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A study by The Chinese University of Hong Kong (CUHK) found that mangroves and other marine wetlands stored 23% more carbon from the atmosphere than previously estimated, which further established the importance of “Blue Carbon” and its contribution to countering carbon emission. This article has been published in the prestigious scientific journal Nature Communications. (https://www.nature.com/articles/s41467-019-14120-2)

“Blue Carbon” Critical to Mitigating the Harmful Effects of Anthropogenic Carbon Emission

Mangroves and other marine wetlands such as saltmarshes and seagrass beds form coastal ecosystems to capture and store carbon dioxide. Scientists refer to them as “Blue Carbon”, in contrast to “Green Carbon” on land such as forests and grasslands. “Blue Carbon” stores carbon in the sediment and the decomposition rate of the accumulated carbon is much lower, meaning less carbon dioxide is released back into air from the carbon pool. Less carbon dioxide in the atmosphere will result in less greenhouse effect, thus a cooler Earth.

Correcting the Bias on Estimating Carbon Density in Tidal Wetlands

Previous studies have suggested that “Blue Carbon” holds 3-5 times higher a carbon density than “Green Carbon”. The estimation relies on factors from other habitats forming sediment organic carbon from organic matter, which is approximated by the “loss-on-ignition” value obtained after heating the sample for several hours. Upon heating, organic matter is released as gases from the sediment sample. The weight difference before and after (the “loss-on-ignition” value) provides a proxy for the organic carbon present, without reference to potential variability associated with specific sediment types.

Professor Shing Yip LEE, Director of Simon F S Li Marine Science Laboratory, and his post-doctoral fellow Dr. Xiaoguang OUYANG at CUHK have combined data from past studies and new field measurements of sediments in tidal wetlands in different countries. Through comparison and analysis as well as inclusion of previously neglected components (e.g. dead wood), the team revealed that carbon stocks in mangrove forests reach 3.7-6.3 Pg (one Pg = 10¹⁵ g or one billion metric tonnes), which shows a previous underestimate of 23%.

Impact of Mangrove Deforestation on Carbon Emissions

Mangrove forests are among the most threatened habitats on Earth. Despite some recent signs of improvement, urban development, coastal aquaculture and agriculture have continued to drive mangrove deforestation, particularly in Southeast Asia. Mangrove deforestation leads to the clearance of biomass and loss of stored carbon from sediments. By combining the extent of global mangrove area estimated by different studies, global mangrove loss rate, and carbon stock in biomass and sediments, the team estimated the annual carbon dioxide emission rate from mangrove deforestation is at 23.5 – 38.7 Tg after Year 2000 (one Tg = 10¹² g or one million metric tonnes). It is equivalent to the amount emitted by 4.5-7.5 million cars circumnavigating the Earth’s equator. Prudent conservation of tidal wetland resources is therefore crucial to managing the harmful effects of future climate change, such as warming and sea level rise.

About Professor Shing Yip LEE

Prof. Lee’s research team focuses on the ecosystem ecology of coastal wetlands, including carbon cycling and conservation. Prof. Lee is also the Chair of the Mangrove Specialist Group of the International Union for Conservation of Nature (IUCN), the world’s largest conservation organisation. The group aims to assess the conservation status of mangroves, identify, quantify and prioritise threats, and develop plans to conserve the most threatened species and habitats.

Dr. Xiaoguang OUYANG studying carbon cycling in Mai Po Marshes:
https://drive.google.com/file/d/1sFnX9bAicGYMtqhYe4gWpQNKUrtfj2GV/view?usp=sharing