Protecting the world from the adverse effects 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.

With 2118 sq km of total mangrove forest cover, the Indian Sunderbans have soaked in 4.15 crore tonnes of carbon dioxide, valued at around USD 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," said Prof Abhijit Mitra, who led the research.

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.