BANGLADESH: The Deep Delta Blues
Guest essay by Kip Hansen
”KUTUBDIA, Bangladesh — Anyone who doubts climate change should come to this lovely low-lying island, lapped by gentle waves and home to about 100,000 people.
But come quickly, while it’s still here.”
“Climate change is destroying children’s futures,” noted Justin Forsyth, the deputy executive director of UNICEF. “In Bangladesh, tens of millions of children and families are at risk of losing their homes, their land and their livelihoods from rising sea levels, flooding and increased cyclone intensity.”
— Swallowed by the Sea — NY Times Opinion Section – Nicholas Kristof
Simple Facts: (with lots of images)
Has the Bay of Bengal experienced “increased cyclone intensity”?
No, there has not been an “increase” in cyclones, neither in intensity nor in number. While I show only the combined monsoon totals, both Pre- and Post-monsoon cyclonic activity peaked mid-20th century, 1920-1970, and have decreased steadily since — the number of depressions, cyclonic storms and severe cyclonic storms have all been downtrending since the mid-20th century (or earlier). Cyclonic storms are currently at similar levels to those recorded for 1890-1900. [Note: “Depressions” may not have been recorded or counted properly — there may have been more than were recorded before the 1910-1920 period when weather services in the region modernized.]
Rising Sea Levels: Is the sea surface height rising in the Bay of Bengal?
Yes, of course it is … sea levels are and have been rising worldwide (but not evenly) since at least the end of the Little Ice Age and, in geological-time, since the end of the last great glaciation (with a lots of variation over the period). How much has the sea surface height been rising in the Indian Ocean and the Bay of Bengal?
That’s the figure from Unnikrishnan and Shankar (2007). And yes, that is less than the generally accepted long-term global average of ~1.8 mm/yr. Unnikrishnan and others re-visited sea level rise in the Bay of Bengal for the National Institute of Oceanography, Goa in 2010 and determined: “Mean sea-level-rise trends along the Indian coasts are about on an average 1.30 mm/year.”
Some of the original text reads as follows:
Long-term, global, tide-gauge records show that changes in sea-level occurred throughout the 20th century (Smith, 2012). Analysis of data for stations in the north of the Indian Ocean with >40 years of records up to 2004 showed rates of rise of 1.06–1.75 mm/year, with a regional average of 1.29 mm/year (Unnikrishnan and Shankar, 2007). The latter authors attributed the considerable inter-annual variation found at all stations to variations in the force of onshore winds in the monsoon season, inflow of fresh water from major rivers and water salinity. Fig. 7 shows the inter-annual variations and overall trends at Diamond Harbour (Calcutta), Hiron Point, Khepupara and Cox’s Bazar. They attributed the differences between their adjusted rate of 5.74 mm/year at Diamond Harbour (Calcutta) and the average rate of 1.29 mm/year in the Indian Ocean to land subsidence.
[top caption and source attribution by the author — kh]
In 2007, Unnikrishnan and Shankar considered the period since collection of satellite altimetric data commenced in 1993 to be too short for estimating long-term trends. However, based on satellite data, Allison et al. (2009) gave a global estimate of 3.4 mm/year, and Smith (2012) stated that global sea-level rise had increased from its 130-year average rate of 1.7 mm/year to about 3 mm/year over the past 20 years. …. However, the analyses [of local and regional data] show no evidence that the rate of global sea-level rise has doubled in the past 20 years. This finding raises questions about the reliability of satellite data interpretations based on the limited number of years since their introduction.
Sea surface height rise?
1.29 – 1.3 mm/yr, far less than the global average
Note that I have asked so far only “How much has the sea surface height been rising?” and not what the local Relative Sea Level Rise numbers are. This is quite intentional, and the difference is very important, particularly for the Bay of Bengal and Bangladesh. If you have been reading my series here at WUWT on Sea Level: Rise and Fall, you know:
Local Relative Sea Level is the only Sea Level data of concern for local governments and populations.
Has there been FLOODING? In Bangladesh?
Yes there has been — catastrophic flooding — literally hundreds of thousands of people drowned. The venerable Wiki reports: “In the 19th century, six major floods were recorded: 1842, 1858, 1871, 1875, 1885 and 1892. Eighteen major floods occurred in the 20th century. Those of 1987, 1988 and 1951 were of catastrophic consequence..” “…The catastrophic floods of 1987 occurred throughout July and August and affected 57,300 km2 of land, (about 40% of the total area of the country)… The flood of 1988, which was also of catastrophic consequence, occurred throughout August and September. The waters inundated about 82,000 km2 of land, (about 60% of the area)… In 1998, over 75% of the total area of the country was flooded”
… and 1999, 2004, 2005, 2010, 2015 and 2017.
