Turning an oil palm plantation back into a mangrove forest or flooding a coastal area to make a saltmarsh should help the land accumulate carbon. But scientists don’t know enough about the amounts of carbon typically involved in such processes to properly account for them. It might sink to very deep water in the open ocean, for example. If plant debris from a coastal habitat is washed out to sea instead of accumulating in the sediment, it could still end up being stored for a long time elsewhere. Other processes which are difficult to quantify might increase rather than diminish the climate benefits of restoring blue carbon habitats. 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. In a study on one saltmarsh, the proportion of 50% imported carbon near the sediment surface increased to 80% in deeper layers. Unfortunately, imported carbon may be more resistant to decay. 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. The proportion of imported carbon can be as little as 10% or as much as 90%. Much of the carbon buried in coastal sediments comes from elsewhere, such as soil swept from the land and carried by rivers. 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. The carbon burial rates reported in studies are usually determined indirectly, by sampling sediment at different depths to estimate its age. Many extra measurements are needed for credible carbon accounting, but these take time and effort, raising the cost of a restoration project. And because there are many low values reported with just a few very high ones, there is a much greater chance of overestimating the climate benefit.ĭifferences in carbon removal rates exist even over distances of just a few kilometers. But the variability means that the expected carbon offsetting could be badly wrong. 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.Īpplying the average value from all these studies for a particular habitat is the easiest shortcut to estimate the carbon sequestration that can be expected from a new restoration project.

A sediment core taken from a saltmarsh at high tide Stephanie Nolte/University of East Anglia Causes of uncertaintyĮstimates of the rate at which blue carbon habitats remove CO₂ from the atmosphere vary widely. So we have a very shaky basis for calculating the future carbon offsets that restoration projects might provide over the next 50 to 100 years. Our new analysis found several reasons why it is extremely difficult to work out a reliable figure for carbon accumulation by coastal ecosystems under current conditions. Whether 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. We are researchers who study how marine life, chemistry and the climate interact, and after examining the processes by which coastal habitats draw down (and release) planet-warming gases, we’re not convinced.

Many academics and private sector groups support the idea, assuming that the rate at which these ecosystems remove CO₂ from the atmosphere can be accurately predicted well into the future. Businesses and states keen to offset their emissions of greenhouse gases such as carbon dioxide (CO₂) are exploring ways to do so by funding the restoration of these so-called blue carbon habitats.

The sediments beneath mangrove forests, saltmarshes and seagrass meadows are rich in organic carbon which has built up over many hundreds of years. Using nature to help achieve that goal, by allowing habitats to regenerate, would seem to offer a win-win solution for the environment and the climate. Removing several hundred billion tonnes of carbon from the atmosphere is now considered necessary to avert the worst effects of climate change.

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