Coastal ecosystems are net sinks of greenhouse gases, new research shows

The new greenhouse gas budget shows that coastal ecosystems globally are the net sinks of the greenhouse gas for carbon dioxide (CO2).₂) but emissions of methane (CH₄) and nitrous oxide (N₂O) counteract some CO₂ uptake, according to international researchers led by Australia’s Southern Cross University.

New findings on coastal greenhouse gas (CO₂ + CH₄ +N₂O) in ten world regions and globally described in the paper, Coastal and estuarine vegetation collectively is a sink for greenhouse gases, published today in Natural Climate Change.

From tropical lagoons to polar fiords, from coastal mangroves to underwater seagrass communities, many coastlines around the world show great variation in greenhouse gas absorption and emissions.

“Understanding how and where greenhouse gases are released and absorbed in coastal ecosystems is an important first step for implementing effective climate mitigation strategies,” said lead researcher, Dr Judith Rosentreter, Senior Research Fellow at Southern Cross University.

“For example, protecting and restoring mangrove and salt marsh habitats is a promising strategy for amplifying CO₂ uptake by these coastal wetlands.”

Other activities to curb human impacts, such as reducing nutrient, organic matter, and wastewater inputs to coastal waterways, can reduce the amount of CH₄ and N₂O is released into the atmosphere.

The global team of scientists surveyed ten different world regions: North America, South America, Europe, Africa, Russia, West Asia, South Asia, East Asia, Southeast Asia, and Australasia (see Figure 1).

They found the strongest coastal greenhouse gas (GHG) sinks to be in Southeast Asia due to their large and productive tropical coastal wetlands that absorb CO₂. The second sinking hotspot is North America, with large areas of coastal wetlands but also CO2₂– Absorbing the fjords.

“Our new research shows that fjords around the world absorb ~40% CO2₂ which would otherwise be released from tidal, delta and lagoon systems. Most (86%) of this important CO₂ uptake by the fjord is from a region of North America, mostly Greenland,” said co-author Professor Bradley Eyre, Professor of Biogeochemistry at Southern Cross University.

Dr Rosentreter added: “Another coastal habitat is a source of greenhouse gases. For example, coastal wetlands such as mangroves, coastal salt marshes, and seagrass beds, release more than three times as much CH₄ than all the estuaries of the world.”

At the same time, coastal wetlands, also called ‘blue carbon’ coastal wetlands, can be strong CO2 sinks₂ and some also take N₂O, which, on balance, makes it a net GHG sink for the atmosphere when all three greenhouse gases are considered.

“In our new study, we show that when we consider all three greenhouse gases (CO₂ + CH₄ +N₂O), eight out of 10 world regions are coastal greenhouse gas sinks,” says Dr Rosentreter.

These findings will inform the Global Carbon Project’s RECCAP2 effort.

“This research was initiated by the Global Carbon Project to establish greenhouse gas budgets in large areas spanning the globe, and the contributions of these coastal ecosystems remain incalculable,” said co-author Pierre Regnier, Professor of Earth System Science at the Université Libre de Bruxelles.

Snapshot: coastal greenhouse gas sinks and sources around the world

Observational data set from 738 sites from studies published between 1975 and 2020 compiled to measure CO2₂CH₄and N₂O flux in estuarine and coastal vegetation in 10 global regions.

The unique coastal features (climate, hydrology, abundance) in each region around the world drive the uptake and/or release of GHGs from coastal systems.

strongest coastal greenhouse gas sinks:

• Above: the island region of Southeast Asia, due to its extensive and productive tropical mangrove forests and seagrass beds which absorb large amounts of CO2₂.
• Next: North America because of its vast areas of salt marshes, mangroves and seagrasses, but also CO2₂– Absorbing the fjords.
• Third: Africa with large CO₂ uptake by mangroves and seagrasses which is substantially reduced by estuarine GHG emissions.

Currently coastal greenhouse gas sinks:

• South America: Moderate CO₂ absorption by coastal wetlands, especially mangroves, and some estuarine GHG emissions.
• Australasia: long stretches of CO2-absorbing coastal wetlands₂however the area also has a large number of estuaries along its coast, many of which are sources of CO2₂CH₄ and N₂HI.
• West Asia: weak estuarine GHG source and moderate CO₂ absorption by coastal wetlands, mostly seagrasses.

Weak coastal greenhouse gas sinks:

• East Asia and South Asia: CO moderate coastal wetlands₂ sinks are largely reduced by estuarine GHG emissions.

Weak source of coastal greenhouse gases:

• Europe and Russia: both regions emit more coastal GHGs than they can remove from the atmosphere. This area has many tidal estuaries which release greenhouse gases; the cooler climate also means they have fewer coastal wetlands (eg mangroves) that would otherwise absorb large amounts of CO₂.