And those are just the rivers flooding … when cyclones (hurricanes) hit, things get much, much worse.
“The Meghna estuary is an unstable area for settlement, and it is badly exposed to cyclones and storm surges; the megacyclone in November 1970 killed an estimated 300,000–500,000 people.” — Brammer (2014)
Take a moment to pause here — what could possibly account for the uncertainty in that figure — 400,000 +/- 100,000 people? — those were men, women, and children – human beings … nevertheless, conditions on the ground following the 1970 cyclone were so bad, only a vague estimate could be made.
1876 — 200,000 deaths, not including the subsequent epidemics and famine.
1897 — 32,000 killed
1960 — 13,000 in two storms
1962 — 11,500
1963 — 11,520
1965 — 19,300
1970 — “The official death toll was 500,000 but the number is likely to be higher.”
2007 — 3,500
2017 — Due to “A multitude of tropical cyclone warnings and watches”, “500,000 people managed to move out of coastal areas before the storm made landfall on May 31”.
How can one country flood so extensively and so often? What is going on down there?
Simple answer: Geography.
Three mighty Asian rivers flow into Bangladesh: The Ganges, the Brahamaputra, and the Meghna. The three river basins are outlined in the map above — they drain most of Northern India, Nepal, and Bhutan — starting high in the Himalayans. The Tsangpo River in what was Tibet which flows east for a thousand kilometers, then turns south, then west into the upper Brahmaputra as well. The total basin area, flowing into the Ganges-Brahamaputra-Meghna (GBM) delta (Bangladesh) is over 1,720,000 sq km.
Then there is this:
Images from NatGeo.
Blue areas are flood plains –the darker blue, the more prone to flooding.
Red areas are Population Density — darker red, more people.
Almost all the land in Bangladesh is prone to river flooding, flash flooding, tidal flooding or tidal surges. Cyclones only make this situation worse. In the following image, the relationship of tides and surge are noted as measured at Polder 32 in Southwest Bangladesh:
In essence, much of Bangladesh consists of river flood plains and tidal mud flats. To combat the repeated flooding by tides, embankments have been built creating polders [definition: An area of low-lying land… that has been reclaimed from a body of water and is protected by dikes. ] In the image above, we see a cross-section of Polder 32 at the lower left.
The situation at Polder 32 is the common state of polderized lands all over Bangladesh: Lands protected from inundation are deprived of soil replenishment (aggradation), and as water is pumped out to keep the lands dry enough to farm, soil levels sink through both enhanced compaction and subsidence due to water extraction. The tides, now restricted to channels between embankments, are higher, having increased by 0.8 meters since 1960. Thus, villages and farms now exist below Mean High Water (the normal high tide mark):
In May of 2009, Cyclone Aila stuck Bangladesh and caused 3.6 meters (12 feet) of storm surge. When storms and accompanying rains raise water levels so high, embankments fail, and the polders become lakes — if the population has not been warned to flee to higher ground, they are trapped. Crops are destroyed and lost livestock drowned.
We saw in the flood map a bit above, almost the entire country is subject to either river flooding or tidal flooding. Fully half of the country is less than five meters above sea level, and many polderized areas are actually below tidal mean high water. Cyclone Sidr, November 2007, produced storm surges as high as 5 meters (16 feet).
And Relative Sea level Rise?
Deltas subside, and, unless replenished by sediment coming downriver, disappear into the sea. Dyking, poldering, and building embankments has proven counter-productive in many areas, leading to further subsidence and threat, while sediments, much needed, flow out to sea. Of the areas surveyed, many in the GBM delta are subsiding at rates of 5-10, and even >20, mm/yr.
There is no quick fix for this situation. Polders and embankments are raised and reinforced with concrete and stone — but the land continues to sink and the sea rises, ever so slightly, inexorably.
There are plans to intentionally breach embankments and let the floods replenish soil levels in polders to allow soil levels to recover, but it is uncertain that this will ever protect the land and citizens from tidal surges caused by severe storms — for those situations, only advanced warning will save lives.
And our farmer in Kutubdia? Has his farm been covered by the rising sea?
No, not covered by the rising sea — though as far as he is concerned, the difference is academic. His farm and home have simply been washed away by the outflow of the three great rivers and the shifting tides that shape and reshape these sandbar/mudflat islands all over the world. As we saw at the start of this essay, sea level in the Bay of Bengal has been rising at only ~1.3 mm/yr – or a total of 2.6 inches over the last 50 years.
