More carbon sequestration in marine meadows and forests?
WOR 8 The Ocean – A Climate Champion? How to Boost Marine Carbon Dioxide Uptake | 2024

Blue carbon – an approach yielding dual benefits

Blue carbon – an approach yielding dual benefits fig. 5.12: Antonio Busiello/Getty Images

Blue carbon – an approach yielding dual benefits

> Vegetation-rich coastal ecosystems such as tidal marshes, seagrass meadows, mangrove and kelp forests are the sites of at least 30 per cent of the organic carbon stored in the seabed. Worldwide, however, the area covered by these ecosystems is shrinking and with it their underground carbon stores. Where humans halt the decline of marine meadows and forests and restore destroyed areas, they not only force carbon uptake in plant communities, but also strengthen physiographic regions whose functioning and health are vital for humankind’s survival.

Using nature’s tools

In the search for ways to increase the ocean’s carbon dioxide uptake, it makes sense to first focus on the key players in the ocean’s carbon cycle. In coastal areas, these include above all the vegetation-rich ecosystems in tidal and shallow waters (up to 50 metres of water depth), i.e. tidal marshes, seagrass meadows, mangrove forests and kelp forests. The combined area of these four ecosystem types accounts for less than one per cent of the world’s ocean area, including the intertidal zone. However, because marine meadows and forests are highly productive ecosystems, they convert a lot of carbon dioxide into biomass and are responsible for at least 30 per cent of the organic carbon stored in the seabed.
Much as terrestrial plants do, marine plants or plants in the tidal zone absorb carbon dioxide in the course of photosynthesis and bind the carbon it contains. However, carbon dioxide is not only taken up from the air, but also from seawater, for example by seagrasses and kelp. Since the plant communities of mangrove forests, seagrass meadows and tidal marshes all form root systems and grow on sandy or muddy substrates, they are able to store a large part of the bound carbon in the marine subsoil – in part as living biomass in their own root systems and in part in the form of plant parts that have died off and which sink to the bottom and become incorporated into the coastal sediment.
5.1 > Mangroves protect the coast from waves, sea-level rise and storm surges. But they cannot withstand all weather extremes. When Hurricane Maria swept across Costa Rica in September 2017, large parts of this mangrove forest died.
fig. 5.1: © Shane Gross/iLCP
Moreover, marine meadows and forests slow down the movement of water. As a result, they filter a lot of suspended particles out of the water and deposit these particles as well as dead animal and plant matter between their stalks and roots. Thanks to this constant input of particles, the plant communities continue to build up the substrate on which they grow. Mangrove forests and seagrass meadows, for example, gain two to five millimetres in height per year on a global average and can thus also buffer the impact of rising sea levels, but only as long as the ecosystems accumulate material faster than the sea levels are rising.
These ecosystems not only store local plant matter, but also plant remains that are deposited from the landward side or washed up from other marine areas. Once the organic material is trapped in the subsoil, it is preserved, as the coastal sediment is saline and low in oxygen. Microbes in the seabed thus lack the oxygen they would need to quickly decompose the biomass.

5.2 > While mangroves occur mainly in the tropics and subtropics, tidal marshes and kelp forests prefer cooler regions. Seagrass meadows, however, are found at both low and high latitudes.
fig. 5.2: after p. I. Macreadie et al., 2021, Nature Reviews Earth & Environment 2, 826–839. doi:10.1038/s43017-021-00224-1 and T. Wernberg et al., 2019, in C. Sheppard, 2019, “World Seas: An Environmental Evaluation” (Second Edition), Academic Press
Both the carbon storage in the root system and the deposition of animal and plant litter in an oxygen-deprived environment result in the tidal marshes, mangrove forests and seagrass meadows accumulating more and more organic material underneath them over time. In some mangrove forests, the upper layer of the seabed contains 95 to 98 per cent carbonaceous material.
These underground carbon stores can be more than ten metres thick and keep growing as long as the ecosystems above them thrive. Ideally, they remain in place for many centuries, sometimes even millennia. Tidal marshes, mangrove forests and seagrass meadows are many times more efficient at carbon uptake and underground storage than terrestrial forests. Compared to tropical rainforests, for example, depending on their location they can store five to 30 times the amount of carbon underground per unit area. In contrast, kelp forests, i.e. forests of brown algae (the Laminariales), cannot store the carbon they bind directly in the subsoil, because brown algae do not have roots but rather grow attached to rocky substrates, so loose or dead algal material is carried away by ocean currents. It washes up on the coasts or sinks into deep waters, where some of it is then deposited in the seabed sediment.
5.3 > Mangroves, tidal marshes and seagrass meadows absorb carbon dioxide from the air, bind that carbon and store it in their biomass as well as underground. This map shows for all coastal countries the average annual carbon sequestration potential for the three ecosystems combined, under the proviso that the ecosystems are healthy.
fig. 5.3: after NOAA, Data: Bertram et al., 2021

