Fjords are coastal systems that act similar to the ocean. They take up large amounts of CO2 from the atmosphere. Some fjords emit also N2O and small amounts of CH4. On balance, fjords are a greenhouse gas sink.
Newswise — International researchers, led by Australia's Southern Cross University, have unveiled a fresh greenhouse gas budget, revealing that coastal ecosystems worldwide act as a net absorber of carbon dioxide (CO2). However, the positive impact of CO2 uptake is partially offset by the emissions of methane (CH4) and nitrous oxide (N2O).
Today, the journal Nature Climate Change published a groundbreaking paper titled "Coastal vegetation and estuaries collectively are a greenhouse gas sink," which presents the latest findings on the balance of greenhouse gases (CO2 + CH4 + N2O) in coastal areas. The research encompasses ten distinct world regions as well as a global analysis. These findings shed light on the crucial role played by coastal vegetation and estuaries in acting as a sink for greenhouse gases.
Coastlines across the globe, ranging from tropical lagoons to polar fjords, and from coastal mangrove forests to underwater seagrass communities, exhibit remarkable diversity in terms of greenhouse gas sinks and emissions. These diverse coastal ecosystems play a significant role in both absorbing and releasing greenhouse gases into the atmosphere.
Dr. Judith Rosentreter, the Senior Research Fellow at Southern Cross University and the lead researcher of the study, emphasized the significance of comprehending the dynamics of greenhouse gas release and absorption in coastal ecosystems. According to Dr. Rosentreter, this understanding serves as a crucial initial step towards developing effective climate mitigation strategies. Recognizing the specific locations and processes involved in greenhouse gas fluxes within coastal ecosystems is pivotal for implementing targeted and impactful measures to address climate change.
As an instance, safeguarding and rehabilitating mangrove and salt marsh habitats emerges as a hopeful approach to enhance the absorption of CO2 by these coastal wetlands.
Additional measures to mitigate human impact include minimizing inputs of nutrients, organic matter, and wastewater into coastal waterways, which can effectively reduce the emission of CH4 and N2O into the atmosphere.
The international team of scientists examined ten distinct regions worldwide: North America, South America, Europe, Africa, Russia, West Asia, South Asia, East Asia, Southeast Asia, and Australasia (as depicted in Figure 1).
The research findings revealed that Southeast Asia exhibited the most significant greenhouse gas (GHG) sink along its coast, primarily due to its expansive and productive tropical coastal wetlands that effectively absorb CO2. Another notable GHG sink hotspot was identified in North America, characterized by its vast coastal wetlands and CO2-absorbing fjords.
Co-author Professor Bradley Eyre, who holds the position of Professor of Biogeochemistry at Southern Cross University, shared significant findings from the research. He explained that the study demonstrated how fjords globally play a crucial role in absorbing approximately 40% of the CO2 that would otherwise be released from tidal systems, deltas, and lagoons. Notably, the North America region, particularly Greenland, accounted for the majority (86%) of this substantial CO2 uptake by fjords.
Dr. Rosentreter further emphasized that while certain coastal habitats act as sinks for greenhouse gases, others function as sources. For instance, coastal wetlands, including mangrove forests, coastal salt marshes, and seagrasses, were found to release more than three times the amount of CH4 compared to all estuaries worldwide. This highlights the significance of considering the diverse contributions of different coastal ecosystems to greenhouse gas emissions and underscores the need for comprehensive approaches in climate change mitigation and management.
Simultaneously, coastal wetlands, often referred to as coastal 'blue carbon' wetlands, exhibit the capacity to serve as robust sinks for CO2. Furthermore, certain coastal wetlands also have the ability to absorb N2O. When taking into account all three greenhouse gases, these wetlands prove to be net sinks for greenhouse gases, effectively mitigating their release into the atmosphere. This recognition underscores the importance of protecting and preserving coastal wetlands for their valuable role in climate regulation and carbon sequestration.
Dr. Rosentreter highlighted the findings of their recent study, emphasizing that when accounting for all three greenhouse gases (CO2 + CH4 + N2O), eight out of the ten world regions examined were identified as coastal net sinks for greenhouse gases. This indicates that these coastal areas have an overall capacity to absorb more greenhouse gases than they emit, underscoring their significant role in mitigating climate change.
Co-author Professor Pierre Regnier, who specializes in Earth System Science at Université Libre de Bruxelles, explained that the research was initiated by the Global Carbon Project. The primary objective was to develop comprehensive greenhouse gas budgets for large regions worldwide, which had previously overlooked the contribution of coastal ecosystems. By including these coastal ecosystems in the analysis, the study aimed to fill the existing knowledge gap and provide a more holistic understanding of greenhouse gas dynamics on a global scale.
Snapshot: Coastal greenhouse gas sinks and sources around the world
To assess the CO2, CH4, and N2O fluxes in estuaries and coastal vegetation across 10 global regions, a dataset comprising observations from 738 sites was compiled. These observations were extracted from studies published between 1975 and 2020.
Distinctive coastal attributes such as climate, hydrology, and abundance vary across different regions worldwide, thereby influencing the uptake or release of greenhouse gases (GHGs) from coastal systems. These unique characteristics play a significant role in shaping the dynamics of GHG emissions and absorption in coastal environments on a regional scale.
Strongest coastal greenhouse gas sinks:
- The top-ranking region is Southeast Asia, characterized by its archipelagic nature. This region boasts expansive and productive tropical mangrove forests and seagrasses that effectively absorb substantial amounts of CO2.
- Following Southeast Asia, North America secures its place due to its significant coverage of salt marshes, mangroves, and seagrasses, along with the presence of CO2-absorbing fjords.
- In the third position is Africa, where mangroves and seagrasses play a pivotal role in absorbing noteworthy levels of CO2. However, the overall CO2 uptake in Africa is moderately influenced by the emissions of greenhouse gases from estuarine areas.
Moderate coastal greenhouse gas sinks:
- South America: moderate CO2 uptake by coastal wetlands, especially mangroves, and some estuarine GHG emissions.
- Australasia: long stretches of coastal wetlands that take up CO2, but this region also has a large number of estuaries along its coasts, many of which are a source of CO2, CH4 and N2O.
- West Asia: weak estuarine GHG source and moderate CO2 uptake by coastal wetlands, mostly seagrasses.
Weak coastal greenhouse gas sinks:
- In both East Asia and South Asia, the moderate CO2 sink provided by coastal wetlands is extensively offset by the emissions of greenhouse gases (GHGs) in estuarine regions.
Weak coastal greenhouse gas sources:
- In Europe and Russia, the emissions of coastal greenhouse gases (GHGs) surpass their capacity to absorb GHGs from the atmosphere. These regions are characterized by numerous impacted tidal estuaries that release significant amounts of greenhouse gases. Additionally, the colder climate in these regions results in a scarcity of coastal wetlands, such as mangroves, which are otherwise efficient at absorbing substantial quantities of CO2.
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