Excess greenhouse gases, such as carbon dioxide, are a major driver of climate change. Mitigating climate change in the future will require both decarbonization – such as transitioning to renewable energy sources – and carbon dioxide removal, which involves extracting already existing carbon dioxide from the atmosphere. A recent study led by Georgia Institute of Technology (Georgia Tech) and Yale University has proposed a unique approach to permanently capture carbon dioxide from the atmosphere through coastal ecosystem restoration.
Specifically, the researchers have focused on blue carbon ecosystems, such as seagrass and mangroves, which naturally capture carbon dioxide through photosynthesis and convert it into living tissue.
“Mangroves and seagrasses extract carbon dioxide from the atmosphere all day long and turn it into biomass,” said senior author Christopher Reinhard, an associate professor of Earth and Atmospheric Sciences at Georgia Tech. “Some of this biomass can get buried in sediments, and if it stays there, then you’ve basically just removed carbon dioxide from the atmosphere.”
According to the experts, in addition to potential benefits for local flora and fauna and the revitalization of coastal economies, restoring these ecosystems can also remove additional carbon dioxide through a novel pathway, while also addressing the issue of increasing ocean acidity.
There are two primary types of carbon in the Earth system: organic and inorganic carbon. Organic carbon is found in living matter and can temporarily remove carbon dioxide from the atmosphere. However, when it becomes buried in seafloor sediments, it leads to permanent carbon dioxide removal. Inorganic carbon, on the other hand, exists in various forms, including as a dissolved component in ocean water.
Approximately 30 percent of the carbon emitted by human activities since the industrial revolution is now stored as dissolved inorganic carbon in the ocean. Unlike organic carbon, carbon dioxide removal through inorganic carbon is more durable and less susceptible to disruption.
“Even if you change the way a coastal ecosystem restoration project is operating, potentially remobilizing previously stored organic carbon, inorganic carbon capture is largely a one-way street,” said lead author Mojtaba Fakhraee, a postdoctoral researcher in Geochemistry and Geobiology at Yale. “So even if a massive ecosystem disruption in the future undoes organic carbon storage, the inorganic carbon that has been captured will still be in the ocean permanently.”
Benefits of restoring coastal ecosystems
Restoring coastal ecosystems offers multiple advantages, including the natural removal of carbon dioxide from the atmosphere and the provision of environmental and economic benefits to coastal communities. Previous research has mainly focused on carbon removal through the burial of organic carbon, neglecting the potential for carbon removal through the formation of inorganic carbon.
Moreover, human activities – particularly the use of fossil fuels – have resulted in ocean acidification due to carbon dioxide dissolving in the water and lowering the ocean’s pH. Storing carbon dioxide as inorganic carbon in the ocean can help mitigate this issue, as the chemical processes involved in inorganic carbon capture alkalinize ocean waters, thereby counteracting the negative ecological consequences of ocean acidification.
“The basic idea here is that you are shifting the acid-base balance of the ocean to drive conversion of carbon dioxide in the atmosphere to inorganic carbon in the ocean,” Reinhard explained. “This means that the process can help to partially offset the negative ecological consequences of ocean acidification.”
To assess the effectiveness of restoring coastal ecosystems for inorganic carbon capture, the researchers developed a numerical model that simulates the chemistry and physics of sedimentary systems, which include solid particles, living organisms, and seawater accumulating on the ocean floor.
This model specifically tracks the benefits of restored blue carbon ecosystems, such as mangroves and seagrass, and their impact on organic and inorganic carbon cycling. It also accounts for the effects of other greenhouse gases, including methane, that may be generated during the restoration process.
“This model comes up with representations for the rates of carbon transformation in the sediment based on how much mangrove is growing above the sediment,” explained co-author Noah Planavsky, a professor of Earth and Planetary Sciences at Yale. “We found that across an extremely large range of scenarios, restoration of blue carbon ecosystems leads to durable carbon dioxide removal as dissolved inorganic carbon.”
The scientists hope that their findings will raise awareness and encourage the protection of existing coastal ecosystems, as well as pave the way toward economic incentives for the restoration of degraded ecosystems, potentially serving as a new form of carbon offset.
“Companies that are trying to offset their own emissions could potentially purchase carbon removal through funding restoration of coastal ecosystems. This could help rebuild these ecosystems and all of the environmental benefits they provide, while leading to durable carbon dioxide removal from the atmosphere,” Reinhard concluded.
The study is published in the journal Nature Sustainability.
Coastal ecosystems are dynamic and complex environments found along the meeting point of the land and sea. They encompass a wide range of physical and biological components, from sandy beaches and rocky shores to estuaries, mangroves, seagrass beds, salt marshes, and coral reefs.
These are coastal wetlands that are flooded and drained by salt water brought in by the tides. They are typically found along protected coastlines in temperate and high-latitudes. Salt marshes play a crucial role in the aquatic food web and the delivery of nutrients to coastal waters. They also serve as protective buffers between the land and ocean, reducing the impact of storms and preventing erosion.
Mangrove forests are tropical and subtropical coastal ecosystems characterized by trees and shrubs that grow in saline or brackish water. They have a complex root system that allows them to withstand strong waves and storms, and they play a critical role in carbon sequestration. Moreover, they provide a habitat for many species and contribute to the livelihoods of local communities through fisheries and tourism.
Seagrass beds are underwater ecosystems typically found in shallow coastal waters. They play a crucial role in nutrient cycling, carbon sequestration, and sediment stabilization. Seagrass meadows also serve as nurseries and habitats for many marine species, including commercially important fish and shellfish species.
These are diverse underwater ecosystems held together by calcium carbonate structures secreted by corals. Coral reefs are home to a variety of species, contributing significantly to biodiversity. They also provide protection to coastlines from the damaging effects of wave action and tropical storms.
Estuaries are where rivers meet the sea, and their brackish water is a mix of fresh water draining from the land and salty sea water. They are one of the most productive ecosystems on Earth, providing many ecological benefits including water filtration, habitat provision, nutrient cycling, and serving as nurseries for many fish and shellfish species.
These are often the most visually recognizable coastal ecosystems. They provide important habitats for a range of species, and are crucial for a variety of ecological processes.
Coastal ecosystems are vital for a variety of reasons, including the provision of ecosystem services, the support of diverse biological communities, and the role they play in the carbon cycle. However, they face a multitude of threats such as climate change, pollution, overfishing, habitat destruction, and invasive species. Preserving and restoring these critical habitats is important for the health of our planet.