fig. 5.4: Graham MacKay


5.4 > Twice daily the Atlantic tidewaters wash over and around the tidal marshes at Northton on the south-western coast of the Scottish island of Lewis and Harris. The species-rich saltmarshes grow in sheltered coastal areas where the tides form sandbanks as a substrate for the plants to grow.

How large are the carbon stores and for how long do they persist?

Currently, vegetation-rich coastal ecosystems remove an estimated 85 to 250 million tonnes of carbon per year from the atmosphere and the sea. The range of this estimate is so wide partly because many processes and interactions within the very complex plant communities and their ecosystems are not yet properly understood. For example, one of the as yet unanswered research questions is how much carbon dioxide mangrove and kelp forests, tidal marshes and seagrass meadows in different regions of the earth absorb and store in the form of organic carbon, and what proportion of this they release again in the course of their life cycle.
Marine meadows and forests release carbon dioxide through respiration. The carbon they have captured is also released when manatees, sea urchins and the many other marine organisms consume the plant matter and convert it into energy and carbon dioxide as part of their metabolism. When microbes decompose the organic material stored in the coastal sediment, not only carbon dioxide is released, but also methane and nitrous oxide under certain conditions. What quantities of these two climate-damaging gases are released from coastal ecosystems under which conditions is not yet well understood. What is certain, however, is that where carbon dioxide, methane or nitrous oxide escape from coastal sediments, the underground carbon stores of coastal ecosystems shrink and drive climate change.
For this reason, it is essential to understand for how long the vegetation-rich coastal ecosystems “lock away” the carbon they absorb. Scientists know that the duration of carbon storage depends on where it is stored. Carbon stored by plants as part of their above-ground biomass in leaves, stalks, twigs and branches is removed from the atmosphere for anything from weeks to decades. In contrast, the underground carbon stores, which are often hermetically sealed, can persist for several centuries or even millennia if the vegetation protecting them remains intact. In the Spanish Portlligat Bay, for example, there are seagrass meadows whose carbon stores are more than 6000 years old.
5.5 > In order to accurately survey the distribution of seagrass meadows off the coast of the Bahamas, scientists equipped tiger sharks with tiny sensors and cameras. The sharks hunt in and above the seagrass meadows. The data they collected helped to reveal that the world’s largest seagrass meadows grow off the Bahamas, covering a total area of 66,900 square kilometres, which roughly equates to 75 times the size of Berlin.
fig. 5.5: Diego Camejo/Beneath the Waves

Carbon sink, coastal protection, nursery − the many services provided by coastal ecosystems

Experts often refer to the carbon sequestered by seagrass meadows, tidal marshes and mangrove and kelp forests as “blue carbon”. However, human societies not only benefit from healthy, vegetation-rich coastal ecosystems because they remove carbon dioxide from the atmosphere and the sea. They are also “ecosystem engineers” that form three-dimensional structured habitats in which numerous other species of marine and coastal flora and fauna find sufficient protection and food. For example, 4000 square metres of seagrass meadows can provide refuge and food sources for about 40,000 fish and around 50 million invertebrates such as lobsters, mussels and shrimp. Moreover, their dense tangle of leaves is a nursery habitat for the young of popular culinary fish species, such as Pacific herring and Atlantic cod.
But that’s not all. Tidal marshes, seagrass meadows and mangrove and kelp forests produce oxygen. They filter out pathogens, suspended matter, dirt and pollutants from the seawater, slow down ocean currents, waves and storm surges and thus protect the coasts from erosion and, through the accumulation of sediment, from rising sea levels. At the same time, they reliably provide food (fish, mussels, crustaceans), offer recreational settings and contribute to people’s health, and attract tourists in many places, thus creating additional jobs and income sources for coastal communities. Moreover, they hold spiritual or mythological significance in many regions of the world.
Through this multitude of services, healthy vegetation-rich coastal ecosystems help coastal communities to adapt to climate change in the best possible way. Measures to protect existing marine meadows and forests and to restore degraded coastal ecosystems are therefore win-win solutions. They help to both mitigate climate change and minimize its impacts.
5.6 > The amount of carbon that coastal ecosystems store underground in the long term depends on a number of factors. These include inputs of material from terrestrial sources or from other marine regions as well as the amount of biomass consumed by animals or decomposed by microorganisms.
fig. 5.6: after NOAA, graphic adapted from original by Sarah Battle, NOAA Pacific Marine Environmental Laboratory

