Coastal ecosystems store much more greenhouse gas than they emit

Scientists led by Southern Cross University in Australia have produced groundbreaking research, revealing that coastal ecosystems worldwide act as net greenhouse gas (GHG) sinks for carbon dioxide (CO2). 

However, their positive contribution is mitigated by emissions of other potent greenhouse gases like methane (CH4) and nitrous oxide (N2O). This game-changing study is the culmination of an international effort to understand our coastal carbon budget.

The researchers painstakingly analyzed greenhouse gas dynamics in a wide range of coastal ecosystems across ten global regions. These included tropical lagoons, polar fjords, coastal mangrove forests, and underwater seagrass communities. 

The results of this global survey, detailed in the paper, “Coastal vegetation and estuaries collectively are a greenhouse gas sink,” appeared today in Nature Climate Change.

What the researchers learned

According to Dr. Judith Rosentreter, the lead researcher of the study, understanding greenhouse gas fluxes in coastal ecosystems is a vital prerequisite for implementing effective climate mitigation strategies.

“Protecting and restoring mangrove and salt marsh habitats can fortify the CO2 uptake by these coastal wetlands,” Rosentreter said, outlining a potential pathway towards curbing global warming.

Alongside this, the researchers highlighted the necessity to diminish human impacts like nutrient overloading, organic matter dumping, and wastewater disposal in coastal waterways. Such measures can help mitigate the release of CH4 and N2O into the atmosphere.

Where and how the study was conducted

The research team, composed of global scientists, scrutinized ten different world regions, including North America, South America, Europe, Africa, Russia, West Asia, South Asia, East Asia, Southeast Asia, and Australasia.

Among these regions, Southeast Asia emerged as the strongest coastal GHG sink, mainly due to its vast, carbon-sequestering tropical coastal wetlands. North America came next, boasting large coastal wetland areas and CO2-absorbing fjords.

Professor Bradley Eyre highlighted the role of fjords in carbon sequestration. “Fjords globally absorb approximately 40% of CO2 otherwise released from tidal systems, deltas, and lagoons. North America, particularly Greenland, is responsible for a whopping 86% of this critical CO2 uptake.”

However, not all coastal habitats are beneficial from a climate perspective. According to Dr. Rosentreter, certain coastal wetlands such as mangrove forests, coastal salt marshes, and seagrasses emit more than three times the amount of CH4 than all the world’s estuaries combined.

Despite this, coastal wetlands, often dubbed ‘blue carbon’ wetlands, still manage to be net GHG sinks. They do so by effectively absorbing CO2 and, in some cases, N2O, thereby compensating for their methane emissions.

“When considering all three greenhouse gases (CO2, CH4, and N2O), eight of the ten world regions studied emerge as net coastal GHG sinks,” said Dr. Rosentreter.

This pioneering research will inform the efforts of the Global Carbon Project’s RECCAP2. According to study co-author Professor Pierre Regnier from Université Libre de Bruxelles, the project was initiated to set up greenhouse gas budgets for vast regions worldwide, considering the overlooked role of coastal ecosystems.

To make this significant breakthrough, the research team analyzed data from 738 sites across ten global regions, with studies spanning from 1975 to 2020. The findings allowed them to quantify the fluxes of CO2, CH4, and N2O in estuaries and coastal vegetation worldwide.

Conclusions from data gathered from 738 global sites

In conclusion, the greenhouse gas dynamics in coastal ecosystems are shaped by specific regional factors like climate, hydrology, and local abundance of flora. The research revealed that the strongest coastal GHG sinks are located in the archipelagic region of Southeast Asia, North America, and Africa. 

The region’s productive mangrove forests and seagrasses in Southeast Asia absorb substantial amounts of CO2, giving it the top spot. North America, with its expansive salt marshes, mangroves, seagrasses, and fjords that uptake CO2, secures the second position. Africa’s large-scale CO2 uptake by mangroves and seagrasses, slightly offset by estuarine GHG emissions, puts it in the third spot.

