Mangrove Forests

The importance of mangroves to protect India’s coast from the adverse effects of climate change

Print edition : July 30, 2021

Some 730 acres of mangroves on the Mithi river were filled in to create the Bandra-Kurla complex in Mumbai. Photo: Vivek Bendre

Garbage dumped around mangroves in the western suburbs of Mumbai, a 2019 picture. Photo: PAUL NORONHA

A home built next to the sea in Mumbai, a 2019 picture. With a population of about 20 million people, Mumbai lies at less than 15 m above sea level. Parts of the city are below or at mean sea level. Photo: PUNIT PARANJPE/AFP

The recent cyclones on the west coast have highlighted the increased climate change–related risks that India is facing and made it clear how essential it is to protect mangroves and other coastal features that act as buffers against storm surges, high winds and flooding.

THE occurrence of two cyclones in consecutive seasons—Cyclone Nisarga in 2020 and cyclone Tauktae in 2021—is highly unusual for the west coast of India but fits in with the warnings of experts. The India Meteorology Department has indicated that rising sea temperatures could mean that the west coast will see annual cyclones. The Indian Institute of Tropical Meteorology has said that the surface temperature of the Arabian Sea has risen by 1.2 to 1.4 °C over the past 40 years. This, it says, can increase wind speeds by 5 per cent and result in cyclones and storm surges.

For reasons not yet fully known, the Indian Ocean is warming faster than other seas. The Arabian Sea, where the average temperature so far has been 24 to 25 °C, saw a dramatic rise during Tauktae’s passing. When the cyclone passed Goa the sea temperature was measured at 30 to 31 °C, which increased the wind speed from 80 to 100 kilometres/hour and it further picked up speed to 175 kmph as it rammed past Mumbai. Classified as “extremely severe”, Tauktae at its peak had a one-minute sustained speed of 220 kmph.

Waves as tall as four metres were generated as it swept past Ratnagiri even though it was some 100 km out at sea. The damage from both cyclones has been well documented, but what has been glossed over is the protective role of mangroves during extreme maritime weather.

How does a mangrove forest protect the coast from a cyclonic onslaught? Mangrove trees live on the boundary between land and sea. They have laterally spreading roots with attached vertical roots that anchor them. The whole plant from the roots to the branches and the canopy absorbs the shock of storm surges. They act as barriers to flood waves through factors such as bottom friction, tree density and the overall width of the mangrove forest. The roots of mangrove trees retain sediments and stabilise the soil of the intertidal region. Many studies have shown that up to 60 per cent of the wave force is dissipated by the first 100 m of mangroves along a coast.

Also read: A breather for mangroves

A 2020 study published in Proceedings of the National Academy of Sciences of the United States of America entitled “Mangroves shelter coastal economic activity from cyclones” by Jacob P. Hochard, Stuart Hamilton and Edward B. Barbier had significant findings. Using spatially referenced data and statistical methods from 2000 to 2012, the authors tracked “the impact of cyclones on economic activity in coastal regions inhabited by nearly 2,000 tropical and subtropical communities across 23 major mangrove-holding countries [including India]”.

Their findings show that “direct cyclone exposure—i.e., within 100 km of the cyclone’s ‘eye’—has a permanent impact on long-run economic outcomes… in the absence of natural protections that buffer winds or reduce stormwater inundation. Five years following exposure, point estimates show a 10% to 22% loss in economic activity when compared with pre-cyclone trends. However, for those coastal communities at high elevation with wide mangroves, we estimate a statistically insignificant point effect of -0.0459. Whereas the cumulative effect of exposure on ‘unprotected’ communities continues to worsen in the fifth year, results suggest that cyclone exposure disrupts economic activity in coastal communities in the year of impact and in subsequent years following exposure. Higher elevation and wide mangroves, together, buffer this initial impact of storm exposure and enable communities to return to pre-exposure growth rates quickly thus avoiding long-term impacts on economic activity.”

Despite their proven benefits, mangroves continue to be wilfully destroyed by official sanction for so-called development. The Indian National Centre for Ocean Information Services in Hyderabad found that the sea levels around India are rising at around 1.6 to 1.7 mm every year. Mumbai, with a population of about 20 million people, lies at less than 15 m above sea level. Parts of the city are below or at mean sea level, making the city’s population vulnerable to climate change–related dangers.

One way to mitigate the risks is to nurture mangroves, natural wetlands and other coastal vegetation and natural rocky shores and sandy beaches. These are all armour for the coast. A paper published in Nature on March 10, 2020, titled “The Global Flood Protection Benefits of Mangroves” by Pelayo Menéndez, Iñigo J. Losada, Saul Torres-Ortega, Siddharth Narayan and Michael W. Beck says that mangroves provide significant protection “from cyclones and the more regular (non-cyclonic) high wave and swell events. However, cyclonic events are when damages are the greatest and mangroves offer the greatest benefit. With climate change the intensity and frequency of the largest events are likely to increase and thus the role of mangroves will therefore be even more relevant in future scenarios.” The paper says “countries such as Mexico, India and Vietnam” draw cyclone-protection benefits from mangroves.