Dying coastal ecosystems

Despite the importance of the ecosystem services they provide, vegetation-rich coastal ecosystems are declining in area worldwide. Once again, humans are responsible. Up to 50 per cent of all tidal marshes, about one third of all seagrass meadows and about 35 to 50 per cent of mangrove forests have been lost over the past 100 years as a result of climate change, coastal development and construction, agriculture and aquaculture, marine degradation, overfishing and other intensive uses. Of the world’s kelp forests, 40 to 60 per cent are experiencing obvious declines in area.
5.7 > Seagrass meadows are hotspots of species diversity, providing shelter, food and habitat for countless marine organisms, including leafy seadragons (a syngnathid fish species), starfish and predators such as the American crocodile.
fig. 5.7 from left to right: Jayne Jenkins/Ocean Image Bank, Jayne Jenkins/Ocean Image Bank, Amanda Cotton/Ocean Image Bank, Dimitris Poursanidis/Ocean Image Bank
When scientists recently analysed satellite images of vegetation-rich coastal ecosystems dating from 1999 to 2019, they realized that in those two decades tidal marshes, mudflats and mangrove forests combined had been lost over a total area of 13,700 square kilometres. Over the same period, however, new coastal ecosystems gained some 9700 square kilometres, either by expanding naturally or by human intervention in the form of plantings. But this did not fully offset the losses. Ultimately, the global extent of the coastal ecosystems studied declined by 4000 square kilometres – an area the size of the Spanish Mediterranean island of Mallorca.
Where ecosystems disappear, their carbon stores also largely disintegrate. For example, between 2000 and 2015 some 30 to 120 million tonnes of stored carbon were lost worldwide as a result of mangrove deforestation. The mangrove forest soil was no longer protected and stabilized by vegetation, resulting in microbes decomposing the material stored underground and releasing the carbon back into the atmosphere in the form of greenhouse gases. Converted into carbon dioxide (carbon mass multiplied by 3.664), this corresponds to greenhouse gas emissions amounting to 110 to 450 million tonnes of carbon dioxide. By comparison, the Federal Republic of Germany emitted greenhouse gases with the warming potential of 746 million tonnes of carbon dioxide in 2022.
5.8 > People benefit in many different ways from ecosystem services provided by vegetation-rich coastal ecosystems, also known as Blue Carbon Ecosystems or BCEs. This overview summarizes the monetary added value that mangroves, tidal marshes, seagrass meadows and kelp forests in south-eastern Australia generate for a coastal community and its visitors.
fig. 5.8: after p. I. Macreadie et al., 2021, doi:10.1038/s43017-021-00224-1

Extra Info Under even greater pressure − how climate change multiplies the risks for coastal ecosystems open Extra Info

