Tropical rainforests are the carbon sink most often cited as a natural way of capturing and storing carbon dioxide. But rainforests are only the fifth most efficient ecosystem in the carbon storage cycle, new research reveals. Growing seagrass on ocean floors comes in second place among the most efficient ways of reabsorbing carbon.
The quantity of carbon dioxide (CO2) already in the atmosphere and the amount that continues to be emitted has led the European Commission, the EU executive body, to conclude that removing and storing it is the only way to keep global warming well below 2°C. In the Commission’s long-term vision for a climate neutral economy by 2050, adopted in November 2018, carbon capture is included among seven strategic actions in A Clean Planet for All. Natural carbon sinks and active carbon capture and storage (CCS) are two ways to the same end.
The slower the decomposition process for vegetation, the longer it acts as a sink. Where seagrass grows on the seafloor, micro-organisms that break down dead creatures and plants work slowly because of a lack of oxygen, says Marianne Holmer from the University of Southern Denmark. The decomposition process is much faster in a forest where there is more oxygen and animals to eat the leaves and other debris. “Tree leaves fall to the ground and decompose almost immediately. Within a year they are gone and part of the [carbon] cycle again,” says Holmer. When a tree falls in the rainforest, the carbon dioxide is also released back to the atmosphere, she adds.
Carbon is locked away for much longer in the seabed. “Leaves may be buried for hundreds, even thousands of years in the seabed of the Mediterranean,” says Holmer. Seagrass plants in the Mediterranean consist of leaves that are as thick as branches, while eelgrass, the most common seagrass type in Denmark, resembles grass and has thin roots and thin rhizomes, Holmer says. The thicker the stems and leaves, the longer the decomposition.
A study by Holmer and her colleagues examines which ecosystems are capable of absorbing and storing carbon dioxide. The most efficient natural carbon dioxide sink is the tundra, where gas is stored in the frozen soil, but with rising temperatures this may change. “Historically, the tundra has been covered by ice, but now we see a melting of the ice, leaving the tundra exposed and releasing CO2 back to the atmosphere,” says Holmer.
Despite the carbon storage properties of seagrass, its meadows are shrinking rapidly around the world. Pollution, discharges of nitrogen from industrial agriculture and warming seas, a result of climate change, are taking their toll. The meadows are losing 7% of their known areas a year globally, says Holmer. The rate of loss is faster than for rainforests.
Deforestation in rainforests is a real concern, but attention should also be given to other habitats, states Holmer. In some areas, seagrass is coming back naturally, especially in waters and coastal areas with open seas, but other places have no parent population to produce and spread the seeds. This is often the case in areas near the coast or with still waters. Seagrass also returns in areas with improved water quality. Fewer nutrients means less algae and more light helping seagrass to grow, says Holmer.
Restoring seagrass meadows is challenging. “In many areas, the sediment is too fine and the seeds sink too deeply into the seabed or are washed away by the waves. The success rate is poor, but all of a sudden, we will get it right and then things can change rapidly,” says Holmer. In Denmark alone, some 75% of original seagrass areas have been lost over the past one hundred years. But their potential to help boost the carbon budget could be significant if scientists succeed in restoring them.
Holmer estimates seagrass could absorb the equivalent of 5-10% of the CO2 from district heating in Denmark. “This is just a rough estimate. We do not know the numbers yet,” she says. In total, district heating accounted for 2.7 million tonnes of CO2 emissions in 2017, according to the Danish District Heating Association. Various ongoing studies in Sweden, Denmark, other European countries, the US and Australia should provide more accurate forecasts of the potential of seagrass to absorb such emissions.
Denmark is one of the countries that has lost most seagrass relative to its size, says Holmer. A nation of islands, Denmark’s extensive maritime territory was at one time home to massive areas of eelgrass, but much has disappeared, not least the victim of intensive agriculture. A project to test if eelgrass can be restored in coastal areas is being conducted by the Danish Environmental Protection Agency with the University of Southern Denmark. The main aim is to improve the marine environment, but a spin-off win is to recreate eelgrass meadows with positive effects for the climate and CO2 reduction, says the agency’s Harley Bundgaard Madsen.
As part of the project, some 20,000 small eelgrass plants were replanted in Horsens fjord on the Jutland peninsula in the summer of 2017. Within 18 months their number increased 20 times. In the inner part of the fjord, where the waters are stiller and murkier because of alga, the replanted eelgrass failed to grow. To take, the plant needs a suitable seabed that is not too soft, says Bundgaard Madsen. The project will be completed by the end of 2019. Further south, in the fjord of Odense, scientists from the agency and the university are studying the seabed and finding ways to stabilise it, such as with layers of sand.
Whether seagrass is ultimately a successful carbon sink in Denmark and elsewhere will depend on whether the right growing condition can be found. Bundgaard Madsen forecasts the plant as having “real potential” if “open coastal areas with fresh and clear water” similar to the outer part of Horsens fjord are available for planting.
Writer: Karin Jensen
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