Protecting the world from the adverse affects of climate change, the Sunderbans forests play a crucial ecological role by acting as a carbon sink and absorbing more than four crore tonnes of carbon dioxide from the atmosphere, according to a recent study.

Having 2118 sq km of total mangrove forest cover, the Indian Sunderbans have soaked in 4.15 crore tonnes of carbon dioxide, valued at around $79 billion in the international market, researchers from the University of Calcutta said.

“Mangrove trees act as a natural tank for carbon dioxide storage. They absorb carbon for their own needs, which is a boon for us. The more such biomass we have on earth, the more CO2 will be pulled from the atmosphere. This will ultimately result in controlling the rise of atmospheric temperature and the subsequent climate change,” Prof Abhijit Mitra, who led the research, told PTI.

This process of removing carbon from the atmosphere and depositing it in a reservoir is known as carbon sequestration.

As a primary greenhouse gas, large-scale CO2 emission is responsible for global warming as it leads to a rise in sea levels and temperature, adversely affecting agriculture, fishery and human health.

With funding from the Union Ministry of Earth Science and the state forest department, the two-year-long study of the carbon sequestration efficiency of the mangroves was done by the varsity’s marine science department.

Out of the total amount of carbon tied up in earthbound forms, an estimated 90 per cent is contained in the world’s forests. For each cubic foot of merchantable wood produced in a tree, it has been estimated that about 15 kg of carbon is stored in total tree biomass.

To evaluate carbon stocks in the above-ground biomass (AGB) of three dominant mangrove species (’Sonneratia apetala’, ‘Avicennia alba’ and ‘Excoecaria agallocha’) in the Sunderbans, carbon content in stem, branch and leaf biomass was estimated using laser beams by the team of ecologists.

The estimates done in the study, however, exclude the below-ground biomass found under the soil.

Atanu Raha, the state’s principal chief conservator of forests, pointed out that the results are positive as there has been no degradation of forest cover in the Sunderbans.

“In the core forest area, there has been no degradation due to human intervention. Only few forest trees have been lost due to natural reasons beyond our control,” he said.

The unique biological productivity, taxonomic diversity and aesthetic beauty of the Indian Sunderbans has been recognised with the crowns of World Heritage Site and Biosphere Reserve in 1987 and 1989 respectively by the UNESCO.

The study also found that the central part of the Sunderbans is a poor carbon sink as compared with the western part of the delta.

“The fresh water of Himalayan glaciers fails to reach the central part due to heavy siltation and clogging of the Bidyadhari channel. This has affected the growth of productive mangrove vegetation,” Mitra said.

Effective soil management, tidal interactions (through artificial canalisation) and sufficient flow of freshwater into the mangroves can improve the biomass production of mangrove species, the study suggested.

If the social forestry project is taken up extensively in the Sunderbans, then it might even help the government to earn carbon credit points and sell them for cash using the carbon emission trading system under the United Nations Framework Convention on Climate Change (UNFCCC).

“Himachal Pradesh has already done it. In the international market one tonne of carbon is valued at USD 19.

So the Sunderbans can be valued at around USD 79 billion in terms of the amount of carbon dioxide it sequesters,” said Mitra.

The carbon trading system has been recognised by the UN as a method to reduce greenhouse gas emissions by giving it a monetary value.

Credits, that can be bought or sold in the market at the prevailing price, gives the owner the right to emit carbon dioxide after paying for it.