Coastal ecosystems play important role in climate mitigation

A team of international researchers, led by Australia’s Southern Cross University, has revealed new insights into the greenhouse gas budget of coastal ecosystems worldwide. Their study, published on May 22 in Nature Climate Change, demonstrates that coastal ecosystems act as a net greenhouse gas sink for carbon dioxide (CO2), but the emissions of methane (CH4) and nitrous oxide (N2O) partially offset the CO2 uptake.

The research findings encompass 10 distinct regions globally, encompassing a range of coastal environments from tropical lagoons to polar fjords and from coastal mangrove forests to underwater seagrass communities. These diverse coastlines exhibit varying capacities for both greenhouse gas absorption and emission.

Dr. Judith Rosentreter, Senior Research Fellow at Southern Cross University and the lead researcher, emphasizes the significance of understanding the release and absorption patterns of greenhouse gases in coastal ecosystems as a crucial first step in implementing effective climate mitigation strategies. Protecting and restoring habitats like mangrove and salt marsh ecosystems emerges as a promising approach to enhance CO2 uptake by these coastal wetlands.

Additionally, curbing human activities that contribute to greenhouse gas emissions in coastal areas can be instrumental in mitigating the impact. Reducing inputs of nutrients, organic matter, and wastewater into coastal waterways can help minimize the release of CH4 and N2O into the atmosphere.

The study analyzed ten world regions: North America, South America, Europe, Africa, Russia, West Asia, South Asia, East Asia, Southeast Asia, and Australasia, providing a comprehensive understanding of the coastal greenhouse gas balance on a global scale.

Figure 1: The coastal greenhouse gas (GHG) budget (CO2 + CH4 + N2O) as CO2-equivalent fluxes (in teragrams of CO2-equivalent per year) in 10 regions around the world: Southeast Asia (9), North America (1), Africa (4) are strong coastal GHG sinks. South America (2), Australasia (10), and West Asia (6) are moderate coastal GHG sinks. East Asia (7) and South Asia (8) are weak coastal GHG sinks, and Europe (3) and Russia (5) are weak coastal GHG sources. Credit: Nature Climate Change (2023). DOI: 10.1038/s41558-023-01682-9

The study revealed that Southeast Asia boasts the most robust greenhouse gas (GHG) sink among coastal regions due to its expansive and productive tropical coastal wetlands, which effectively absorb CO2. Another significant GHG sink hotspot is located in North America, characterized by extensive coastal wetlands and CO2-absorbing fjords.

Co-author Professor Bradley Eyre, a Biogeochemistry expert at Southern Cross University, highlighted the crucial role of fjords worldwide in sequestering approximately 40% of the CO2 that would otherwise be released from tidal systems, deltas, and lagoons. Notably, the North America region, particularly Greenland, contributes 86% of this vital CO2 uptake in fjords.

Dr. Rosentreter emphasized that various other coastal habitats serve as sources of greenhouse gas emissions. For instance, coastal wetlands like mangrove forests, salt marshes, and seagrasses release over three times more CH4 than all estuaries worldwide.

However, when considering all three greenhouse gases (CO2 + CH4 + N2O), referred to as the coastal greenhouse gas balance, eight out of the ten world regions studied function as net sinks for greenhouse gases. This is primarily due to the ability of coastal wetlands, also known as “blue carbon” wetlands, to absorb CO2 and some extent of N2O, ultimately mitigating their overall impact on the atmosphere.

The findings of this research will contribute to the Global Carbon Project’s RECCAP2 initiative, which aims to establish comprehensive greenhouse gas budgets for large regions worldwide, accounting for the previously unassessed contributions of coastal ecosystems.

Co-author Professor Pierre Regnier from Université Libre de Bruxelles highlighted that the study was initiated by the Global Carbon Project to fill the gap in understanding the greenhouse gas budgets of major regions globally, with coastal ecosystems’ contributions previously overlooked.

Snapshot: Coastal greenhouse gas sinks and sources around the world

The researchers compiled a comprehensive dataset encompassing 738 observation sites from studies published between 1975 and 2020 to assess the fluxes of CO2, CH4, and N2O in estuaries and coastal vegetation across ten global regions.

Distinctive coastal characteristics such as climate, hydrology, and abundance play a significant role in driving the uptake and release of greenhouse gases in coastal systems worldwide.

The strongest sinks for greenhouse gases in coastal areas were identified as follows:

  1. Southeast Asia, particularly its archipelagic region, demonstrated significant CO2 uptake due to the presence of extensive and productive tropical mangrove forests and seagrass meadows.
  2. North America exhibited a strong greenhouse gas sink, attributed to its vast areas of salt marshes, mangroves, and seagrass ecosystems, along with fjords that absorb CO2.
  3. Africa showcased notable CO2 uptake by mangroves and seagrasses, although it was moderately offset by greenhouse gas emissions from estuaries.

Moderate sinks for greenhouse gases were observed in the following regions:

  1. South America demonstrated moderate CO2 uptake by coastal wetlands, primarily mangroves, along with some emissions from estuaries.
  2. Australasia encompassed extensive coastal wetlands with CO2 absorption capabilities. However, this region also featured a significant number of estuaries that served as sources of CO2, CH4, and N2O.
  3. West Asia displayed a weak source of greenhouse gases from estuaries, coupled with moderate CO2 uptake by coastal wetlands, predominantly seagrass meadows.

Weak sinks for greenhouse gases were observed in the following regions:

  1. East Asia and South Asia exhibited a moderate CO2 sink in coastal wetlands, which was largely offset by greenhouse gas emissions from estuaries.

Weak sources of greenhouse gases were identified in the following regions:

  1. Europe and Russia showcased higher coastal greenhouse gas emissions compared to their uptake. The presence of impacted tidal estuaries releasing greenhouse gases, coupled with a colder climate that limits the extent of coastal wetlands such as mangroves, contributed to this outcome.

These findings provide valuable insights into the variations in greenhouse gas fluxes in coastal regions worldwide, enabling a better understanding of their role in the global carbon cycle.

Source: Southern Cross University

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