Strategies to increase carbon dioxide removal by marine forests and meadows

There is also some good news: Damaged or lost mangrove forests and tidal marshes can be restored, as a number of exemplary restoration projects have shown. The replanting of seagrass meadows, in contrast, is very costly and far less likely to succeed. There is still much need for research and development in this regard, just as there is for the restoration of kelp forests.
Nevertheless, researchers hope to increase carbon dioxide uptake and carbon storage by tidal marshes, seagrass meadows, mangrove forests and kelp forests in the long term through three sets of measures. What is common to all three of these sets is that they promote the plant communities’ growth and thus their ability to photosynthesize, sequester carbon and store it in the seabed for the long term.
These measures embrace:
  • The protection and improved management of existing vegetation-rich coastal ecosystems: If rivers can flow freely towards the sea, their water is no longer polluted by fertilizers and other nutrients or pollutants, and dams do not prevent them from carrying sand and other sediments into the coastal waters, mangroves and seagrasses find much better conditions than in coastal regions where these conditions do not exist. Intact food webs are also needed to ensure that, for example, there are enough predators to keep the number of potential pests low.
  • The restoration of marine meadows and forests that were lost due to human intervention: This includes, for example, the replanting of mangrove forests and seagrass meadows and the removal of dikes so that salt marshes can be re-established in newly created intertidal areas.
  • he expansion of existing ecosystems: This would require the creation of new mangrove forests, seagrass meadows, kelp forests and tidal marshes, including in areas where they do not naturally occur and may never have occurred in the past. In addition, plant species would have to be selected and assembled that, as a community of species, would most efficiently deliver the desired ecosystem services.
5.9 > At the southern tip of San Francisco Bay, researchers and environmentalists are working hand in hand to restore more than 60 square kilometres of salt marshes that were destroyed during the gold rush and in the course of industrial development. Their approach appears to be paying off, as this comparison of satellite images from 2002 and 2015 shows.
fig. 5.9: NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey
fig. 5.9: NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey
Experts refer to the approach of expanding or creating new ecosystems as ecosystem design. It is believed that ecosystem design can meet three objectives at the same time:
  • To increase the carbon dioxide uptake of vegetation-rich coastal ecosystems and offset part of the residual carbon dioxide emissions caused by humans.
  • To increase species diversity in coastal waters, provided correct approaches are taken.
  • To offer humans and nature significantly better opportunities to adapt to climate change and defy the dangers it causes, thanks to the many additional ecosystem services provided by coastal ecosystems (nutrition, water quality, coastal protection, etc.).
However, an expansion of vegetation-rich coastal ecosystems would always be at the expense of other neighbouring local ecosystems, such as sandy beaches or tidal flats, if they were planted under mangroves or converted into tidal marshes.
Moreover, an expansion would entail disruptions to the lives of coastal populations, precisely because people around the globe use coastal areas intensively, and in many populated regions there is little open space left.
On German coasts, for example, it would be conceivable that dikes would have to be dismantled and the pastureland behind them abandoned to create more space for tidal marshes. Bays where seagrass meadows are newly planted would have to be closed to bottom trawling and perhaps also to boat traffic, at least temporarily. In order to establish new kelp forests along the North Sea coast, many tonnes of rock would have to be moved into the sea, because brown algae only grow on rocky substrates.
Previous experience with restoration projects has shown that measures aimed at nature conservation and climate change mitigation can only be successfully implemented together if the interests of the local communities are taken into account from the outset, if the local communities are involved in all decision-making processes, if they can contribute their own knowledge and expertise, and if they derive particularly strong benefits from the conservation measures.

A useful tool for climate change mitigation?

Investments in the protection, restoration and expansion of marine meadows and forests only pay off in terms of climate policy if they actually lead to additional carbon uptake and long-term storage in the seabed. This effect must be quantifiable and attributable to tangible measures. Otherwise it will be difficult to reward those in charge of the measures taken – for example by issuing carbon credits, i.e. tradable certificates for the additionally sequestered carbon dioxide.
Moreover, it must be ensured that the additionally sequestered carbon remains permanently in the seabed and is not released again after a few years as a result of microbial decomposition. Climate experts define “permanently stored” as carbon that is securely removed from the atmosphere for at least 25 years, at best several hundred years. Whether vegetation-rich coastal ecosys- tems are capable of this would need to be monitored by means of sophisticated observation systems – and over equally long periods of time.
It is already known that after the restoration or replanting of a seagrass meadow or mangrove forest it takes at least ten or 20 years for the new ecosystem to absorb and store as much carbon annually as healthy extant ecosystems. For every newly created vegetation-rich coastal ecosystem, this means that only after one to two decades can it be verified whether the actual performance of this new or expanded ecosystem in terms of carbon removal matches the initial expectations.
Apart from these challenges, there are seven other serious arguments that have so far made it difficult to realistically classify and soundly evaluate carbon dioxide removal processes based on the restoration, creation or expansion of vegetation-rich coastal ecosystems. These include:
  1. huge regional differences in carbon uptake and sequestration by individual ecosystems,
  2. lack of standards for measuring carbon sequestration,
  3. unresolved questions as to the origin of the stored organic material,
  4. lack of knowledge regarding the generation and release of methane and nitrous oxide,
  5. uncertainties as to the amount of carbon dioxide that is released or sequestered when calcifying inhabitants of coastal ecosystems build up their calcareous shells and exoskeletons and when these dissolve again,
  6. lack of detailed knowledge about the future effects of climate change impacts and other human-induced stressors on marine meadows and forests, and
  7. unanswered questions as to the cost and scalability of potential restoration and expansion measures.

