Our home is girt by sea; our land abounds in nature’s carbon sinks

Simple seagrass can answer some complex climate problems. Joanne Saad

Reducing carbon emissions is necessary, but what about the carbon that has already been released into the atmosphere? Many countries are turning to “biosequestration” for the answers: using nature - including plants and soil - to capture and store carbon. And Australia’s greatest storage potential may be in our oceans.

Australia is one country hoping biosequestration will get us out of the climate change mess. The irony is that biosequestration is the same process that created fossil fuels: the carboniferous forests, which gave rise to the coal measures, and the rich deposits of microalgae which gave rise to oil-rich strata.

Soil and trees are important, but have their drawbacks

In June of this year the Labor government outlined its plans to use biosequestration to offset Australia’s carbon emissions. The scheme is known as the Carbon Farming Initiative, or CFI, and is due to face the Senate later this year.

Under the CFI, farmers can convert farmland back into forests, earning them carbon credits. These can be sold to individuals or businesses that have a mandate to offset their emissions.

As acknowledged by Ross Garnaut, the initiative has some practical and social limitations. Are farmers willing to change their land use practices? Is there enough land to make meaningful offsets? Could there be adverse impacts on biodiversity, such as expansion of monoculture forests and the clearing of native vegetation for forest establishment?

There are also technical concerns. How long can carbon be stored? What is the potential for large amounts of carbon to be released if systems become degraded? Do we really understand the environmental factors that influence biosequestration rates?

A study recently published in Nature has also revealed some unexpected consequences of using terrestrial systems for capture and store carbon.

The authors of the study found that increasing CO₂ levels in terrestrial soils stimulated production of other greenhouse gases. Methane (CH₄) went up by 45% and nitrous oxide (N₂O) by 20%. These gases are 25 times and 298 times (respectively) more dangerous than CO₂ in terms of their global warming potential.

Look to the sea

As the prospects of using terrestrial systems to sequester carbon become increasingly limited, a new hope has emerged in “blue carbon”. This is carbon captured by vegetated coastal habitats, particularly mangroves, seagrasses, and saltmarshes.

To see Peter Macreadie talking about seagrass, skip to the five minute, 22 seconds mark.

Although these habitats only occupy about 1% of the seafloor, it is estimated that they capture and store up to 70% of the carbon in the marine realm.

Terrestrial systems typically bind carbon over decades, and can become saturated with carbon. Blue carbon is stored over millennia and can accrete carbon vertically as sea levels rise.

In a land that is girt by sea, and blessed by a rich coastal vegetation, Australia is in one of the best positions to capitilise on its blue carbon resources, particularly its seagrasses.

Australia has vastly more seagrass than any other country in the world. Along its 32,000 km coastline, Australia has around 90,000 square km of seagrass. This is enough seagrass to cover the state of Victoria.

In terms of carbon abatement value, Australia’s seagrass is worth around $AU45 billion at a price of $AU23 per tonne.

Protect it or lose it

Our blue carbon resources, however, are continually under threat from human activities such as coastal development and run-off from agriculture. This can lead to direct loss or modification of their carbon sink potential.

The risk here is that these habitats could switch from being carbon sinks, to being carbon sources. If their stored carbon is released, it will further acidify our oceans and contribute to the ever growing atmospheric CO2 burden.

The challenge for Australia is to maintain the abatement potential of its blue carbon resources and prevent their future loss. We also have to consider restoring blue carbon habitats from where they have disappeared.

The challenge for science is to better understand the causes of variability in the carbon sequestration rates of blue carbon habitats, and to understand how they might be affected by future changes in our climate.

Author Peter Macreadie has been nominated for a Eureka Prize for his work with seagrass: you can see his nomination and vote here.