CUHK eNews Sep 2020

Methane a Culprit of Reducing Climatic Cooling Effects of Mangroves

Methane Shown to Reduce Climate Benefits of Subtropical Mangrove Wetlands

The research team collects data at Mai Po Nature Reserve to produce the world’s first-ever multi-year dataset of ecosystem-scale methane emissions from a subtropical estuarine mangrove.

An international collaborative study has found that sustained methane emissions from subtropical estuarine mangroves can reduce the mangroves’ climatic cooling effects by over 50%, over a period of 20 years.

The study, which involved the University of California, Berkeley and Stanford University, was led by Professor Derrick Yuk Fo Lai from the Department of Geography and Resource Management at CUHK.

Methane is a greenhouse gas over 30 times more powerful than carbon dioxide on a unit mass basis. While natural wetlands are known to be the single largest source of atmospheric methane globally, coastal wetlands are generally assumed to have negligible methane emissions owing to the dominance of high salinity conditions that suppress methane-producing microorganisms. Until Professor Lai’s study, there had been no long-term data on methane emissions from mangroves, especially in estuarine zones with lower salinity, to challenge this longstanding assumption.

The research team collected data at Mai Po Nature Reserve in Hong Kong to produce the world’s first-ever multi-year dataset of ecosystem-scale methane emissions from a subtropical estuarine mangrove. The data set was based on direct and continuous measurements over a three-year period using an eddy covariance system installed at the Mai Po Marshes in Hong Kong. (An eddy covariance system measures gas exchanges between land and the atmosphere.)

Professor Lai said, ‘We are the first ever team to operate an eddy covariance system at a subtropical mangrove to monitor large-scale methane emissions continuously throughout the year. As a result, the findings are a valuable reference for the study of the role of coastal wetlands in climate change.’


The Role of Coastal Wetlands in Climate Change

Vegetated coastal wetlands such as mangroves and salt marshes sequester more carbon than any other ecosystem. They exhibit high rates of photosynthetic carbon dioxide uptake and sediment trapping, leading to significant carbon storage in the ecosystem and hence reduced accumulation of carbon dioxide in the atmosphere. The potential of managing this ‘blue carbon’ for climate change mitigation has received increasing attention in recent years in the international community.

According to Professor Lai, ‘The magnitude of methane emissions from this estuarine mangrove is expected to increase in future owing to the combined effects of global warming, increased river discharge, and human induced eutrophication (the process in which a body of water becomes overly enriched with nutrients), which could further aggravate the greenhouse effect. It is crucial to consider the simultaneous exchange of carbon dioxide and methane between the mangrove and the atmosphere for an accurate assessment of the overall climatic impacts of subtropical mangroves.’

The findings were recently published in Global Change Biology, a leading journal in the field of biodiversity conservation and environmental science.


Read more: