped create the very deposits of coal and oil we tap into for fossil fuels today.

Nearby on the ecology colour palette are the better-known green carbon systems of trees and forests. While important, they aren't nearly as efficient at storing carbon as their blue counterparts.

"We know that forests are pretty good at [carbon sequestration], but their carbon stores are bound to the lifetime of the trees, for only 100 or so years, and then it is released back into the atmosphere," Dr Macreadie said.

As well as being a temporary carbon store, trees can only soak up so much carbon before they become "saturated".

Blue carbon ecosystems, on the other hand, can store more carbon for longer --- thousands of years --- and at a far quicker rate.

"So you need a lot more green carbon habitat to do the same amount of carbon offsetting."

Conservation gets weird

Some ecologists are worried that these ecosystems don't receive the attention they deserve and are now being lost faster than we can conserve them.

Dr Macreadie estimated that about half of the world's blue carbon ecosystems have already disappeared, thanks to human activities.

Seagrass meadows have shrunk at a rate of 1 per cent every year since the start of the 20th century.

In 2016 there was extensive die-back of mangroves in the Gulf of Carpentaria, but it coincided with the mass coral bleaching on the Great Barrier Reef, which received more attention.

To spur people into action to protect these "ugly duckling" habitats, Dr Macreadie and his team are getting creative with tea bags.

Around the world, citizen scientists have been burying tea bags in the soil of blue carbon ecosystems to find out how well the area stores carbon.

The tea leaves inside your everyday tea bag are carbon-based, which makes them a handy addition to the team's experimental toolkit.

If, after a few months of being buried in mud, the tea leaves are still there, then that might be a good spot for locking away carbon.

But if you dig the tea bags back up and the tea leaves have gone, it means the carbon has decomposed --- indicating that area's not capable of carbon storage.

The project is uncovering how carbon storage ability varies even within blue carbon ecosystems.

Ticking carbon bombs: sinks become sources

Coastal development is the major danger to blue carbon habitats, and is now raising the issue that stored carbon will be emitted as CO2 back into the atmosphere.

"The really big threat is that the damaged ecosystems will release their ancient carbon stores," Dr Macreadie said.

An international team this week reported that a marine heatwave off Western Australia in 2010-11 that damaged seagrass meadows may have released of millions of tonnes of ancient carbon stores back into the atmosphere as CO2.

Oscar Serrano, a marine ecologist at Edith Cowan University involved in the research, said that the Shark Bay seagrass meadows accumulated around 144 million tonnes of carbon over the past 4,000 years.

"We estimated that around 1,000 square kilometres of seagrass was lost due to the heatwave, which could have released between 2 and 9 million tonnes of CO2," Dr Serrano said.

It's a climate change double-whammy; losing carbon sequestration habitat while adding to our CO2 emissions at the same time.

However, scientists acknowledge that measuring CO2 emissions from blue carbon habitat loss is very challenging.

"We have to base our estimates on a number of assumptions, and the main uncertainty is the fate of the carbon stored in the system," Dr Serrano said.

"It's very hard to study mainly because of the time it takes for stored carbon to be converted back to CO2, and because it's such a complex system."

Dr Macreadie has been studying blue carbon ecosystems since the term was first coined around 9 years ago.

The field has grown exponentially since, but there's plenty more to discover about our coastal armpits, the benefits they provide and how to make sure they keep squirrelling away CO2.

"If we're going to keep burning fossil fuels, we need to find ways to pay for our carbon sins," he said.

"We need people to recognise the importance of these ecosystems and the weird things they do for us, and how they can form a sort of green infrastructure along our coasts."