13 Blue Carbon

Carbon burial in seagrass meadow sediment

Johannesen

Blue carbon will not solve climate change. The effect is too small; existing sediment carbon stock is a liability; and there is a timescale mismatch between ancient fossil fuel emissions and uptake by vegetation. Clearer communication would support informed decision-making.

The protection or expansion of blue carbon ecosystems can only make a very limited contribution to solving the problem of excess atmospheric CO2. In this Comment, we address three issues that relate to this point: (1) the magnitude of the effect, (2) the security of the existing sediment carbon stock, and (3) the mismatch in timescales. Despite these limitations, blue carbon ecosystems are important ecologically and can play a role in short-term carbon sequestration.

The global rate of blue carbon burial in seagrass meadow sediment has been greatly overestimated as a result of systematic methodological problems. Briefly, most global estimates neglect the effects of sediment mixing (wave mixing or bioturbation), which overestimates sedimentation rates; neglect remineralization of organic carbon in surface sediment, which overestimates carbon burial rates; include terrigenous organic carbon, much of which would have been buried even in the absence of the seagrass meadow; and extrapolate from a few sites with tropical species that have extensive, carbon-rich root mattes to the whole global extent of seagrass habitat

Also, many estimates of organic carbon accretion do not consider the effect of CaCO3 formation, which releases carbon to the atmosphere and negates a variable fraction of the drawdown associated with organic carbon burial or the release of CH4 or N2O from seagrass meadows and salt marshes.

Even the most optimistic estimates suggest that full restoration of mangrove, salt marsh and seagrass ecosystems would only provide an ongoing sink equivalent to 3% of current global anthropogenic emissions. Full restoration is unlikely, and the 3% estimate relies on carbon burial rates that are almost certainly too high.

Existing sediment carbon stock is a liability, not an asset

Blue carbon stock refers to the inventory of organic carbon stored over a defined depth (often 1 m) in the sediment of vegetated coastal ecosystems. Most blue carbon papers quantify sediment carbon stock, rather than ongoing burial rates.

The existing stock is buried in sediment but no longer draws down any more carbon dioxide from the atmosphere. In fact, existing sediment carbon stock represents a potential liability, i.e., an insecure reservoir of carbon that could be released into the atmosphere in the future. This is an important factor that has been largely overlooked in the public discussion of offsetting schemes.

When a seagrass meadow dies or a forest burns, some of the stored carbon is re-released into the atmosphere. Existing stocks are increasingly threatened as a result of climate change, both by sea-level rise and by episodic marine heatwaves. The magnitude of the re-release of carbon as a result of these processes is unknown, but integrated over a long enough time, it could easily become as large as or larger than ongoing burial. Accretion is gradual and incremental, while release is episodic and highly variable.

Timescale mismatch

Even if we exclude the possibility of avoided emissions offsets, a fundamental problem with the idea of blue carbon offsets for fossil fuel emissions is the orders of magnitude difference in timescales. The modern carbon cycle acts on timescales of days to about a century, or up to a few thousand years in the case of equilibration with the deep ocean Carbon exchanges readily among the atmosphere, surface ocean, vegetation and surface sediment: the expansion of a seagrass meadow moves some carbon from the atmosphere into vegetation and surface sediment; a forest fire releases carbon from the trees back into the atmosphere. These processes only represent exchange among the compartments of the active, modern carbon system, and not a true removal from the system.

By contrast, fossil fuels have been isolated from the active carbon cycle for hundreds of millions of years. Burning fossil fuels adds ancient carbon into the modern carbon cycle, increasing the total amount to be distributed among the atmosphere, vegetation, etc. The timescale for the removal of the excess carbon dioxide by natural processes is tens of thousands to hundreds of thousands of years for silicate rock weathering and hundreds of millions of years for the re-formation of fossil fuels. Moving carbon from one short-term reservoir to another does not remove it from the actively cycling modern system.

Important role of blue carbon ecosystems

Blue carbon ecosystems do serve important functions. They provide critical habitat for juvenile fish and other marine species; they protect shorelines from erosion; they provide food security for coastal communities; and they protect existing stocks of organic carbon.

Johannesen (2023) Why blue carbon cannot truly offset fossil fuel emissions