Why Coastal Habitat Restoration Can’t Be Relied On to Slow Climate Change

Removing billions of tons of carbon from the air is now necessary to avoid the deleterious impacts of climate change. Using nature could be a win-win solution for the environment and the climate by allowing habitats to regenerate.

The sediments under mangrove forests, salt marshes and seagrass beds have been rich in carbon over the past few centuries. Companies and States concerned with offsetting their emissions of greenhouse gases such as CO2 explore the possibility of doing so by financing the restoration of these so-called “blue carbon” ecosystems.

Researchers and private sector companies could possibly subtract this option, which however assumes that the rate at which these ecosystems could subtract CO2 of the atmosphere can be estimated accurately and over time.

In our research, we study how marine life, chemistry and climate interact. After examining the mechanisms by which coastal habitats absorb (and release) greenhouse gases, we are not convinced of the climate benefit of restoring blue carbon ecosystems. There is a significant risk that their capacity to mitigate emissions has been greatly overestimated.

Our new study identifies several reasons for the great difficulty in accurately estimating the amount of carbon stored by blue carbon ecosystems in the short term. Scientific knowledge on the basis of carbon offsetting restoration programs in the next 50 to 100 years is very uncertain.

Extraction of a sample of salt meadow sediments at high tide.
Stephanie Nolte/University of East Anglia

Sources of uncertainty

There are many estimates of the rate at which blue carbon ecosystems absorb CO2 atmospheric. The analysis of several hundred studies reveals, for salt marshes, a difference of a factor of 600 between the lowest and highest absorption rate. The factor is 76 for seagrasses and 19 for mangroves. Using an average value for each ecosystem category would be the quickest way to estimate the uptake that can be expected from restoration programs.

However, the very high variability could make the estimate of expected carbon credits totally wrong. Since many low uptake values ​​have been published with just a few very high values ​​(skewed or non-normal distribution in statistical jargon), the risk of overestimating the climate benefit is greater than the risk of overestimating it. underestimate.

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Such variability is also found at the local scale, at distances of only a few kilometres. It is therefore necessary to multiply the measures to obtain credible carbon accounting. This requires significant time and funding, increases the cost of restoration projects.

But there are other problems. The rate of carbon sequestration is generally estimated indirectly, by taking sediment from several depths in the soil to estimate its age. Organisms that dig burrows disturb the sedimentary column and mix recent layers with older layers, which distorts the dating making the sediments younger than they really are and overestimating carbon storage.

Some of the carbon sequestered in coastal sediments is not produced locally, but is imported from surrounding environments, for example from terrestrial soil washed away by rivers. The proportion of imported carbon can be very low (10%) or the majority (90%). Imported carbon should not be counted in the allocation of carbon credits for the restoration of blue carbon ecosystems. It is important to be able to distinguish the local carbon stored by the restoration from the imported carbon that could have been sequestered in the absence of restoration.

Imported carbon can be particularly resistant to degradation. A study on a salt meadow shows that the proportion of imported carbon is 50% on the surface and 80% in the deeper, and therefore older, sediment. It is the non-imported carbon present in the deep sediment that must be taken into account to estimate the long-term sequestration. Basing the estimate on the younger sediment, without subtracting the imported carbon, would lead to a very strong overestimation of the stored carbon.

Other hard-to-estimate processes may increase rather than reduce the climate benefit of restoring blue carbon ecosystems. Plant fragments exported rather than locally accumulated in the sediment may be sequestered long-term elsewhere. They can, for example, be exported and stored in the deep ocean. The quantities of carbon necessary in this process are too poorly known to be able to be taken into account.

Many mangrove forests have been destroyed in favor of palm oil plantations. Turning a plantation into a mangrove or flooding it to promote the establishment of a salt marsh can help sequester carbon. But it can also increase the production of methane and nitrous oxide, two powerful greenhouse gases. This is the consequence of a lack of oxygen in the sediment but which also provides excellent conditions for the preservation of carbon. The production of these gases, which is difficult to measure in a simple way, can annihilate the climatic benefit of the restoration.

Measurement of gas exchange in Australian mangrove sediments.
Judith Rosentreter/Southern Cross University

Calcifying (limestone-producing) plants and animals that live in blue carbon habitats, especially plant beds, should also be considered. The fronds, kinds of leaves, of marine plants are often covered with a layer of worms and calcareous algae. The production of this limestone layer leads to the production of CO2. In a Florida seagrass, the amount of CO₂ released in this way is greater than the amount of CO₂ absorbed by the plant. Conversely, it is also possible that the limestone in the sediment is dissolved, leading to CO₂ absorption. Again, accurate measurements are essential at each restoration site to determine the role of these chemical reactions in the net uptake of CO2.

Herbarium of marine plants of the Mediterranean Sea.
David Luquet/CNRS and Sorbonne University

Finally, it is also necessary to take an interest in the future of the restored ecosystems. Will they withstand the ravages of climate change, particularly heat waves, storms and sea level rise? Will they be managed well enough to protect them from competition with agriculture, aquaculture, tourism and other activities responsible for the disappearance of these habitats?

Every effort should be made to halt, and if possible reverse, the overall loss of coastal vegetation. Blue carbon ecosystems are more than carbon sinks. They insure coastal communities from storms, preserve biodiversity, serve as nurseries for commercial species and respond to water quality.

We want the protection of blue carbon habitats to be effective, and global warming to be kept below critical levels for their survival between +2.3°C and +3.7°C. Unfortunately, this is far from certain. If these temperature limits are exceeded, the carbon recently caused by these ecosystems could return to the atmosphere when vegetation is no longer present to protect the sediment from erosion.

Since the amount of carbon that can be removed from the atmosphere and stored by blue carbon ecosystems is so uncertain, it is too risky today to rely on it as an offsetting mechanism for CO2 emissions.2. The consequences of failure would be too damaging. The priority must therefore be on reducing emissions; CO removal approaches2 air can help achieve carbon neutrality, but you should only rely on those whose effectiveness is certain.

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