The mangroves are there for a reason

Why are we destroying our last line of defence against natural calamities?

Coastal mangrove forests are resilient, tolerating salt, powerful waves and even rising seas. But new research suggests that when sea levels rise too quickly, the mangroves can drown. BALLLLAD/ISTOCK/GETTY IMAGES PLUS

Mangrove forests can only take so much. The famously resilient, salt-tolerant and twisty trees have so far managed to keep pace with rising sea levels, providing a valuable buffer to coastal communities against pounding storm surges. Now, researchers have found the forests’ limit.  Mangroves cannot survive in seas rising faster than about 7 millimeters per year, the scientists report in the June 5 Science.

Sea levels are rising globally at an average rate of about 3.4 millimeters per year, according to the Intergovernmental Panel on Climate Change (SN: 9/25/19). But over the next few decades, that rate is projected to accelerate to between 5 millimeters per year and 10p millimeters per year by 2100, scientists say.

That could drown the forests, which act as a buffer protecting many coastlines around the globe by reducing erosion from tides and dampening the energy of storm waves sweeping ashore. And mangroves come with additional boons, says Neil Saintilan, a biogeographer at Macquarie University in Sydney. They provide a safe nursery habitat for tropical fish and help reduce atmospheric levels of the climate-warming gas carbon dioxide.

Mangroves are carbon-sequestering engines, drawing carbon dioxide out of the atmosphere and swiftly burying it in soils. From about 8,600 to 6,000 years ago — a period of particularly rapid expansion for the mangroves — this coastal ocean–based “blue-carbon” storage by the mangrove forests amounted to about 85 petagrams of carbon, enough to lower atmospheric carbon dioxide levels at the time by about 5 parts per million, Saintilan and colleagues estimate. Currently, the average concentration of carbon dioxide in Earth’s atmosphere is about 417 ppm.

These valuable forests are typically resilient to changes in sea level, holding their ground by building up sediment amongst their tangled roots. Scientists have observed this in the modern era, Saintilan says, by recording how quickly sediment accumulates and the land surface elevation within the forests rises.

But those data span only a few years to perhaps a decade or two, he says. As a result, there have been two big unknowns: How long mangrove forests might be able to keep up this balancing act; and at what point the seas might simply rise too quickly for the trees, drowning the forests.

New mangroves form from slender seedlings called propagules, which drop from the trees into the shallow water and float until they take root in a new location. Coastal mangrove forests can buffer nearby communities from storm surges and accumulate new sediment around their roots, building up new land and sequestering carbon. N. SAINTILAN

How quickly the seas rise over the next century will depend on the rate of global warming, which causes seawater to expand and ice sheets to melt — and that, in turn, depends on rates of greenhouse gas emissions.

To understand how mangroves may respond to faster-rising seas, Saintilan and colleagues turned to the past. The peak of the most recent ice age was between about 26,000 and 20,000 years ago. After that, the ice sheets began to retreat as the world warmed, and sea levels began quickly rising, at rates faster than 12 millimeters per year.

Saintilan and colleagues focused on a time period between 10,000 and 7,000 years ago, as sea level rise began to slow and mangrove forests began to appear. The researchers examined previously published data on 78 organic carbon-rich sediment cores collected from coastal sites around the planet, and compared those with computer simulations of sea level rise rates for each site, to assess when the waters rose slowly enough for mangrove forests to grow.

The forests did not grow until the sea level rise had slowed to an average global rate of 6.1 millimeters per year, the team found. Today, under a scenario of high greenhouse gas emissions, sea level rise will accelerate to about 6 or 7 millimeters per year within the next 30 years. Even under mid-range scenarios that include cuts in greenhouse gases, the rate of rise will exceed that threshold by the end of the century, the researchers note. At that point, the mangrove forests sheltering many coastal communities will be unable to keep up, the researchers say.

Mangroves have always been like a mother to Bangladesh, protecting it from the periodic onslaught of natural calamities in the coast. One may recall the role played by the Sundarbans during cyclones like Sidr, Aila, and Bulbul. Even though these tropical cyclones caused significant damage to both lives and properties in the southwest, extensive devastation was prevented largely because the Sundarbans acted as a natural shield against them. With climate change increasing the likelihood of calamities, such mangroves have become extremely important. The question is, why are we destroying them then?

According to a report by this daily, far from doubling down on initiatives to save our mangroves or reforest battered coastal areas with trees, the authorities are allowing them to be ravaged under various pretexts. In Chattogram and Cox’s Bazar, for example, the government has reportedly leased out around 41,000 acres of mangrove forests over the last decade to establish economic zones, tourism parks, and energy structures. Besides, in Moheshkhali and Chakaria upazilas, around 12,500 acres of mangroves were grabbed by politically influential people for shrimp farming. However, the amount of forestland grabbed is higher than what’s shown in official data as encroachment continued unabated over the last two decades.

The account of how an entire forest known as “Chakaria Sundarbans” was wiped out from the south-eastern landscape – following a policy of promoting shrimp farming – makes for depressive reading. Equally depressing is the fact that the government built the Mirsarai Economic Zone over an area of 30,000 acres of mangroves, felling 5.2 million trees. More trees had to be felled for the eco-tourism park on Sonadia island. We can go on and on about how such destruction, with official seals or without, for public purposes or private, has been commonplace across the coastline. In districts bordering the Sundarbans, forest clearance is routinely heard of. In fact, forests are being ravaged everywhere. If you ask the relevant department, they will cite lack of resources to justify their failure to stop it. But what about official encroachment or destruction?

Who will stop government agencies from going back on the government’s own commitment to end deforestation – and increase forestland by 25 percent – by 2030? We must stop this trend. As climate change continues to heighten risks for Bangladesh, we must take bold action against any attempt to destroy our trees and forests, including the mangroves protecting the coast.

Source: Agencies

The views expressed in this article are the author’s own and do not necessarily reflect Coverpage’s editorial stance

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