Blue carbon is “uncertain” and “unreliable” says new report

By Chris Lang

Mangrove forests store vast amounts of carbon, particularly in the sediments below the mangroves. Along with seagrass meadows and saltmarshes, mangroves form part of “blue carbon” – the carbon stored in coastal and marine ecosystems. Blue carbon is being touted as yet another nature-based solution to the climate crisis that in reality is a distraction from the need to leave fossil fuels in the ground.

Predictably enough, Conservation International is one of the so-called environmental organisations that is promoting blue carbon. Conservation International has teamed up with IUCN and UNESCO to form the Blue Carbon Initiative. And, of course, it is a carbon trading initiative.

New research published in Frontiers in Climate finds that carbon removal using blue carbon ecosystems is “uncertain and unreliable, with questionable climate cost-effectiveness”.

In an article on The Conversation website, the co-authors, Phil Williamson of the University of East Anglia and Jean-Pierre Gattuso of Sorbonne Université write that,

the climate benefits from restoring these habitats – by planting mangrove trees, for example – is far from certain, and there’s a real risk that the scale at which they can mitigate emissions has been massively oversold.

In a press statement, Williamson says that,

“We have looked into the processes involved in carbon removal and there are just too many uncertainties. The expected climate benefits from blue carbon ecosystem restoration may be achieved, yet it seems more likely they will fall seriously short.”

Williamson and Gattuso’s analysis uncovered several reasons why it is extremely difficult to calculate a reliable figure for the carbon storage in coastal ecosystems under current conditions. “So we have a very shaky basis,” they write, “for calculating the future carbon offsets that restoration projects might provide over the next 50 to 100 years.”

Estimates vary widely

Estimates of the rate of removal of CO₂ from the atmosphere by blue carbon ecosystems vary widely. Williamson and Gattuso write that,

Across several hundred scientific studies, there was a 600-fold difference between the highest and lowest estimates for carbon burial in saltmarshes, a 76-fold difference for seagrasses and a 19-fold difference for mangroves.

Using the average value from these studies is one option, but “the variability means that the expected carbon offsetting could be badly wrong,” Williamson and Gattuso note.

Blue carbon ecosystems just a few kilometres apart can have large differences in carbon removal rates. In order to achieve credible carbon accounting, many extra measurements are needed. But this takes time and adds to the cost of establishing a mangrove restoration project, for example.

Then there’s the fact that carbon burial rates are usually determined indirectly, by taking sediment samples at different depths to estimate its age. The problem here, Williamson and Gattuso explain, is that, “Burrowing organisms disturb and mix younger and older layers, causing errors in this dating process by making sediments seem younger, and carbon burial rates greater, than they really are.”

Imported carbon

Somewhere between 10% and 90% of the carbon buried in coastal sediments comes from elsewhere. Soil washed from fields and carried by rivers, for example. This “imported carbon” should be excluded from estimates used in offset accounting, “to clarify how much was buried as a result of restoring the habitat and how much might have simply been buried regardless,” Williamson and Gattuso write.

Unfortunately, imported carbon may be more resistant to decay. In a study on one saltmarsh, the proportion of 50% imported carbon near the sediment surface increased to 80% in deeper layers. Since the deeper value represents the habitat’s long-term carbon burial rate, the direct contribution of a restored habitat to removing carbon may be much less important than thought.

There are other processes involved in the coastal carbon cycle that are difficult to quantify and therefore make it difficult to know the exact climate benefits (or otherwise) of restoring blue carbon ecosystems.

For example, plant debris from a coastal habitat could be washed out to sea instead of accumulating in sediment, where it could end up sinking into very deep water in the ocean. “But scientists don’t know enough about the amounts of carbon typically involved in such processes to properly account for them,” Williamson and Gattuso note.

Marsh gas

Turning an oil palm plantation back into a mangrove forest or flooding a coastal area to form a saltmarsh sounds like a no-brainer. Surely mangrove forests and saltmarshes are better for the climate than oil palm plantations?

Well, it depends. Not if the restored land ends up releasing more methane and nitrous oxide, both of which are powerful greenhouse gases. Methane is also called marsh gas because it is found at the surface of marshes.

Williamson and Gattuso explain that,

That’s because these gases are formed when there is insufficient oxygen in the soil or sediment, the same conditions that favour carbon accumulation. Technically demanding measurements are needed to find out exactly what is going on.

Calcifying animals

Calcifying animals and plants grow in blue carbon ecosystems, particularly seagrass meadows. Leaves of seagrass are often covered in a white crust of shelled worms and coralline algae. CO₂ is produced when these organisms make their calcium carbonate covering.

One study found that in one of the biggest seagrass dominated estuaries in the world, in Florida Bay, USA, CO₂ emissions from calcification far exceeded the carbon removed by the seagrass.

Another study found that a chemical reaction between dissolved CO₂ and carbonate in the sediment can result in more carbon uptake.

In the first case, the amount of CO₂ absorbed by blue carbon is overestimated, and in the second it is underestimated. “Again, sophisticated measurements are needed at each site to sort out the importance of these effects,” Williamson and Gattuso write.

The future

A huge uncertainty is what will happen to blue carbon ecosystems in the future as the climate crisis intensifies. Blue carbon ecosystems are threatened by heatwaves, storms, and sea level rise. They will also have to be sufficiently well managed to survive encroachment by agriculture, tourism, and other industies and activities.

Of course it is important to reverse the destruction of coastal ecosystems worldwide. Blue carbon ecosystems are not just carbon sinks. They protect communities from storms, protect biodiversity, and improve water quality.

But Williamson and Gattuso conclude that,

Since the scale of long-term carbon removal and storage by blue carbon habitats is so uncertain, it is too risky to rely on as a means of offsetting continued emissions. The consequences of failing to deliver are too great. The priority must therefore be to double down on emission reductions, only using carbon removal methods to help achieve net zero where we are confident that they will work.