Don’t rely solely on regenerating coastal habitats to reduce carbon emissions

coastal ecosystem

It is now believed that to prevent the worst effects of climate change, hundreds of billions of tons of carbon must be removed from the atmosphere. Allowing habitats to regenerate appears to offer a win-win solution for the environment and climate by using nature to help achieve this goal.

Organic carbon that has accumulated over hundreds of years is abundant in the sediments that lie beneath mangrove forests, salt marshes and seagrass beds. Companies and nations looking for ways to fund the restoration of these so-called blue carbon habitats are looking for 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 widely. There was a 600-fold difference between the highest and lowest estimates for carbon burial in salt marshes, a 76-fold difference for seagrass and a 19-fold difference for mangroves, according to several hundred of scientific studies.

The easiest shortcut to calculating the carbon sequestration that can be anticipated from a new restoration effort is to apply the average value of all these studies for a specific habitat. But due to the variability, the anticipated carbon offset could go wrong. Moreover, the probability of overestimating the climate benefit is considerably higher because many low values ​​rather than a few very high values ​​are recorded.

Even over a few kilometres, there are variations in the rates of carbon removal. Credible carbon accounting requires many additional measures, but they are time and labor intensive, increasing the cost of a restoration operation. More serious problems exist. The methods used to establish the carbon burial rates reported in research often involve sampling sediments at different depths to determine their age. By disturbing and combining younger and older layers, burrowing creatures introduce errors into the dating process by making sediments appear younger and carbon burial rates higher than they actually are.

Much of the carbon found in coastal sediments comes from other sources, such as soil that rivers have brought downstream. Imported carbon may represent as little as 10% or as much as 90% of the total. To distinguish between the amount of carbon that has been buried as a result of habitat restoration and the amount that might otherwise have been buried, imported carbon must be removed from the estimates used in offset accounting.

Unfortunately, imported carbon might be less likely to deteriorate. When researching a salt marsh, the percentage of imported carbon increased from 50% near the top of the sediment to 80% in deeper layers. The immediate role of restored habitat in carbon removal may be much less than expected because the deeper value represents the ecosystem’s long-term carbon burial rate.

The climate benefits of restoring blue carbon habitats may be enhanced rather than diminished by other more difficult to quantify processes. It is nevertheless possible that plant debris from a coastal habitat ends up being retained elsewhere for a very long time even if it is washed out to sea rather than accumulating in the sediments. On the high seas, for example, it could dive into very deep waters. But in order to fully account for these processes, scientists need sufficient knowledge of the amounts of carbon that are typically involved.

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

Indeed, these gases are produced when there is not enough oxygen present in the soil or sediments, which are the same circumstances that favor the accumulation of carbon. To determine precisely what is happening, technically difficult measurements are required.

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

(Source: The Conversation)

First published: 05 August 2022, 01:08 IST

Teresa H. Sadler