Saturday, January 10, 2009

Bangladesh Sea Levels - The land sinks more than the sea rises

When the effects of Global Warming are discussed, the ongoing saga of the people that live in Bangladesh is quite frequently used as a graphic illustration of the fate that faces all those who live close to the ocean, especially those in low-lying areas.

Two problems are generally used to illustrate the problems, the fact that the water is rising, both covering and eroding the area in which people can live forms one part, and secondly the invasion of salt water into the ground beneath the settlements displaces drinking water. And while the situation is made worse each year when the monsoons come and rivers rise (for example 2000; 2002; 2004; 2007) it is an ongoing problem throughout the year.

However, while the blame for the rise in sea level (it is around 5.4 mm per year) is almost always ascribed purely to the rising sea levels due to global warming, in actual fact the sea is rising at only 1.4 mm/year, a rate which it has stayed the same in the Bay of Bengal for over forty years, and the majority of the problem is caused because the area in question is a river delta, and the ground is subsiding by about 4 mm/year. Neither of these is directly caused by greenhouse gas effects.

Now this is a mildly controversial statement, so let me try and give a little more background to these facts. First of all let me borrow a map of Bangladesh, so that you can see the location at the top of the Bay of Bengal, and that it is a country that is relatively flat and permeated with rivers.

Map of Bangladesh, showing the Ganges Delta

The part of the country that is most affected is the south, particularly the area known as the Sundarbans. This is largely a mangrove swamp in the delta of the Ganges River, and an area of natural beauty, but, as the Wikipedia article notes,
During each monsoon season almost all the Bengal Delta is submerged, much of it for half a year.
It also notes
A 1990 study noted that there "is no evidence that environmental degradation in the Himalayas or a 'greenhouse' induced rise in sea level have aggravated floods in Bangladesh"; however, a 2007 report by UNESCO, "Case Studies on Climate Change and World Heritage" has stated that an anthropogenic 45-cm rise in sea level (likely by the end of the twenty-first century, according to the Intergovernmental Panel on Climate Change), combined with other forms of anthropogenic stress on the Sundarbans, could lead to the destruction of 75% of the Sundarbans mangroves.

The scale of the ground settlement in the area has been recognized for a long time, for example (from Hunter and Kisch’s “A Statistical Account of Bengal,” in 1875)
Before I pass from the physical aspects of the Sundarbans, it may be well to reproduce the following paragraphs from the Revenue Survey Report of Colonel J.E. Gastrell, showing that a remarkable depression has at some time taken place in the general levels: - “What maximum height the Sudarbans may have ever formerly attained above the mean tide level is utterly unknown, that they were much higher that at present is, I think, more than doubtful. But that a general subsidence has operated over the whole extent of the Sundarbans, if not of the entire delta, is, I think, quite clear from the result of examination of cuttings or sections made in various parts where tanks were being excavated. At the village of Khulna, in the Jessor District, about twelve miles north of the nearest Sundarban lot, at a depth of eighteen feel below the present surface of the ground, and parallel to it, the remains of an old forest was found . . . “
(Note a tank is an Indian underground water reservoir).

Historically the land subsidence, which can reach up to 5 mm/year was surpassed by the sediments laid down in the seasonal flooding, with the sediments adding perhaps 7 mm to the land, however the increased population density, means that instead of allowing the river access to the land, dykes are built instead to keep the rising water away, and this stops the sedimentation, but not the subsidence, and thus the rate at which the land is sinking increases. This is exactly the same sort of thing that is happening in the Mississipi Delta in the United States, with potential explanations due to sediment weight and compaction, of a layer that started out as a water-saturated mud, as time progresses.

OK, so having established that the land is settling, I should probably also give some sources for the level of water level increase (saving the causes of that for a subsequent post). The average value of 5 mm can be taken for example from a Jadavpur University study, which includes:
The JU wing had conducted a 10-year study in and around the Bay of Bengal and concluded that the sea is rising at 3.14 mm a year in the Sunderbans against a global average of 2 mm, threatening low-lying areas of India and Bangladesh.

"In places like east Sagar Island the sea is rising more due to easterly tilt of subsidence. The rise near the tiger habitat at Pakhiralay is at an average of 5 mm annually, while towards Khulna in Bangladesh it is 10 mm," said Sanyal who is a member of the National Coastal Zone Management Authority.

Now, as to the overall rise (as opposed to land sinking) this is a difficult thing to establish, due to tidal action, seasonal variations and similar complications, taken with only a limited amount of historical data. However the Indian National Plan on Climate Change (pdf) states (on page 15)
Using the records of coastal tide gauges in the north Indian Ocean for more than 40 years, Unnikrishnan and Shankar have estimated, that seal level rise was between 1.06 and 1.75 mm per year. These rates are consistent with 1-2 mm per year global sea level rise estimates of IPCC.

To go back further in time significantly reduces the amount of record available, but if one goes to the National Institute of Oceanography, in India, one gets this curve for the last 135 years, which suggests that there is nothing particularly remarkable about the current changes in sea level, nor that there is any effect due to the GHG Global Warming that has been made the point of so much commentary.

Sea-level variability at Mumbai, which has the only more-than-century-long sea-level record in the Indian Ocean. Annual sea level (cm) is plotted as a function of time to reveal the variability of the annual sea level over the length of the data record (1878 to 1988 in this figure) (red), showing the inter-annual variability in sea level at Mumbai. Filtering annual sea level with a 10-year boxcar filter (10-year running mean) (blue) reveals inter-decadal variations. At Mumbai, these inter-decadal variations are as large as are the variations from year to year.


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