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June 2, 2023

Coastal ecosystems key GHG sinks

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Salt marsh mangrove on Assateague Island, USA. Photo Sara Cottle

The ability of coastal ecosystems to absorb and release greenhouse gas emissions (GHG) as part of the world’s GHG budget has demonstrated that the majority are a net greenhouse gas sink. However, emissions of methane (CH4) and nitrous oxide (N2O) counteract some of the carbon dioxide (CO2) uptake, according to international researchers led by Australia’s Southern Cross University.

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

These new findings of the coastal greenhouse gas balance (CO2 + CH4 + N2O) in ten regions around the world are outlined in the paper, Coastal vegetation and estuaries collectively are a greenhouse gas sink, published recently in Nature Climate Change.

Estuary at Huskisson, Australia. Photo Chris King

Australasia moderate

The strongest coastal greenhouse gas sinks identified were the archipelagic region of Southeast Asia, because of its extensive and productive tropical mangrove forests and seagrasses; North America because of its large areas of salt marshes, mangroves, and seagrasses, but also CO2-uptaking fjords; and Africa with large CO2 uptake by mangroves and seagrasses that is moderately reduced by estuarine GHG emissions.

Fjord at Trolltunga, Norway. Photo Dong Zhang

‘Canada and Greenland have huge areas of really pristine, natural fjords systems,’ said lead researcher, Dr Judith Rosentreter, Senior Research Fellow at Southern Cross University.

‘They act very much like the ocean in the way that they absorb greenhouse gases. But we don’t have as many in Europe and Russia.’ 

Australasia rated as a moderate coastal greenhouse gas sink with 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.

Europe and Russia are the two regions that release more coastal GHG than they can take up from the atmosphere. 

‘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. At the same time 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 GHG sink for the atmosphere when all three greenhouse gases are considered,’ said Dr Rosentreter.

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

Global GHG budget in ten regions. Image Rosentreter et al

‘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 Dr Rosentreter.

Dr Rosentreter said that it is often human activities that contribute to negative impacts on these systems, for example nutrients and waste water inputs into coastal waterways can lead to the death of plants such as mangroves etc and the decline of these systems. 

‘Curbing human impact can reduce the amount of CH4 and N2O released to the atmosphere,’ she explained to The Echo

‘For example, protecting and restoring mangrove and salt marsh habitats is a promising strategy to strengthen the CO2 uptake by these coastal wetlands.’

The findings will inform the efforts of the Global Carbon Project’s RECCAP2.

‘The research was initiated by the Global Carbon Project to establish greenhouse gas budgets of large regions covering the entire globe, and for which the contribution of these coastal ecosystems remained unaccounted for,’said co-author Pierre Regnier, Professor of Earth System Science at Université Libre de Bruxelles.

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