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What’s a swamp worth?

While the representatives of over 200 nations meet at Cancun — a Mexican beach resort borne out of destroyed mangrove forests — to take stock of the changing global climate, experts are reiterating that these wetlands can help cool the earth to a large extent. As the search for carbon sinks — natural bodies that can capture and store carbon linked to climate change — intensifies, mangroves are increasingly being seen as an ecological asset worth saving.

Closer home, a group of marine scientists has estimated the value of the Sundarbans, the world’s largest mangrove forest. The forest is worth around $79 billion if one considers just the carbon dioxide it sequesters or removes from the atmosphere, concludes a study by scientists at the University of Calcutta. “This, without taking into account the importance of these wetlands in preserving biodiversity, buffering for devastating cyclones and as nurseries for commercially important fish,” says Abhijit Mitra, the lead author. Accounting for these services would push the swamp’s value up several notches.

In 1997, a team led by environmental economist Robert Costanza — director of the Gund Institute for Ecological Economics at the University of Vermont in the US — estimated the value of the world’s mangroves at approximately $181 billion. “The figure, however, does not include the carbon value,” says Mitra. The ability of these forests to absorb carbon generated such huge interest only after the Kyoto protocol — an international agreement linked to the United Nations Framework Convention on Climate Change (UNFCC) aimed at fighting global warming — was adopted in late 1997. The Kyoto UNFCC meeting fixed a price for carbon spewed into the atmosphere as a way of motivating countries, businesses and individuals to reduce carbon emissions.

“Mangroves have been recognised as an important component of the clean development mechanism [an arrangement under the Kyoto protocol] to fight global warming,” says Atanu Kumar Raha, principal chief conservator of forests, West Bengal. “They act as better carbon sinks than terrestrial forests because they include a significant part of estuarine water bodies that help bloom phytoplanktons (minute free-floating aquatic plants), which are known to sponge up huge amounts of atmospheric carbon.”

So how did the scientists estimate the carbon-storing capacity of the forests? “We measured the biomass (amount of living matter in a particular area or volume) and carbon stock of three dominant species in the Sundarbans — Keora (Sonneratia apetala), Bain (Avicennia marina) and Geoan (Excoecaria agallocha) — using laser beams,” says Mitra.

The project was funded by the ministry of earth sciences, Government of India. It was found that 45-50 per cent of the biomass of these trees is made up of carbon. “Our data are comparable to the results of studies on mangroves in Indonesia, Sri Lanka and the Andaman and Nicobar Islands,” says Mitra.

The capacity for stored carbon content, however, is different for different species. For instance, Keora trees store 120 tonnes of carbon per hectare, while Geoans absorb as little as 25 tonnes. “We arrived at the dollar value using the price yardstick fixed by Carbon Neutral, a US-based firm that provides carbon reduction solutions,” says the Calcutta researcher. The forest density was taken as 26 trees per square metre and the average age of a tree 20 years.

The study also found that the central part of the Sundarbans is a poor carbon sink. “That’s because of the low growth rate of these species owing to siltation and too much salinity in the Bidyadhari canal,” says Mitra. The team had earlier worked on the salinity fluctuation of the Sundarbans with scientists at the University of Massachusetts. Its report was published in Current Science last year. High salinity has an adverse impact on fish and phytoplankton. Kakoli Banerjee, another team member, investigated the natural fish nurseries and found that the number of trash fish has risen considerably in the central region.

According to Raha, the study is significant in terms of understanding carbon sequestration through the Sundarban mangroves. “The results will enrich our database and help us form strategies for afforestation,” he says.

However, not everyone thinks highly of such an ecological valuation. “Such studies do not take into account other intangible benefits of mangroves,” says Sugata Hazra, director of the School of Oceanographic Studies, Jadavpur University. “For instance, the study does not consider the enormous medicinal value of certain species. Some of the plants have proven anti-cancer or painkiller properties,” he explains. Although protecting and replanting mangroves is a way out, Hazra says experts should focus on sea level rise, loss of habitat and recurrent storm surges in the Sundarbans. “Islands are going under the sea in a fast warming world. Developing nations like India should fight for a climate fund for environmentally displaced people in Cancun,” he says.

Developed nations, on their part, should reduce carbon emissions drastically rather than pin hopes on the mangroves, he suggests.

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