Restoring coastal vegetation — so-called “blue carbon” habitat — may not be the natural climate solution it’s claimed to be, according to new research.
In their analysis, researchers from the University of East Anglia (UEA), the French National Center for Scientific Research (CNRS) and the Prince Albert II of Monaco Foundation’s OACIS initiative challenge the widely held belief that restoring areas such as mangroves, salt marshes and sea grasses can remove large amounts of carbon dioxide (CO2) from the atmosphere.
The results of their review are published today in the journal Limits in climateidentify seven reasons why carbon accounting for coastal ecosystems is not only extremely difficult, but also risky.
These include high variability in carbon burial rates, vulnerability to future climate change, and fluxes of methane and nitrous oxide. The authors, who also reviewed information on recovery costs, caution that additional measurements may reduce these risks, but will mean significantly higher costs.
However, they stress that blue carbon habitats should still be protected and where possible restored, as they provide benefits for climate adaptation, coastal protection, food security and biodiversity.
Lead author Dr Phil Williamson, Honorary Reader in UEA’s School of Environmental Sciences, said: “We have studied the processes involved in removing carbon and there are too many uncertainties. The expected climate benefits of restoring the blue carbon ecosystem may be achieved, but so far it seems more likely to be sorely missed.
“If you want to get additional carbon removal, you need additional habitat, and the opportunities for recovery are limited. Many of these sites were developed for coastal settlements, tourism and port development.
“However, we believe that every effort should be made to halt and, where possible, reverse the global loss of coastal vegetation. That’s because blue carbon habitats are more than carbon stores – they also provide protection from storms, support biodiversity and fisheries, and improve water quality.”
Sediments beneath mangroves, tidal flats, and seagrass meadows are rich in organic carbon that has been accumulated and stored over many hundreds of years.
Many recent studies and reviews have positively identified the potential of these coastal blue carbon ecosystems to provide a natural climate solution in two ways: through conservation, reducing greenhouse gas emissions resulting from the loss and degradation of such habitats, and through restoration to increase carbon sink and its long-term storage.
This new review focuses on the latter, assessing the possibility of achieving quantitative and safe carbon removal (negative emissions) through coastal vegetation restoration.
Increasingly, businesses and states are committing to offset their emissions by restoring these ecosystems through carbon credits, assuming reliable knowledge of the amount of CO2 they will be removed from the atmosphere in the future.
However, Dr Williamson and co-author Professor Jean-Pierre Gattuso of the CNRS and the Prince Albert II of Monaco Foundation’s OACIS initiative say the policy issue is more subtle. That means CO2 removal using coastal blue carbon has questionable cost-effectiveness if considered only as a climate mitigation action, either to offset carbon emissions or to be included in countries’ Nationally Determined Contributions that define their efforts to reduce emissions and adapt to the impacts of climate change compliance with the Paris Agreement.
“If we use these ecosystems to offset carbon emissions to a large extent, expecting them to remove, say, up to 100 gigatons of carbon dioxide over the period 2025-2100, only to find that they only remove 10, or maybe only one gigatonne CO2then climate tipping points could be crossed with very serious consequences,” said Dr Williamson.
“However, if such ecosystems are restored to protect biodiversity, we will find that they also remove several gigatonnes of CO2then it would be a bonus to assume that other means are used for climate mitigation.
“Therefore, recovery must be in addition to, not a substitute for, near-total emissions reductions. Where projects to restore coastal blue ecosystems are undertaken primarily for carbon sequestration, they should include comprehensive long-term monitoring to ensure that the intended climate benefits are achieved.”
Professor Gattuso said: “Many important issues related to the measurement of carbon flows and conservation are still unresolved, affecting certification and leading to potential over-crediting.
“Restoring coastal blue carbon ecosystems is nevertheless highly beneficial for climate adaptation, coastal protection, food security and biodiversity conservation. Thus, such actions may be socially justifiable in very many circumstances, based on the many benefits that such habitats provide at the local level. .”