Moderate coastal GHG sinks were identified in South America, Australasia, and West Asia. South America’s coastal wetlands, particularly mangroves, demonstrate moderate CO2 uptake, mitigated somewhat by estuarine GHG emissions. 

Australasia features lengthy stretches of CO2-absorbing coastal wetlands, offset by estuaries along its coasts that release CO2, CH4, and N2O. West Asia, with a moderate CO2 uptake by coastal wetlands, primarily seagrasses, and a weak estuarine GHG source, also falls into this category.

In the case of East Asia and South Asia, their coastal wetlands’ moderate CO2 sink is considerably reduced by estuarine GHG emissions, categorizing them as weak GHG sinks.

Interestingly, Europe and Russia emerged as weak GHG sources, meaning these regions emit more greenhouse gases than they absorb. These regions host many impacted tidal estuaries that emit greenhouse gases. In addition, their colder climate limits the extent of coastal wetlands, like mangroves, which could otherwise absorb significant amounts of CO2.

This research lays a foundation for understanding how unique regional characteristics and ecosystem types contribute to the global GHG budget. It not only offers a comprehensive understanding of coastal GHG sources and sinks but also emphasizes the crucial role these ecosystems play in mitigating climate change. 

As humanity grapples with the escalating climate crisis, efforts to conserve and restore these coastal ecosystems should be amplified, harnessing their potential as powerful tools in our fight against global warming.

More about coastal ecosystems

Coastal ecosystems are biologically diverse habitats found along the coastlines of continents and islands. They play crucial roles in the environment due to their high productivity, the ecological services they provide, and their role in carbon sequestration. Coastal ecosystems include a variety of habitats, such as estuaries, mangroves, salt marshes, seagrasses, coral reefs, and coastal uplands.

Estuaries

These are bodies of water where freshwater from rivers mixes with saltwater from the sea. Estuaries are rich in nutrients and support a wide variety of aquatic plants, fish, and shellfish. They act as important nurseries for many species of fish and invertebrates that live in the ocean.

Mangroves

These are forest ecosystems found in tropical and subtropical tidal areas. Mangroves can tolerate saltwater and have unique root systems that allow them to thrive in low-oxygen soil conditions. They protect coastlines from erosion, storm surges, and tsunamis. Mangroves are also important nursery grounds for many fish species and support a high level of biodiversity.

Salt Marshes

These are coastal wetlands that are flooded and drained by salt water brought in by the tides. Salt marshes are highly productive habitats that provide important ecosystem services including water filtration, storm protection, and serving as nursery grounds for many species of fish and invertebrates.

Seagrasses

These are flowering plants that live in shallow marine environments. They form extensive beds or meadows, which provide important habitats and nursery areas for many marine species. Seagrasses also play a key role in the carbon cycle, sequestering significant amounts of carbon dioxide from the atmosphere.

Coral Reefs

These are diverse underwater ecosystems held together by calcium carbonate structures secreted by corals. Coral reefs are home to a quarter of all marine species and thus have extremely high biodiversity. They provide numerous ecosystem services, including food, tourism opportunities, coastal protection, and even potential medicinal compounds.

Coastal Uplands

These are areas of land adjacent to the coast, which can include dunes, bluffs, and cliffs. These habitats provide important nesting sites for many species of birds and can act as buffers against coastal erosion.

Coastal ecosystems are critically important to the global carbon cycle. Coastal wetlands, mangroves, and seagrasses sequester carbon dioxide from the atmosphere and store it in their sediments in what is often referred to as “blue carbon.”

Despite their ecological importance, coastal ecosystems worldwide are under significant threat. Human activities, including land development, pollution, overfishing, and climate change, pose serious risks. Rising sea levels and increasing ocean temperatures, in particular, threaten these delicate ecosystems.

Conservation of coastal ecosystems is critically important, not only because of their biodiversity and the services they provide, but also because of their role in mitigating climate change. 

Efforts to protect and restore these ecosystems can contribute significantly to the global fight against climate change while preserving biodiversity and supporting human communities that rely on these ecosystems for their livelihood.

Image Credit: Dong Zhang on Unsplash

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