The global climate is changing dramatically as a result of excess carbon dioxide in the atmosphere. Nature-based solutions, such as the expansion or protection of forests or coastal blue carbon ecosystems—mangroves, salt marshes, seagrass meadows, seaweed—have been proposed as a means to offset emissions and reduce the effects of climate change. However, there is considerable public confusion about the mitigation potential of such nature-based solutions.

Media articles use extravagant language to describe the potential for blue carbon to mitigate climate change. Headlines include phrases like: “the secret weapon in the fight against climate change,” “the carbon-fixing superpower of wetlands,” and “the hidden CO₂ sink that… could save the planet” (Box 1a). These headlines reflect the enormous carbon sequestration rate claimed for blue carbon ecosystems in the scientific literature, e.g., that, while occupying only 0.5% of the seafloor, these ecosystems are responsible for more than half of the carbon burial in the ocean¹, or that seagrass meadows alone bury 10–18% of the organic carbon buried in the ocean, in <0.1% of the area².

The carbon sequestered by vegetated ecosystems is often presented as being equivalent to the carbon released by fossil fuel burning. This equivalency appears in media articles, blogs by non-governmental organizations, and even scientific papers (Box 1b). It is implicit in statements that protecting or expanding a particular blue carbon ecosystem will offset the emissions of a certain number of cars, coal-based power plants, etc. This is the basis for carbon offset and carbon credit schemes.

However, 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.

We discuss each of these points in turn and then suggest some important messages to communicate to the public, including how blue carbon and other nature-based solutions fit into the range of mitigation options available.

The global rate of blue carbon burial in seagrass meadow sediment has been greatly overestimated as a result of systematic methodological problems^3,4. 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 habitat³. 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 burial⁵ or the release of CH₄ or N₂O from seagrass meadows and salt marshes⁶.

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 emissions¹. Full restoration is unlikely¹, and the 3% estimate relies on carbon burial rates that are almost certainly too high³.

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 rates⁷. 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 atmosphere⁸. Existing stocks are increasingly threatened as a result of climate change, both by sea-level rise⁹ and by episodic marine heatwaves⁸. 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.

Carbon offsets for “avoided emissions” offer to balance additional emissions, such as airplane flights, against no change in carbon stock, which almost inevitably leads to an increase in net emissions. Protecting blue carbon ecosystems also protects their capacity to continue to absorb and bury carbon dioxide. One could argue that offsets for protecting the opportunity for future burial are different from offsets for protecting existing stock, but even the former cannot meaningfully offset fossil carbon emissions for the various reasons discussed in this Comment.

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 (Fig. 1). 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 ocean¹⁰ (Fig. 1). 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 system¹¹.

Carbon exchanges among atmosphere, surface ocean, vegetation and soils on a timescale of days to ~100 years. Equilibration with the deep ocean takes years to thousands of years. These processes collectively comprise the Modern Carbon Cycle. The release of fossil carbon has caused the build-up of excess CO₂ in the atmosphere. The excess CO₂ is only removed from the modern cycle by silicate rock weathering (10,000 s–100,000 s of years) or by the re-formation of fossil fuels (hundreds of millions of years).

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By contrast, fossil fuels have been isolated from the active carbon cycle for hundreds of millions of years¹¹. Burning fossil fuels adds ancient carbon into the modern carbon cycle (Fig. 1), 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^12,13 and hundreds of millions of years for the re-formation of fossil fuels¹¹. Moving carbon from one short-term reservoir to another does not remove it from the actively cycling modern system.

It might seem obvious that fossil fuel emissions and uptake by vegetation operate on very different timescales. However, there is a real communication gap on this subject. Although the idea of different timescales has begun to appear in the literature (e.g., ref. ¹⁴), it is common to treat these processes as directly equivalent, as evidenced by carbon offset programs that balance planting vegetation against fossil fuel emissions, and by the comparisons made in many scientific papers and media articles (Box 1b).

Despite the limitations discussed above, 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^1,8,15.

Expanding the area or increasing the carbon burial efficiency of blue carbon ecosystems could draw down some additional CO₂ from the atmosphere in the short term, buying time to implement other actions. Protecting existing blue carbon ecosystems could also help to stabilize the organic carbon already stored in the underlying sediment, preventing future losses.

There is a great deal of confusion over the role of blue carbon in climate change mitigation. Geoscientists whose work extends over a wide range of timescales might find the points raised in this article to be obvious and the explanation unnecessary. However, the expansion of blue carbon offsetting schemes implies that these points are not widely understood, and some misperceptions are perpetuated by the scientific literature on blue carbon. Whenever the mitigation potential of a vegetated ecosystem is linked to a particular number of cars, tons of coal, etc. (presumably out of an understandable desire to present the information in units that are accessible to the reader), it reinforces the false equivalence between emissions of ancient, fossil carbon and the movement of carbon among compartments within the modern system.

Which mitigation actions to take will not be, ultimately, a scientific decision. The decision will be taken by policy-makers and the general public. The role of scientists is to offer clear information about the different types of actions and what might be achieved by each.

In particular, it would be useful for scientists to communicate how blue carbon and other nature-based solutions fit into the range of options for mitigation. Potential climate change mitigation actions can be divided into three categories.

1. 1.

   Eliminating fossil fuel emissions. Stopping the emissions of ancient carbon would stop the increase in the total amount of carbon in the modern system and stop making the problem worse. This would not, by itself, reverse any of the effects of previous emissions.

2. 2.

   Expanding blue carbon ecosystems (and/or terrestrial equivalents). This could move some carbon from the atmosphere into vegetation, soils and sediments in the short term. Storage in vegetation and surface sediment is not a secure, long-term sink, but it could buy time to consider further options.

3. 3.

   Enacting large-scale technological solutions. These include bioenergy with carbon capture and storage¹⁶; the injection of liquified CO₂ into basaltic rocks, where it becomes a solid mineral¹⁷; enhanced weathering; and large-scale alkalinization of the ocean¹⁸. These options could possibly reverse some of the climate change effects already experienced. However, their risks and effectiveness are still largely unknown¹⁹.

Expanding blue carbon ecosystems provides ecological and social benefits, as well as some short-term carbon sequestration, but it cannot truly offset fossil fuel emissions. Clearer communication of the true mitigation potential of different types of management actions would support informed decision-making.