Blue carbon for climate mitigation: Looking a gift horse in the mouth

There are not many good-news stories in environmental science, particularly where climate is concerned. The recent awareness of large-scale carbon accumulation and storage by vegetated coastal habitats—primarily saltmarshes, seagrass meadows, and mangroves—is therefore rightly celebrated. Carbon removal is not the only natural service that such blue carbon ecosystems (BCEs) freely provide: they also help protect shorelines from storms, support commercially important fisheries, provide habitat for many other organisms, and reduce coastal eutrophication by removing nutrients.

Scientific interest in BCEs has grown from just a handful of papers a year in the 1990s to more than 600 in 2023. BCEs have also attracted the attention of governments, as a ‘natural solution’ in formal climate policies—on the basis that BCE restoration will help achieve the global climate goal of net zero emissions by mid-century. Indeed, a win-win opportunity is envisaged, since both climate and biodiversity would benefit.

Carbon offsets are fine in theory

The proverbial advice is that gift horses should not be closely examined for potential weaknesses; nevertheless, we need to be confident that all is as it seems. That is because a lose-lose outcome is also possible: if the climate benefits of BCEs should prove illusory, then depending on them for mitigation purposes would allow warming and sea-level rise to continue, directly jeopardizing the future of BCEs (as well as ourselves).

Carbon offsets are now a multi-billion-dollar industry, building on the net-zero concept developed by the 2015 Paris Agreement on climate change. The basic science is sound: if global anthropogenic emissions of greenhouse gases are balanced by their removal, there should be no further increase in their atmospheric concentrations. All recent Intergovernmental Panel on Climate Change (IPCC) scenarios that avert dangerous climate change (between 1.5 and 2.0°C of surface warming) now include gigatonne-scale carbon removal by 2050, as well as the rapid change from fossil fuels to renewable energy sources.

The challenge of additionality for BCE carbon removal

The validity of carbon offsets does, however, depend on two core premises: there must be measurable additionality, i.e. net climate benefit that would not have otherwise occurred, and the carbon removed must be securely stored on a long-term basis (at least 100 years). For international recognition that these criteria are being met, formal monitoring, reporting, and verification (MRV) are needed. The carbon removal process should also be sufficiently scalable to provide non-trivial climate mitigation, and the costs should be societally acceptable, generally considered to be less than $100 per tonne CO2 removed.

Afforestation and enhanced forest protection have been the main way of deriving carbon offsets to date, mostly via the private sector. But forest carbon stores are inherently insecure: they are vulnerable to fire and disease, and these risks are increasing in a warmer world. Scalability is also a major constraint, with forestation competing with food production and/or natural habitat protection over much of the world. Hence the growth of interest in ocean-based carbon removal, with focus on BCE restoration as an apparently low-risk and societally acceptable climate mitigation action.

There is, however, a fundamental problem in BCE carbon accounting, due to high natural variability in sediment carbon accumulation rates. I looked at this issue in some detail and found a 19-fold range between the lowest and highest reported values per unit area by mangroves, with a highly skewed distribution (many more low results than high ones). For seagrasses, the range of reported values is greater, with a 76-fold range; for saltmarshes, greater still, with a 600-fold range.

This variability arises from a complex combination of environmental factors that are not well understood. Using a global ‘central tendency’ specific to each BCE hides the problem but doesn’t solve it: important regional and local differences are then ignored. A further issue is whether the effect of high outliers should be reduced by using the geometric mean or median rather than the arithmetic mean. A recent meta-analysis of saltmarsh carbon dynamics led by Victoria Mason (see Further reading) used the arithmetic mean; however, re-calculating their carbon accumulation data for restored saltmarshes as a geometric mean gave a global value that was 41 per cent lower.

Three other major uncertainties affect the reliability of BCE carbon accounting:

• BCEs don’t just take up CO2 but can also release methane (CH4) and nitrous oxide (N2O). These greenhouse gases don’t last so long in the atmosphere but have a much stronger warming effect than CO2. Their emissions can be highly variable, changing with local conditions, tidal cycle, and season.

• A highly variable but potentially large proportion (up to 50–80 per cent in estuaries) of the carbon accumulating in BCE sediments originates from elsewhere, primarily from land. Such non-local carbon may include soot, micro-plastics, and other highly recalcitrant forms that arguably would have been preserved anyway, regardless of the restoration. On the basis that carbon credits should only be awarded as an unambiguous consequence of management action, the ratio of local to nonlocal carbon needs to be determined, and the latter excluded.

• BCEs often support high abundances of molluscs and crustaceans: their calcium carbonate formation (calcification) is therefore enhanced, releasing CO2. The opposite effect can also occur, with carbonate dissolution. Site-specific measurements are needed to determine which process dominates, and whether it significantly decreases or increases net carbon removal and storage.

Can risks and uncertainties be allowed for?

There is a solution to the above uncertainties: factor in the risks when estimating BCE carbon offsets. Thus, instead of assuming that planting mangroves will remove 45.1 kg of CO2 per tree per year (a recently published estimate), scale that back to, say, 4.5 kg per year. The potential error of an order of magnitude may seem high, but is actually conservative, since there is also a relatively high risk of restoration failure and reduced future carbon removal as a result of climate change impacts. The question, then, is whether BCE restoration is still cost-effective as a climate mitigation action, or whether it can still be justified on the basis of the many non-climatic benefits that coastal wetlands provide.

• Phil Williamson (P.Williamson@uea.ac.uk), University of East Anglia. Further reading

Further reading

Williamson, P. and Gattuso, J-P. 2022. Carbon removal using coastal blue carbon ecosystems is uncertain and unreliable, with questionable climatic cost-effectiveness. Frontiers in Climate, 4, 853666.

Mason, V.G., Burden, A., Epstein, G., Jupe, L.L., Wood, K.A. and Skov, M.W. 2023. Blue carbon benefits from global saltmarsh restoration. Global Change Biology, 29: 6517-6545.

Bach, L.T., Vaughan, N.E., Law, C.S. and Williamson, P. 2024. Implementation of marine CO2 removal for climate mitigation: the challenges of additionality, predictability, and governability. Elementa: Science of the Anthropocene, 12, 00034.