Major regional differences in carbon sequestration

Carbon uptake and storage by marine meadows and forests is influenced by various biological, chemical and physical environmental factors. These not only affect the photosynthetic performance of local plant communities, but also determine the amounts of organic material that are filtered, deposited, decomposed or permanently trapped in the coastal sediment. This dependence on local environmental conditions has a major bearing on the amount of carbon that individual marine meadows and forests actually absorb and store. Experts speak of a high variability of carbon storage in this context. There are, for example, highly productive salt marshes that store up to 600 times more carbon than less productive salt marshes. In the case of seagrasses, the differences can be 76-fold, and 19-fold in the case of mangroves.
Based on this knowledge, scientists conclude that the restoration or expansion of vegetation-rich coastal ecosystems for the purpose of increased carbon dioxide removal from the atmosphere will only make sense and be expedient at those sites where the conditions for high sequestration rates are met or can be established by means of targeted human intervention. This, however, calls for detailed data sets on the carbon storage rates of all marine meadows and forests. But such measurements have so far only been taken at a small number of selected sites.

Lack of standards for measuring carbon sequestration

Measuring carbon uptake and sequestration directly, both on land and in coastal regions, is a difficult and lengthy endeavour and technically complex. For this reason, most data on carbon storage in vegetation-rich coastal ecosystems has so far been collected by means of indirect measurements. This means that researchers took coastal sediment samples – usually down to a depth of one metre – analysed their carbon content and then calculated the average carbon storage using a variety of parameters such as current velocity and sedimentation rate.
However, the error rate of these indirect methods can be very high for various reasons. For example, if one day a dam is built in a river containing large mangrove forests in its delta, the water’s flow velocity and sediment load are reduced. For the mangroves in the river delta, this change means that from that point forward they have significantly less material available to trap animal and plant remains in the seabed. As a result, the mangroves grow more slowly. At the same time, the total size of their carbon stores will be ever less indicative of their current carbon sequestration rate – unless the relevant measurements are taken using methods that have not yet been established as a global standard.
The same is true for coastal wetlands where humans begin to practise arable farming, or if the water quantity or quality in river deltas and coastal waters change as a result of climate change or human use. Another factor to be taken into account is bioturbation, i.e. the extent to which organisms living on or in the seabed burrow through the subsoil and thus the carbon stores. As a result, the trapped organic material is more likely to decompose and degrade. Moreover, intensive bioturbation makes it more difficult for researchers to determine the sediment deposition rate. If they leave bioturbation out of their calculations, the carbon deposition rate may be overestimated by 50 to 100 per cent. Underestimation is also possible. Carbon sequestration data from soil samples should therefore always be interpreted with great caution, experts note.

The question as to the origin of the stored organic material

In order to one day be able to determine the quantity of carbon that has been extracted and stored in the subsoil as a result of an individual blue carbon measure, it is important to know where the organic material trapped in the coastal sediment originated. Was it produced by the seagrass meadows or tidal marshes on site or transported by wind and ocean currents from far away? A number of different studies show that the proportion of material brought in from afar can be high. In mangrove forests in Vietnam, for example, it was found to account for 24 to 55 per cent of the carbon stored below ground. In the case of Australian seagrass meadows, it was as high as 70 to 90 per cent. Some experts argue that if that much material comes in from the outside, there is a risk that the carbon dioxide removal potential of local coastal ecosystems may be overestimated. After all, the carbon was absorbed from the atmosphere elsewhere and stored in the form of organic material. Admittedly, this attribution detail is more of a statistical problem. It is irrelevant to the question of how much organic material is stored. It does become relevant if one day there is a debate as to who can take credit for the carbon drawdown.
5.10 > The restoration of seagrass meadows is complex and often costly because the grasses have to be transplanted by hand. In a restoration project on the Atlantic coast of the US state of Virginia, the organizers use laundry baskets to transport the seagrass seedlings from the propagation tanks to their future growth site.
fig. 5.10: Shane Gross

