Global Warming: Don't Rely Solely on Regrowing Coastal Habitats to Reduce Carbon Emissions

It is now believed that in order to prevent the worst effects of climate change, hundreds of billions of tonnes of carbon must be removed from the atmosphere. Allowing habitats to regenerate would seem to offer a win-win solution for the environment and the climate by utilizing nature to help achieve that aim.

The organic carbon that has accumulated over many hundreds of years is abundant in the sediments that lie beneath mangrove forests, saltmarshes, and seagrass meadows. Businesses and nations looking for ways to fund the restoration of these so-called blue carbon habitats are researching ways to do so in order to offset their emissions of greenhouse gases like carbon dioxide (CO2).

The rate at which blue carbon habitats absorb CO2 from the atmosphere is estimated to vary greatly. There was a 600-fold discrepancy between the greatest and lowest estimations for carbon burial in saltmarshes, a 76-fold difference for seagrasses, and a 19-fold difference for mangroves, according to several hundred scientific studies.

The most straightforward shortcut to calculate the carbon sequestration that may be anticipated from a new restoration effort is to apply the average value from all of these studies for a specific habitat. But because of the variability, the anticipated carbon offsetting could go horribly wrong. Additionally, the likelihood of overestimating the climatic benefit is substantially higher because numerous low values rather than just a few very high ones are recorded.

Even over a few kilometres, there are variations in carbon removal rates. Credible carbon accounting necessitates numerous additional measures, but they require time and labour, increasing the cost of a restoration operation. More serious issues exist. The methods used to establish the carbon burial rates reported in research often involve sampling sediment at various depths to determine its age. By disturbing and combining younger and older layers, burrowing creatures introduce mistakes into the dating process by making sediments appear younger and carbon burial rates higher than they actually are.

A large portion of the carbon found in coastal sediments originates from other sources, such as soil that rivers have brought downstream. It is possible for imported carbon to make up as little as 10% or as much as 90% of the total. To distinguish between the amount of carbon that was buried as a result of restoring the habitat and the amount that may have been buried otherwise, imported carbon should be eliminated from estimations used in offset accounting.

Unfortunately, imported carbon might be less susceptible to deterioration. In research on one saltmarsh, the percentage of imported carbon went from 50% near the sediment top to 80% in deeper layers. The immediate role of restored habitat in removing carbon may be much less significant than expected because the deeper value represents the ecosystem's long-term carbon burial rate.

The advantages of restoring blue carbon habitats to the climate may be increased rather than decreased by other processes that are harder to quantify. It is nevertheless possible for plant debris from a coastal habitat to wind up being retained for a very long period elsewhere even if it gets washed out to sea rather than building up in the sediment. In the open ocean, for instance, it might submerge into very deep water. But in order to fully account for such processes, scientists need sufficient knowledge of the amounts of carbon that are generally involved in them.

The land should be able to absorb carbon if an oil palm plantation is converted back into a mangrove forest or if a coastal region is flooded to create a salt marsh. However, the same area might also generate more nitrous oxide and methane, two potent greenhouse gases, negating any potential climate benefits.

This is because these gases are produced when there is not enough oxygen present in the soil or sediment, which are the same circumstances that encourage the accumulation of carbon. To precisely determine what is occurring, measurements that are technically challenging are required.

However, relying on blue carbon ecosystems to balance ongoing emissions is too risky because the extent of long-term carbon reduction and storage by these habitats is unknown. The consequences of failing to deliver are far too severe. As a result, increasing emission reduction efforts must be prioritized, and only carbon removal techniques that we are confident will be effective should be used to help achieve net zero emissions.

(Source: The Conversation)