But [and there is always a “but”…]:
Had our farmer lived elsewhere in Bangladesh, he might have gained new fields to farm, as we see from this image of land gain and loss. Little Kutubdia Island is just off the map on the right, south of Chittagong, and was not part of the survey done. It is a coastal island whose unfortunate geography leaves it in the path of the outflow of the entire GBM delta and the seasonal storms that blow up into the Bay of Bengal. And while the sea surface is not rising much here, about 1.3-1.4 mm/yr [see the tide gauge graphs far above — Cox’s Bazaar is very near Kutubdia], repeated storms and associated tidal surges erode the beaches of this little island, as they do everywhere in the world. If the farmer’s land faced the Bay, then it has been washed away.
Overall, however, for Bangladesh:
“Rapid geomorphological changes are taking place in the Meghna estuary … The Google Earth image in Fig. 2 shows the 1943 land boundaries superimposed on the 2013 land boundaries. Comparison of Landsat images taken in 1984 and 2007 showed a net land gain of 451 km2 in the Meghna estuary within that period, representing an average annual growth rate of 19.6 km2 (Fig. 3) Brammer, in press. Earlier, Allison (1998) had calculated annual net gains of 14.8 km2 between 1792 and 1840 and of 4.4 km2 between 1840 and 1984. This historical evidence of large-scale net annual land gains in the Meghna estuary suggests that land gain might exceed land loss resulting from the slow rates of sea-level rise projected for the 21st century.” Brammer(2013)
The important point, for our poor Kutubdia Island farmer, and every other individual, is not whether the sea surface is rising or the land sinking — it is the relationship between the height (elevation) of his home and land and the height of the river or sea adjacent — the Relative Sea Level — and not just the Mean Relative Sea Level but, far more importantly, the extreme heights to which it can rise — Spring Tides, Storm Surges and the every-threatened great river floods.
And child brides? Is climate change is destroying children’s futures?
Short answer: No.
The cultural practice of marrying off young daughters (children, we would call them) is not caused by Climate Change — it has been part of the culture in this area of the world for all of recorded history. Its cause is poverty. Poverty is always exacerbated by bad weather, flooding rivers, and cyclonic damage — all which bring with them destroyed crops, lost livestock, damaged and wrecked homes, spoiled cropland and, with the unavailability of clean of potable water, disease and epidemic. Any societal stressor worsens poverty in Bangladesh — and parents faced with the inability to feed their children follow, as they always have, the cultural solutions available to them. The suggestion that climate change is forcing poor farmers to marry off their young girls is based on a single “women’s studies” research paper that, although it found no evidence of a relationship between climate change and the practices of early marriage for Bangladeshi girls, insists that it might be happening.
Those of you who are outraged by this practice can and should volunteer to serve with charitable NGOs in Bangladesh to help them solve the grinding poverty that is the norm there. [Hint: Just turning down your thermostat, biking to work, and monitoring your carbon footprint will not help.]
The Bottom Line:
0. Bangladesh comprises the delta of 3 great rivers, draining between them a total of 1.7 million square kilometers. When the monsoons come, immense amounts of water flow down these rivers, flooding Bangladesh and rearranging the land of the delta.
1. Nearly the entire land surface of Bangladesh is subject to flooding of various types, and does so with great regularity. Some of these floods have been catastrophic with unimaginable loss of life. Fully half of the land is less than 5 meters above normal sea level. Cyclonic storm surge has been as much as 6 meters in recent times.
2. Subsidence (land sinking) in the GBM delta ranges from 5-7 mm/yr to >20 mm/yr: “…after five decades of polderisation, the difference in height between natural and artificial landscapes equated to approximately a metre [ 1 meter ] , or an average 2 cm/yr. This is an order of magnitude greater than global sea-level rise over this period.” (Brown 2015)
3. As a result of the building of embankments to create “polders” (reclaimed land surrounded by dikes) much of the enclosed land is already below Mean High Water.
4. Sea level in the Indian Ocean and the Bay of Bengal is rising — but at the low rate of 1.3 mm/yr — far less than the global average.
5. Cyclonic activity in the Bay of Bengal peaked in the mid-20th century and has declined since then to 1890-1900 levels. These cyclones pushing up into the Bay of Bengal necessarily reshape the coastline of the delta and coastal islands.
6. There will be no end to the suffering in Bangladesh which has been brought about by its poverty and geography — with or without any climate change.
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Author’s Comment Policy:
I make an effort respond to all comments addressed to me (by leading them with “Kip…”). Your questions help me to fill in information that I may have left out inadvertently and to clarify where I have failed to be clear.
The poverty of Bangladeshis — and their suffering resulting from flooding and severe weather — is heartbreaking. If you are so moved, there are good charitable NGOs that focus on Bangladesh.
If you have personal experience there — or expertise on the area, let us hear from you.
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