The generation and release of methane and nitrous oxide

When animal and plant remains are trapped in oxygen-free coastal sediment, microbial decomposition of this organic material produces the climate-damaging greenhouse gases methane (CH4) and nitrous oxide (N2O). It is estimated that the world’s vegetation-rich coastal ecosystems together emit more than five million tonnes of methane per year. If this were true, it would be sufficient to cancel out the positive climate effect of marine meadows and forests due to carbon uptake and sequestration. However, it is as yet impossible to say whether coastal ecosystems actually emit that much methane, because important baseline knowledge about the degradation and release processes in coastal sediments under marine meadows and forests is lacking. Studies investigating these aspects are currently being conducted as part of various research projects. For decisions on the possible use of these ocean-based CDR processes, it is essential to understand whether and, if so, how the restoration or expansion of vegetation-rich coastal ecosystems may change their methane and nitrous oxide emissions. Moreover, if such measures were to be implemented one day, fine-meshed monitoring networks would have to be established to monitor the emissions balance of newly created or expanded marine meadows and forests on a full-coverage basis.

Emissions balance of calcification and dissolution in vegetation-rich coastal ecosystems

When calcifying organisms such as corals, calcareous algae, foraminifera, mussels or true conchs form their exoskeletons and shells from calcium carbonate (lime, CaCO3), the corresponding chemical reaction generates carbon dioxide, which then dissolves in the water. This release causes the carbon dioxide concentration in the water to rise and the greenhouse gas to escape into the atmosphere when at some point the water rises to the sea surface. The reverse happens when lime dissolves in seawater. In the course of the corresponding chemical reaction, those solution products are released that are needed to chemically bind carbon dioxide dissolved in the water. As a result, the carbon dioxide concentration in the water decreases and the ocean can absorb new carbon dioxide from the atmosphere.
Vegetation-rich coastal ecosystems are habitats for many calcifying organisms. However, scientists are currently still discussing how their calcification (which releases carbon dioxide) and possible dissolution processes of the calcareous shells and exoskeletons (which bind carbon dioxide) affect the overall carbon balance of coastal ecosystems and what consequences this may have for the climate. Measurements taken off the coast of the US state of Florida, for example, have shown that marine organisms in one of the world’s largest seagrass meadows formed more calcium carbonate during the study period than was dissolved again through chemical reactions. As a result, the coastal ecosystem was estimated to have released three times more carbon dioxide than it was able to remove from the atmosphere by storing the shell and skeletal remains in the coastal sediment.

Uncertain climate change impacts on marine meadows and forests

The Intergovernmental Panel on Climate Change notes that climate-related changes such as rising temperatures, more frequent and more intense ocean heatwaves, ocean acidification, storms and sea-level rise have mostly detrimental effects on coastal ecosystems, threatening their continued existence as carbon stores and providers of many other ecosystem services.
A potentially increased uptake of carbon dioxide from the atmosphere can probably only be expected where marine meadows and forests shift inland – if there is room for them to do so – and then possibly form larger ecosystems than before. If large-scale spatial shifts are not possible due to space constraints and ecosystems decline in area or disappear, their carbon stores in the coastal sediments would also be at risk.
Worst-case estimates indicate that carbon stores amounting to 3.4 gigatonnes could be lost this way by 2100.
Of the four vegetation-rich coastal ecosystems under discussion, seagrass meadows react most sensitively to rising temperatures, so that even today, with global warming of 1.15 degrees Celsius, marine heatwaves in particular cause them great harm. For example, as a result of such temperature extremes lasting for weeks or even months, 36 to 80 per cent of the local seagrass meadows in the US Chesapeake Bay, in the western Mediterranean and in Sharks Bay in Western Australia have died in recent years. Because heatwaves occur more frequently, last longer and reach higher temperatures with increasing climate change, the climate risks and the extent of the damage caused will continue to escalate in the coming years. Researchers predict that many of the existing seagrass meadows will die if the global surface temperature rises by more than 2.3 degrees Celsius.
5.11 > Climate risks for coastal ecosystems increase with global warming. Kelp forests and seagrass meadows are more temperature-sensitive than salt marshes and mangroves and are therefore already exposed to moderate to high risks with 1.5 to two degrees Celsius of warming.
fig. 5.11: after IPCC, 2022, Climate Change 2022: Impacts, Adaptation and Vulnerability. doi:10.1017/9781009325844, Chapter 3, Supplementary Material, Figure 1 and Figure TS.4
Climate change impacts on tidal marshes are at a medium level with a warming of 1.2 degrees Celsius, but at 3.1 degrees Celsius or more, experts predict that these will also suffer severe damage. One of the effects will be that plant communities are going to die out where they are permanently flooded in the future as a result of sea-level rise.
For mangroves, the thresholds for moderate and severe impacts are two and 3.7 degrees Celsius of global warming respectively. In Australia there are however mangrove forests that are already being affected by climate change, especially when heatwaves, droughts and a short-term drop in sea level, such as due to changes in currents, occur simultaneously. In some other areas, mangroves have been spreading polewards for decades, mingling with or overgrowing tidal marshes. New research results from the central tropics also indicate that a warming of up to two degrees Celsius is likely to lead to increased carbon storage by mangroves, at least in that region.
Current and future climate impacts must be taken into account from the outset when restoring and expanding coastal ecosystems. However, experts still find it very difficult to make predictions about temperature-related species migration.
They therefore recommend that projects to restore or re-establish marine meadows and forests should be carried out primarily at the cooler margins of their current range.
fig. 5.12: Antonio Busiello/Getty Images


