Study examines coastal ecosystems globally as potential greenhouse gas sinks

Researchers have showed that coastal ecosystems globally are a net greenhouse gas sink for carbon dioxide (CO2), but emissions of methane (CH4) and nitrous oxide (N2O) counteract some of the CO2 consumption by these sinks.

From tropical lagoons to polar fjords, from coastal mangrove forests to underwater seagrass communities, many coastlines around the world show high diversity in greenhouse gas sinks and emissions.

The international team of researchers, led by Australia’s Southern Cross University, said that understanding how and where greenhouse gases are released and absorbed in coastal ecosystems was an important first step to implement effective climate mitigation strategies.

“For example, protecting and restoring mangrove and salt marsh habitats is a promising strategy to strengthen the CO2 uptake by these coastal wetlands,” said lead researcher Judith Rosentreter, Senior Research Fellow at Southern Cross University.

Other activities to curb human impact, like reducing nutrients, organic matter, and wastewater inputs into coastal waterways, could reduce the amount of CH4 and N2O released to the atmosphere, the researchers said.

The findings of the coastal greenhouse gas balance (CO2 + CH4 + N2O) in ten world regions and globally are outlined in a paper published in the journal Nature Climate Change.

The researchers looked at ten different world regions: North America, South America, Europe, Africa, Russia, West Asia, South Asia, East Asia, Southeast Asia and Australasia.

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

They found the strongest coastal greenhouse gas (GHG) sink was in Southeast Asia because of its extensive and productive tropical coastal wetlands that take up CO2.

A second sink hotspot was found to be North America, with its large areas of coastal wetlands but also CO2-uptaking fjords. A fjord is a long narrow inlet of the sea between steep cliffs.

The research also showed that fjords around the world take up about 40 per cent of CO2 that would otherwise be released from tidal systems, deltas and lagoons.

“Most (86 per cent) of this important CO2 uptake by fjords comes from the North America region, mostly Greenland,” said co-author Bradley Eyre, professor of Biogeochemistry at Southern Cross University.

“Other coastal habitats are sources of greenhouse gases. For example, coastal wetlands such as mangrove forests, coastal salt marshes and seagrasses, release more than three-times more CH4 than all estuaries in the world,” said Rosentreter.

At the same time, the researchers said, coastal wetlands, also called coastal ‘blue carbon’ wetlands, can be strong sinks of CO2 and some also take up N2O, which, on balance, makes them a net greenhouse gas sink for the atmosphere when all three greenhouse gases are considered.