CHENNAI: Tamil Nadu’s mangrove forests collectively store 8.73 lakh tonnes of carbon stock, making them a crucial asset in combating climate change through carbon sequestration. According to a recent study by the Centre for Climate Change and Disaster Management (CCCDM) at Anna University, submitted to the Tamil Nadu Biodiversity Conservation and Greening Project for Climate Change Response (TBGPCCR), the state’s mangrove cover expanded from 4,500 hectares in 2021 to 9,039 hectares in 2024. This increase has been driven by both natural regeneration and afforestation efforts. New plantations account for 40.1% of the total mangrove cover established over the past decade.
The study used advanced techniques like remote sensing and field measurements to map mangrove areas and calculate their carbon storage capacity across 13 coastal districts. Thiruvarur leads with 2,142 hectares, followed closely by Thanjavur at 2,063 hectares, together making up nearly half of the state’s mangrove area. Cuddalore (1,117 hectares), Nagapattinam (1,021 hectares), and Ramanathapuram (712 hectares) also contribute significantly, while smaller patches exist in places like Chengalpattu (11 hectares) and Kanniyakumari (32 hectares).
When it comes to carbon storage, Cuddalore stands out, locking away 249 tonnes of carbon per hectare, thanks to dense tree cover and rich soils. Thiruvarur follows with 145 tonnes per hectare, and Thanjavur stores 77.5 tons per hectare. These figures come from detailed calculations of above-ground and below-ground biomass, using an equation that factors in tree diameter, height, and wood density, combined with soil carbon measurements. Smaller or less dense mangrove areas, like Villupuram (2.59 tonnes per hectare) and Tiruvallur (13.1 tonnes per hectare), store less carbon, often due to lower tree numbers or invasive species like Prosopis juliflora. Nagapattinam and Ramanathapuram hold 39 and 65.1 tonnes per hectare, respectively, showing moderate but meaningful contributions.
Soil analysis further reveals the mangroves’ carbon-storing power. Samples from 172 locations across the state reveal that Cuddalore’s soil organic carbon (SOC) reaches up to 1.3%, while Thiruvarur and Thanjavur range from 0.54% to 0.96%.
Districts like Tiruvallur and Villupuram, however, show SOC levels below 0.1%, indicating less fertile soils. Total Dissolved Solids (TDS), a measure of salinity, vary widely from 222 mg/L to 564 mg/L, with higher levels in Cuddalore and Nagapattinam reflecting their coastal conditions. The Muthupettai mangrove forest, spanning Thiruvarur and Thanjavur, offers a closer look, with SOC between 0.24% and 0.98% and water salinity from 1.3% to 31%, conditions that support species like Avicennia marina.
Chief Minister M K Stalin has announced major funding for the restoration and increasing the mangrove cover in the state. Mangroves not only sequestrate carbon, but also act as bio-shield against extreme weather events. In the last three years, Thiruvarur forest division has taken-up mangrove restoration works in over 2,000 hectares.
Despite their promise, challenges remain. The invasive Prosopis juliflora competes with native mangroves in several districts, stunting growth and reducing carbon storage. In Nagapattinam, efforts by local communities to block seawater flow to protect groundwater have unintentionally harmed mangroves, which need saline environments to thrive.
Past events like Cyclone Gaja in 2018 also left scars, causing dieback in Muthupettai. Yet, the study points to solutions: community-led efforts through 252 Village Marine Councils and Eco-Development Committees in Ramanathapuram and Thoothukudi, along with restoration projects like planting Rhizophora saplings in Thoothukudi, show progress. The report recommends expanding these initiatives, enforcing anti-encroachment laws, and promoting sustainable livelihoods like eco-tourism and carbon credit programmes.
Interestingly, the mangrove cover in Chennai and Chengalpattu were not taken into consideration for carbon stock assessment. Chennai has 141 hectares of mangroves and Chengalpattu 11 hectares.
Kurian Joseph, director of CCCDM, told TNIE the carbon holding capacity of mangroves in Chennai and Chengalpattu were ‘not significant’. This would be because of poor soil quality, degradation etc.
“We have assessed the soil organic carbon for the top 30 cm of soil. We are now analysing the soil organic carbon below 30 cm. So, the cumulative carbon stock in the Tamil Nadu mangroves will further go up,” Kurian Joseph said.
Supriya Sahu, Additional Chief Secretary, Environment, Climate Change and Forest Department, told TNIE there is still a whole amount of work to be done. “We are in the process of accessing carbon sequestration potential in the state. There are 4-5 sources and mangroves is one of them. The Thoothukudi based Suganthi Devadason Marine Research Institute (SDMRI) had done a similar assessment for seagrass. The Blue Carbon Agency, which we established, will look into all these assessments and prepare proposal for obtaining carbon credits.”