5.12 > More than 1000 species of flora and fauna live in the kelp forests around the Channel Islands, a group of islands off the Pacific coast of the US state of California. Of the 27 species of kelp worldwide, nine can be found in this marine area alone, including the largest of all brown algae, the giant kelp or bladder kelp (Macrocystis pyrifera).

Vulnerability to other man-made disruptions and stressors

Even if humankind were to succeed in limiting climate warming to well below two degrees Celsius, the continued existence of many coastal ecosystems and the success of restoration projects or new plantings would be threatened by many other man-made disturbances and stressors. These include, above all, land-use change such as coastal construction in the course of the expansion of coastal cities, mangrove deforestation, for example for the construction of aquaculture installations, the diking and agricultural use of tidal marshes, and the eutrophication of coastal waters through fertilizer and wastewater inputs.
Whether or not measures to restore or replant tidal marshes, seagrass meadows, kelp forests and mangrove forests succeed also depends on whether appropriate sites and plant species were chosen and on whether the rights, needs and knowledge of the local communities were taken into account during planning and implementation. After all, local people bear the responsibility for ensuring that marine meadows and forests are protected in the long term and utilized in a sustainable manner. Experts are also calling for sufficient funds to install monitoring systems and implement protective measures to ensure that the vegetation-rich coastal ecosystems continue to fulfil their important climate function for a long time to come.

Extra Info Blue carbon as a component of emissions trading – a difficult undertaking open Extra Info

Would extension and restoration measures be economically viable and widely applicable?

Whether measures to expand or restore tidal marshes, seagrass meadows, kelp and mangrove forests are economically worthwhile depends on the standpoint from which experts evaluate the services provided by coastal ecosystems. Do they focus solely on the potential increased carbon dioxide uptake of restored or expanded marine meadows and forests, or do they also take into consideration the many other services that ecosystems provide to humans? There are numerous uncertainties associated with both approaches. These include the difficulty of providing evidence of actual additional carbon dioxide uptake. At the same time, the costs of new plantings or extensions vary significantly by vegetation type and coastal region. This is mostly due to different methods being employed, the different wages for the requisite divers, experts and support workers, and whether or not the long-term monitoring costs for the restored or expanded coastal ecosystem are taken into account.
Moreover, there is the question as to the proportion of the marine meadows and forests destroyed by human activities that could realistically be restored – experts refer to the scalability of restoration measures in this regard. Large stretches of coastline where tidal marshes, seagrass meadows or mangrove forests once grew are now built on, diked off or used for farming. So if these former habitat sites cannot be reclaimed, there is simply no room for new plantings. One argument against such reclamation for nature restoration is that in many areas coastal land with high restoration potential is used by smallholder farmers whose entire income depends on precisely that land. If farming families had to give up their land, they would lose the resource base on which their livelihoods depend. For these and other reasons, some experts believe that in Southeast Asia, for example, the area on which mangrove forests could actually be restored or replanted is much smaller than generally held. Depending on the region, their proportion is a mere 5.5 to 34.2 per cent of the theoretically available coastal area, if all socioeconomic arguments against restoration are taken into account.
Other experts are more optimistic about the restoration potential.
In a global analysis of the status and restoration potential of mangrove forests, researchers concluded in 2018 that there are only two types of areas where mangroves cannot be replanted: locations that have been urbanized (0.2 per cent of the mangrove area lost in 1996 to 2016) and locations where former habitat has become permanent open water (16 per cent of the mangrove area lost in 1996 to 2016). According to the study, the restorable area of mangrove forests totals 8120 square kilometres, with 81 per cent of these areas being considered highly restorable.
5.14 > Three of the four coastal ecosystems grow on soft substrates, form roots and are therefore able to accumulate carbon in the substrate. In contrast, kelp grows on rocks and can only store the carbon they take up in their algal biomass.
Tab. 5.14: after CDRmare


