A warmer globe~I

The Statesman, 04 Nov 2014

KISOR CHAUDHURI

It was almost time for the ‘bore tide’ to hit the small river island, Chamta, located deep inside the core area in the Indian part of Sunderban Tiger Reserve. The local administration had warned of the possibility of a three-metre high wave that could pass through the channels in this delta where the Ganga meets the Bay of Bengal. Till 30 years ago, such high tides, locally known as sarasari gon, used to roll upstream of the Hooghly river with a distinct roar for almost 90 km and on their way, they crossed Kolkata. Such upsweeps, that are not often repeated these days, brought distress to riverside populations by sinking their boats and flooding the man-made dykes to inundate crop fields with saline water.

The space between the Hooghly river at the western edge and the Padma in Bangladesh across the “Bengal Fan” measures 342 km and is marked by a number of tributaries of the Ganga and Brahmaputra. This creates a labyrinth of channels and creeks. These narrow twisting and turning channels deposit huge quantities of silt on the higher grounds splitting the flow into two only to rejoin after a distance ~ thus forming tiny river islands. Sooner than one can imagine mangrove and a few other halophytic species like the salt tolerant ‘Kankra’ (Bruguiera gymnorhyza) dominate these lands and arrest passing silt in their stilt roots to help expand the new-born landmass. In due course of time, high rainfall ensures the proliferation of early “colonisers” like the endemic Dhani grass (Oryza coarctata) followed by the pioneer species Sundari (Heritiera fomes) that help stabilise the surface to create a refuge for primary consumers like the deer, wild boar and the giant Bengal monitor lizard ~ to be followed by their predator ~ the tiger.

As many as 200 such islands make up the physical profile of the Sunderbans that remains among the most dynamic land formations on earth. These low-lying muddy islands where the surface height varies between 0.9 metre and 2.11 metre above sea-level remain highly vulnerable to the constant threat from indundation by surrounding channels that, during high tide, flow at a level higher than the island surface.

Alongside a great diversity of animals, human settlements have come up in these islands where virgin land with loads of nutrients attracted people from places as far as Midnapore to populate such hostile lands, in defiance of swirling waters and the forbidding tiger. Here, tides change every six hours, forcing the inhabitants to regulate their way of life in rhythm with the changing tides.

The will for survival has resulted in the creation of a lifeline of earthen dykes that surrounds all human habitations including the crop fields to keep away brackish water. Construction of the dykes was initiated by the zamindars of Bengal way back in 1700 to bring in settlers for paddy cultivation, a source of revenue. A mind-boggling length of more than 4400 km of these dykes now act as a cradle for protecting humanity in the “Bengal Fan”. If attached end to end, the dykes will make a high wall all the way from Kolkata to Mumbai and then to Delhi and finally complete the triangle by ending up in Kolkata. 

Defying all speculation and contrary to my expectations for a spectacle of an awesome bore tide, a docile wave of no more than a metre high passed by Chamta where I stood. On its way it washed away a large part of the already crumbling west bank of the channel. Later, several river engineers confirmed that due to “natural” reasons as well as the creation of several barrages upstream of the Ganga, the flow rate of the channels has declined dramatically, resulting in silt deposits on the bed. This has decreased the depth of the rivers. Reverse flows during high tide bring back further loads of sediment from the river mouth to aggravate the situation still further. Uplift of the bed leads to spilling of the channels, enhancing the scope for indundation of islands and surrounding areas.

A mysterious depression in the mouth of the Hooghly on the Indian side of the Sunderbans has for long challenged the imaginations of geographers and mariners. This massive submarine canyon acts as a barrier for further westward transport of the rivers’ sediment and a sink for about a third of the rivers’ sediment discharge. As one of those yet-to-be understood geographical phenomenon, in all suddenness this sub-terrennian chasm of great depth empties its water and silt content from the 500-metre level to 20 metres, with the resultant devastation. As an aftermath of such geological confusion an enormous amount of water and silt is thrown back towards the east, further lifting the river beds and inundating the riverside landmass.

The Sunderbans estuary, a part of the Ganga-Brahmaputra delta, is dominated by sub-aqueous levees and covers a geographical space of 10 000 km2, of which only 40 per cent remains in India and the rest in Bangladesh. Climate experts claim that the water level of the Bay of Bengal in the estuary of Ganga-Brahmaputra river system shows a disturbing trend of an annual rise by 25 mm against the global average of 3.1 mm. They fear, if this rate persists then by mid-21st century global warming-induced climate change will be responsible for rise in sea-level to such extent that five million people in the low-lying southern regions of Bangladesh will possibly be inundated. New York Times reported that by 2050, the rising sea level will submerge 17 per cent of the land area of southern Bangladesh displacing 18 million people.

Contradicting such frightening predictions, other learned authorities report that over the last 100 years the delta warmed by 0.50 C and the sea level rose by a mere 0.50C, and hence, such predictions for a doomsday scenario seem a little too farfetched.

The Intergovernmental Panel for Climate Change (IPCC) has predicted that sea levels could rise 10 to 23 inches by 2100. The report adds that in recent years sea levels have been rising faster than the upper end of the range predicted. According to IPCC, the current rate of sea-level rise is expected to increase as a result of thermal expansion of the oceans and melting of most mountain glaciers, permafrost and partial melting of the West Antarctic and northern polar ice caps.

(To be concluded)

The writer is a fellow of the Royal Geographical Society, London and a wildlife consultant