Abundance of evidence

This is just one paper that has researched mangroves. There is an abundance of such hard evidence, but Mumbai’s treatment of its mangroves goes against all the scientific proof of their benefits. The island city had enormous mangrove forests until three decades ago. In Mumbai, development has always been linked to reclamation, and this automatically spelt doom for mangroves. Towards the southern tip of the city, what used to be a huge mangrove forest is now a bus depot and a sprawling illegal slum. The forest was destroyed before the Coastal Regulation Zone (CRZ) laws came into existence in 1991 and at a time when trucks full of construction rubble would line up at night and brazenly dump their loads into the waters of the intertidal region. In this way Mumbai’s topography expanded into the sea.

After the CRZ laws were passed, mangroves continued to be killed but in a more insidious fashion. The inlets that bring in seawater, which is essential for the survival of mangroves, are blocked so that the plants wither and die. Then the dead mangroves are sold as firewood and the rubble dumping begins. In this way the city lost about 9,000 acres (one acre is 0.4 hectare) of mangrove between 1991 and 2001.

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Infrastructure projects are a big threat to mangroves. To construct an overpass along a long stretch of Mumbai’s eastern seaboard in 2019, about 8,000 mangrove trees were killed by depriving them of seawater. When the work for Mumbai’s new international airport started in Navi Mumbai, satellite images showed that hundreds of acres of mangroves were destroyed. The bullet train project, which will run between Mumbai and Ahmedabad, will claim at least 35,000 mangrove trees. More recently, the Mumbai Metropolitan Region Development Authority (MMRDA) is all set to clear-cut a one-hectare plot of mangroves to build an elevated station for the metro rail service.

In 2015, the Bombay High Court declared mangroves as protected forests, and soon after that the State government implemented a mangrove conservation plan to be carried out on government land. One happy outcome of this has been the notification of 1,036 ha of mangroves on government land as reserved forest and the handing of the same to the Forest Department on June 9. This way mangrove land held by authorities such as the Jawaharlal Nehru Port Trust and the MMRDA will finally get transferred to the Forest Department. The non-governmental organisation Vanashakti is fighting for this. More than 3,000 ha of mangrove forest is supposed to have already been transferred to the Forest Department under orders from the Chief Conservator of Forests but this has not been done. Vanashakti has filed a public interest litigation petition seeking orders for the restoration of all mangrove forests destroyed illegally since 2005. Satellite imagery will be used to establish the before-after scenario.

In June, the Mangrove Cell of the Forest Department released a glossy publication on mangrove conservation called “Bio-Sentinels of Maharashtra”. The Mangrove Cell was established in 2012, and a year later a special unit called the Mumbai Mangrove Conservation Unit was created and headed by a Divisional Forest Officer. Ecotourism involving local people and creating sustainable livelihoods are some of the conservation efforts of the cell, but the point is that the nemesis of the mangrove forest is politician-supported development and on this the cell is quiet.

There was a well-meaning plan to replant mangrove trees, but there is a strong reason why this will not work: developers and mangroves both vie for the coast, and it is well established that developers will win the game. All the above-mentioned infrastructure projects are ones that have been officially sanctioned but without a thought being given to the fact that Mumbai is on the red list of climate change–affected cities. A 2019 report entitled “Flooded Future: Global vulnerability to sea level worse than previously understood” brought out by the American research institute Climate Central predicts that “many of the world’s coastlines are far lower than has been generally known and that sea level rise could affect hundreds of millions of more people in the coming decades than previously understood”.

The report says that Asia is particularly vulnerable because of the vast numbers of people who live along the coast. It goes on to say: “Based on sea level projections for 2050, land currently home to 300 million people will fall below the elevation of an average annual coastal flood. By 2100, land now home to 200 million people could sit permanently below the high tide line.” The report specifically mentions Kolkata as vulnerable in the Indian context, but environmentalists say that all coastal cities, including Mumbai, are at risk.

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A June 2021 working paper of the Overseas Development Institute (ODI) written by Angela Picciariello, Sarah Colenbrander, Amir Bazaz and Rathin Roy entitled “The costs of climate change in India: a review of the climate-related risks facing India, and their economic and social costs” further corroborates this: “Looking towards the end of the century, global sea levels are projected to rise by at least 44-74 cm relative to the mid-1990s, excluding the risk of ice-sheet collapse. Sea levels along the Indian coast are not forecast to rise quite as much as the average, increasing by 20–30 cm compared with current levels (Swapna et al., 2020). Even so, this will have a severe impact on infrastructure and property, particularly in low-lying and densely settled cities such as Mumbai, Chennai and Kolkata. It will also affect low-income rural communities that depend on coastal ecosystems for food and livelihoods, as the disappearance of coral reefs, degradation of mangroves and saline intrusion into the water table affect the productivity of agricultural land and natural ecosystems.”

Flooding is endemic to the low-lying areas in Mumbai and now with storm surges and cyclones the whole city is vulnerable. Mangroves are proven buffers against storm surge flooding. The ODI paper quoted a 2013 study in which Mumbai was ranked fifth in the world for flood-related losses. It quoted the catastrophic floods of 2005 in the city that killed 5,000 people and caused economic losses of $690 million. It says: “Floods will only get worse when combined with the heavier rains, higher sea levels and more severe storms associated with climate change.” With the risks of climate change increasing, saving Mumbai’s mangroves becomes more important than ever.

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