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A research team led by Professor Derrick Lai Yuk-fo from the Department of Geography and Resource Management at The Chinese University of Hong Kong (CUHK) has identified subtropical brackish fishponds as one of the global ecosystems with the highest greenhouse gas (GHG) emissions, based on a two-and-a-half-year study. The team emphasises that protecting coastal wetlands, particularly mangroves, from conversion to aquaculture ponds could be an effective, nature-based climate solution. If the conversion could be completely halted by 2030, it could cumulatively reduce carbon emissions by 90.2 gigatons of carbon dioxide (CO₂) equivalent by the end of this century. The study has been published in the American Geophysical Union’s transdisciplinary journal Earth’s Future.

Significant greenhouse gas emissions from fishponds, high cost of mangrove conversion

Coastal wetlands such as mangroves, salt marshes and seagrass meadows are crucial “blue carbon” ecosystems, capable of long-term carbon sequestration and storage. However, approximately 25% to 50% of global coastal wetlands have disappeared over the past century due to development.

The subtropical brackish fishponds in northwestern Hong Kong, which this study looked at, were natural mangroves in the early 20^th century but were later converted for aquaculture. This study used the eddy covariance technique to measure the fluxes of CO₂, methane (CH₄) and water vapour from these fishponds continuously for over two and a half years. The results indicate that the brackish ponds emit 687 and 102 grams of carbon per square metre annually in the form of CO₂ and CH₄ respectively (approximately 8.6 kg of CO₂ equivalent), making them one of the top GHG emission hotspots among the world’s ecosystems. The figures rank in the top 0.1% and 0.5% in the global ecosystem-scale CO₂ and CH₄ flux databases, respectively (see figure 1).

The study further assessed the cumulative net carbon emissions impact under three conservation scenarios (see figure 2). If the current mangrove deforestation rate persists, the cumulative net carbon impact resulting from mangrove conversion to aquaculture would reach 109 gigatons of CO₂ equivalent by the end of the century. If mangrove deforestation stops by 2050, the cumulative net carbon impact by then will fall to 47.5 gigatons of CO₂ equivalent. But if mangrove deforestation halts by 2030, the cumulative net carbon impact by then would drop significantly to 18.8 gigatons of CO₂ equivalent, equating to a saving of up to US$16.7 trillion in economic losses.

The conversion of coastal wetlands to aquaculture not only releases the carbon stored in existing wetland vegetation and soil but also eliminates the future carbon sequestration potential of these natural wetland ecosystems, as well as incurring additional GHG emissions from the establishment and operation of fishponds. The study emphasises that preserving mangroves is among the most effective nature-based solutions to mitigate climate change.

“Protecting coastal wetlands is crucial not just for biodiversity but also for combating climate change,” said Professor Lai. “Our findings underscore the importance of immediate conservation action to halt mangrove deforestation.” The research team calls on policymakers and stakeholders to prioritise the protection of coastal wetlands and to consider the long-term environmental costs associated with converting these vital ecosystems into aquaculture ponds.

The full text of the published paper can be accessed at: https://doi.org/10.1029/2024EF005277