A composite Landsat image of the Ganges delta taken in 1999 and 2000. The Sundarbans is deep green. Image: NASA.
On May 20, Cyclone Amphan made landfall over the coast of West Bengal, Odisha and Bangladesh. The first tropical storm of the 2020 cyclone season, Amphan devastated buildings, roads, livelihoods and lives in its path across the eastern part of the subcontinent.
Tropical cyclones are fast-blowing storms that have a low-pressure centre, strong winds and thunderstorms at the periphery. When they cross over land, these storms rapidly lose power. But as their low-pressure centre moves over land, they cause intense storm surges, inland flooding, high waves, wind gusts and very heavy rain.
Only a week after the India Meteorological Department (IMD) had reported the formation of a low-pressure region over the Bay of Bengal, Amphan had intensified into a super-cyclonic storm – the first since 1999 – and struck Bakkhali, in West Bengal, with 155 km/hr winds.
Bakkhali is in the Sundarbans delta. Here, the rivers Ganga and Brahmaputra join and empty into the Bay of Bengal, depositing Himalayan silt and sediment in this zone. The Sundarbans is a UNESCO world heritage site and a critical wetland. More importantly, it is the largest intact tract of mangrove swamp forest in the world and is very important as a wildlife habitat. And perhaps most importantly in the context of cyclones, it acts as a dampener.
When Amphan struck the Ganges delta, it flooded coastal communities and pushed water up to 15 km inland in parts of the Sundarbans. IMD recorded winds of 150-160 km/hr. However, the cyclone wrought much less damage because of the delta’s mangroves.
Mangroves are swamp forests. They provide many ecosystem services to coastal communities. The density of their trees, together with the variety of tree species, attenuate the inflow of water and create a sort of buffer zone against floods and storm surges.
A 2013 study of mangroves in Florida estimated that a mangrove forest could reduce the effects of a Category 5 storm to the intensity and effects of a Category 3 storm. Mangroves are also more effective when the storm is more violent. A series of studies in the early 2000s discovered that mangroves with an average height of 6-10 metres could shorten a cyclone’s waves by 60%. Their roots form a complex interweave and jut above the soil. Together with the trees’ trunks, they work like speed-breakers to slow the tides.
Sadly, deforestation, land-use change and human activities – including aquaculture and tourism – have reduced the extent and health of the Sundarbans’ mangroves. Experts already think this deterioration played a part in Amphan’s effects being as deadly as they were.
The cyclone has directly affected over 10,000 people in Bangladesh. In India, 70% of West Bengal’s population has reportedly been affected. Mamata Banerjee, the state’s chief minister, has said the government has incurred a cost of Rs 1 lakh crore as a result.
Climate change is expected to render bad storms worse in the future, and more frequently so, as well as elevate the sea level. And as the Sundarbans, and in fact mangroves across India, continue to shrink in the face of infrastructure development, urbanisation and agricultural land conversion, the country’s coastal and near-coastal communities will only fare worse.
Sea-level rise in particular poses a grave threat. Mangroves need freshwater to maintain the estuarine conditions they thrive in. But as sea-levels rise, the swamps become more saline, prompting some tree species to move further inland, which in turn affects their roots. The increasing prevalence of barrages, dams and upstream diversions only makes this problem worse. Dams and barrages also reduce the amount of sediments reaching the mangroves, contributing to coastal erosion.
Preserving what mangrove tracts we already have can dramatically reduce the damage to our coasts due to tropical storms. And for its lavish legal protections, Sundarbans continues to be threatened by both human and natural causes; the former is of course more immediate.
If the state can rope in coastal communities to manage and help conserve mangroves, these coastal forests will repay the investment manyfold. In 2017, researchers estimated that 23% of India’s fish catch already comes from mangrove aquaculture. Such tropical inland fisheries are a $5.38 billion a year business around the world, and in India employ over 55 lakh.
Priya Ranganathan is a landscape ecologist and geologist working with the Ashoka Trust for Research in Ecology and the Environment, Bengaluru, on wetland conservation and ecosystem services.