Natural CO2 storage plays an important role in efforts to slow down man-made climate change. Researchers have now evaluated numerous studies on this topic and come to the conclusion that wetlands such as moors, salt marshes, mangrove forests and seagrass meadows are of particular importance. In absolute terms, they store less CO2 than oceans and forests, but they are particularly efficient in relation to their area. According to the authors, it is therefore important for climate protection to preserve and restore wetlands.
The burning of fossil raw materials releases large amounts of CO2 into the atmosphere and causes the global climate to warm up as a result of the greenhouse effect. In order to curb climate change, it is relevant on the one hand to reduce greenhouse gas emissions. On the other hand, it is also about separating CO2 from the atmosphere and storing it for as long as possible. In addition to various technical options, natural CO2 sinks in particular play a role. Plants absorb CO2 and use it to form biomass. If this is later decomposed or burned, the CO2 is released back into the atmosphere. Depending on the environmental conditions, however, the CO2 can also remain bound in the biomass in the long term.
CO2 storage hotspots
A team led by Ralph Temmink from the University of Utrecht in the Netherlands has now analyzed the available scientific literature to find out what role wetlands play in this context. “Wetlands cover only 1 percent of the earth’s surface, but store 20 percent of the carbon sequestered by ecosystems worldwide,” the researchers write. “This disproportionate share is favored by high carbon sequestration rates and effective storage in peatlands, mangrove forests, salt marshes and seagrass beds.”
According to the authors’ analysis, wetlands rank third in absolute terms of stored carbon dioxide, after oceans and forests. “However, if you look at the amount of CO2 stored per square meter, it turns out that wetlands store about five times more CO2 than forests and even 500 times more than oceans,” says Temmink. “Peat bogs, salt marshes, mangrove forests and seagrass meadows are therefore global hotspots of CO2 storage.”
Landscape forming plants
The reason for this are special properties of plants and their environment, which lead to positive feedback loops: the more plants grow, the better they can grow. The peat mosses found in moorland absorb rainwater and in this way fuel their own growth. Dead plant remains are deposited under the living peat moss and form a layer that thickens over time. Since this is constantly under water, the dead plants hardly decompose, so that the CO2 bound in them remains stored.
In this way, the mosses gradually build up the moor landscape, leaving large amounts of organic carbon in the soil. The situation is similar with the plants in marshland, in seagrass meadows and in mangrove forests. With their dense network of roots, the plants hold back dead organic material. The nutrients it contains, in turn, drive the growth of the plants. In this case, too, the positive feedback leads to the formation of an increasingly dense soil layer in which a large amount of CO2 is bound.
CO2 sink can become a CO2 source
According to the authors, these results give hope, on the one hand, that wetlands can be a powerful tool for CO2 capture. On the other hand, however, they also emphasize the danger that destroyed wetlands themselves will become a significant source of CO2. If, for example, a bog is drained in order to use the area for other purposes, the CO2 stored in it is released back into the atmosphere. “Globally, around one percent of these ecosystems are lost to human intervention every year,” the researchers write. “The CO2 released as a result accounts for around five percent of all man-made CO2 emissions every year.”
It is therefore particularly important to preserve existing wetlands, according to the authors. Restorative measures could also be worthwhile. However, compared to reforestation measures, the renaturation of CO2-storing wetlands is very expensive and previous attempts have failed in more than half of the cases. In order to increase the success rate of future projects, Temmink and his colleagues have also analyzed the causes of failure and provide tips on how problems can be avoided.
“Previous approaches have been influenced by agriculture and forestry, which plant plants at a distance to avoid competition,” the researchers explain. However, when it comes to landscape-forming wetland plants, it is important to remember that they grow better together. “Restoration is much more successful when the plants are planted in large, dense clumps, when their landscape-forming characteristics are mimicked, or when simply restoring very large areas in one go,” says co-author Tjisse van der Heide from the University of Groningen. “The good news is that with this knowledge, large-scale restoration of these important wetlands is now within reach.”
Source: Ralph Temmink (University of Utrecht, The Netherlands) et al., Science, doi: 10.1126/science.abn1479