Tab. 5.14: after CDRmare

Extra Info Green, red and brown algae – part of the climate solution? open Extra Info

Not a panacea, but a useful tool in the right place

Blue carbon experts are still arguing about what conclusions should be drawn from the uncertainties mentioned above regarding the feasibility and long-term effectiveness of the large-scale restoration and expansion of vegetation-rich coastal ecosystems. Sceptics describe the existing blue carbon approaches as too immature to be used as a basis for national removal targets or to be included in carbon offset trading.
In support of their position, they point to the comparatively wide range of additional carbon dioxide removal potential of marine meadows and forests. The wider that range, the more uncertain the actual potential for carbon removal.
Other experts, however, are encouraged by that range to take a closer look. Studies indicate that protected and restored coastal ecosystems could remove an additional 0.06 to 2.1 gigatonnes of carbon dioxide per annum from the atmosphere.
This removal quantity is roughly equivalent to 0.02 to 6.6 per cent of global carbon dioxide emissions in 2020 and would be far from sufficient to offset the projected residual emissions of several billion tonnes of carbon dioxide and other greenhouse gases.
Blue carbon approaches alone would therefore not achieve the goal of global greenhouse gas neutrality even if all known measures that could prevent man-made greenhouse gas emissions were implemented in parallel.
However, current research on the carbon uptake and storage by tidal marshes, seagrass meadows, mangrove and kelp forests also proves that there are indeed coastal areas where marine meadows and forests store a great deal of carbon and in this way contribute significantly to reducing greenhouse gas concentrations in the Earth’s atmosphere.
The size of this contribution, however, is determined by local environmental conditions, which vary greatly from site to site and explain the major differences in carbon dioxide removal potential. It would therefore be wrong to dismiss the ability of coastal ecosystems to absorb significant amounts of additional carbon dioxide, the experts argue. Instead, research is tasked with investigating the extent to which each individual coastal ecosystem absorbs, stores and, if necessary, releases carbon and to what extent it would also be able to fulfil this removal and storage function in a warmer world.
Only when sufficient data on the carbon cycle of local tidal marshes, seagrass meadows, mangroves and kelp forests are available could a decision be made as to whether new plantings for the restoration or expansion of marine meadows and forests in these areas would be socially equitable and actually promising from an emissions perspective, i.e. whether they would result in additional carbon dioxide removal. Optimistic estimates indicate that this would be the case in so many coastal areas that, in a best-case scenario, the current area of marine meadows and forests worldwide could be expanded by 30 to 50 per cent by 2050.
Should this hope not be fulfilled and the vegetated areas gained ultimately prove to be smaller, both humans and nature would still benefit from healthy and productive coastal ecosystems in many different ways.
Their many co-benefits make tidal marshes, seagrass meadows, mangroves and kelp forests an invaluable guarantor of survival for millions and millions of people and even more marine organisms. Protection and restoration measures therefore tend to enjoy broad societal support.
The scientific community refers to blue carbon approaches as measures with very few downsides which therefore give rise to few concerns (low-regret measures). Moreover, the restoration methods at least for mangroves and tidal marshes are technically mature enough that their use would be theoretically feasible and could be well controlled by local administrations and political institutions.
Investments in effective and science-based conservation and restoration projects for tidal marshes, seagrass meadows, mangroves and kelp forests are therefore already paying off today. Measures of this kind are needed more urgently than ever in a warming world. Textende