95% of Mumbai’s mangroves could perish from rising sea levels: State
Permanent submergence could drown Avicennia marina tree roots; experts suggest more resilient speciesUpdated: Nov 16, 2019 23:48 IST
Most mangrove species in Mumbai’s wetlands will not survive rising sea levels, the Maharashtra forest department has said. The department’s warning comes on the heels of a US study which predicted that large parts of the island city could be inundated in around 30 years.
The grey mangrove (Avicennia marina), which makes up 95% of Mumbai’s mangrove trees, has aerial breathing roots that rise 15cm to 20cm over the soil. The forest department said that rising sea levels could permanently drown the roots, killing the trees.
“The present mangroves species distribution (breathing roots of Avicennia marina) will be under permanent submergence due to the height of its roots, against the current predicted sea-level rise,” said N Vasudevan, additional principal chief conservator of forest (mangrove cell). “The examples of this are already being witnessed at Thane creek where Avicennia marina trees are getting submerged during high tide and dying. In isolated cases, other species with higher roots like Sonneratia alba or Sonneratia apetala [roots ranging between 30-80cm] are taking over. The grey mangrove’s dominance is a boon and a bane as the species is resilient to high salinity and pollution but will not survive rising water levels.”
According to the United Nations’ Intergovernmental Panel on Climate Change (IPCC), global sea levels have risen by around 15cm during the 20th century and is currently rising more than twice as fast at 3.6mm per year. It will reach around 30-60cm by 2100, even if greenhouse gas emissions are sharply reduced and global warming is limited to well below 2 degrees Celsius.
Last month, a study by US-based researchers used satellite maps to issue an alert for coastal cities like Mumbai. According to their predictions, almost entire south Mumbai, large portions of the western and eastern suburbs, coastal areas along Navi Mumbai, Thane, entire Vasai-Virar and Mira Bhayander in the Mumbai Metropolitan Region (MMR) will be inundated by 2050.
Experts confirmed the forest department’s prediction. “Existing mitigation plans for coastal cities are not up to the mark to address the latest alarming predictions, and Avicennia marina is not resilient enough to sustain the impact of sea-level rise. Mangrove cell needs to plant more resilient species at mudflats behind existing plantations, to build a second line of defence,” said E Vivekanandan, former principal scientist at Central Marine Fisheries Research Institute (CMFRI).
Mumbai has the largest mangrove cover (6,600 hectare) for any urban area in the world. Mangrove forests are vital in reducing the effects of sea erosion and cyclones. “Mangroves have the potential to cope with the sea-level rise through vertical accretion [growth through the gradual accumulation of silt]. However, an increase in flooding duration for trees with small breathing roots can lead to their death at the seaward margins and will result in shifts in species composition. This will reduce productivity and ecosystem services,” said P Ragavan, mangrove ecologist with Mangrove Foundation of Maharashtra, an autonomous society that assists the state government in marine conservation. “We need location-specific information for Mumbai, which is imperative to understand the vulnerability of mangroves against sea-level rise.”
Vasudevan said mitigation measures need to be looked at urgently. “We need to plant and expand mangrove species that are resilient and will withstand rising water levels. It has to be planned through genetic alterations for species which will sustain rising water, salinity, turbidity, and overall coastal pollution,” he said.
“Maharashtra needs to explore the fishbone channel plantations (see box) that have been successful in addressing rising coastal waters in Gujarat and Tamil Nadu. Under this technique, channels are dug similar to a fishbone structure leaving adequate space for slightly elevated plantations (trees with taller roots) at a gap of 20 m from each other on mudflats. On one hand, excess water will enter these channels and the trees will cope with rising water,” said V Sundararaju, retired Indian Foreign Service (IFS) officer and president, Society for Conservation of Nature, Trichy, Tamil Nadu.