by Judith Curry
On the complexity of sea level rise in the Bangladesh delta – global warming may be the least of their concerns.
Bangladesh is often put forward as the poster child for the adverse impacts of global warming. A World Bank press release last June states:
Bangladesh will be among the most affected countries in South Asia by an expected 2°C rise in the world’s average temperatures in the next decades, with rising sea levels and more extreme heat and more intense cyclones threatening food production, livelihoods, and infrastructure as well as slowing the reduction on poverty, according to a new scientific report released today by the World Bank Group.
“Bangladesh faces particularly severe challenges with climate change threatening its impressive progress in overcoming poverty,” said Johannes Zutt, World Bank Country Director for Bangladesh and Nepal. “Bangladesh has demonstrated itself as a leader in moving the climate change agenda forward.
In Bangladesh, 40% of productive land is projected to be lost in the southern region of Bangladesh for a 65cm sea level rise by the 2080s. About 20 million people in the coastal areas of Bangladesh are already affected by salinity in drinking water. Rising sea levels and more intense cyclones and storm surges could intensify the contamination of groundwater and surface water causing more diarrhea outbreaks.
An important new paper puts Bangladesh’s sea level rise into the appropriate context:
Rapid rise in Effective Sea-Level in southwest Bangladesh: its causes and contemporary rates
John Pethick and Julian Orford
Abstract. Evidence is presented from three estuarine tide gauges located in the Sundarban area of southwest Bangladesh of relative sea level rise substantially in excess of the generally accepted rates from altimetry, as well as previous tide-gauge analyses. It is proposed that the difference arises from the use of relative mean sea level (RMSL) to characterise the present and future coastal flood hazard, since RMSL can be misleading in estuaries in which tidal range is changing. Three tide gauges, one located in the uninhabited mangrove forested area (Sundarban) of southwest Bangladesh, the others in the densely populated polder zone north of the present Sundarban, show rates of increase in RMSL ranging from 2.8 mm a- 1 to 8.8 mm a- 1. However, these trends in RMSL disguise the fact that high water levels in the polder zone have been increasing at an average rate of 15.9 mm a- 1 and a maximum of 17.2 mm a- 1. In an area experiencing tidal range amplification, RMSL will always underestimate the rise in high water levels; consequently, as an alternative to RMSL, the use of trends in high water maxima or ‘Effective Sea Level Rise’ (ESLR) is adopted as a more strategic parameter to characterise the flooding hazard potential. The rate of increase in ESLR is shown to be due to a combination of deltaic subsidence, including sediment compaction, and eustatic sea level rise, but principally as a result of increased tidal range in estuary channels recently constricted by embankments. These increases in ESLR have been partially offset by decreases in fresh water discharge in those estuaries connected to the Ganges. The recognition of increases of the effective sea level in the Bangladesh Sundarban, which are substantially greater than increases in mean sea level, is of the utmost importance to flood management in this low-lying and densely populated area.
[Link] to abstract. Additional excerpts:
Forecasts of eustatic sea level rise due to accelerated global warming at a given location must, however, be recalibrated to take account of local effects. This is an essential step before identifying any headline rate of change of sea level for this area, as mis-directed planning for future environmental events can involve the livelihoods, if not lives, of millions of inhabitants of the delta lands. Syvitski et al (2009) estimated that the Ganges/Brahmaputra delta, within which Bangladesh is located, is subsiding at a rate of between 8 and 18 mm a-1 due to a combination of deltaic subsidence, compaction and dewatering. The relative sea level changes (RSLC) associated with such subsidence will be in addition to future global warming effects. Evidence is presented from three tide gauges along the Pussur estuary in South West (SW) Bangladesh that show rates of increase in relative mean sea level (RMSLR) of 7.9 mm a-1 at the mouth of an estuary and 2.8 mm a-1 at 100 km inland. These figures differ slightly but not significantly from the generally accepted rate of sea level rise for Bangladesh which is put at between 3 mm a-1 and 6 mm a-1 in most published sources. However, the figures for the RMSLR disguise the fact, shown by the tide gauge records, that the high water levels are rising much more rapidly: at 10.7 mm a-1 at the mouth of the estuary and at 17.2 mm a-1 at 100 km inland. The latter figure gives cause for some concern since it refers to the highly populated and protected polder zone of southwest Bangladesh.
Although our analysis indicates that relative sea level rise in the Pussur Estuary is a response to several independent factors including land subsidence and eustatic sea level, the dominant process responsible for increased ESLR is the amplification of tidal range. This process alone is responsible for 57 % of the average observed increase in ESLR. Increases in tidal range within an estuary can be the result of a number of factors including channel constriction by embankments and increased water depth due to sea level rise or dredging. The Hiron Point tide gauge is located on a natural channel within the extensive Sundarban mangrove forest with no anthropogenic influences. Tidal range amplification here contributes only 28.6 % of the observed increase in ESLR, so the observed tidal range amplification at Hiron Point of 5.6 mm.a-1 is likely to be a response to sea level rise alone. In contrast, Khulna is located within the polder area of the SIZ where channels have been constricted by embankments (Islam 2006) and deepened by dredging (IWM 2004), so that, in addition to the natural response to sea level rise, tidal range will be amplified as a result of these anthropogenic activities. At Khulna the total contribution of tidal range to observed ESLR is 83 %. If the rate of tidal range response to sea level rise at Khulna is assumed to be the same as that at Hiron Point, then the anthropogenic contribution to total observed increase in ESLR at Khulna can be put at c. 54 %.
In view of these results we adopt a worst case (precautionary) approach and assume a linear trend over the next century resulting in an average increase in tidal range of 1.4m by the year 2100. Existing tidal range in the Pussur is between 3m and 4m, so that by 2100 the tidal range could have increased, on the basis of these figures, to between 4.4 and 5.4 m.
In contrast to the impact of tidal range amplification on ESLR, the gradual reduction in fresh water inputs to the Pussur Estuary from the Ganges has offset the rate of sea level rise to a significant degree. Proposals now under consideration to partially restore this fresh water flow via the Gorai River (Haskoning 2001) would result in an increase in the rate of rise in ESLR. These results indicate that any restoration to pre-1950 Ganges discharges would increase the rate of increase in ESLR in the Pussur Estuary by an average of 40 %.
Eustatic sea level rise will, of course, remain essentially spatially constant throughout the region. Subsidence rates will vary locally but are not expected to be significantly different from those in the Pussur Estuary. Since polders have constricted all of the upper to mid estuaries in southwest Bangladesh, it seems reasonable to conclude that the increase in tidal range noted in the Pussur estuary could apply elsewhere. The reduction in rates of sea level rise due to loss of fresh water inputs, however, applies only to the Pussur and its adjacent estuary, the Baleshwar, which alone are connected to the Ganges. Thus, in the remaining estuaries, the mitigating effect of fresh water reduction will not have been felt and it is to be expected that ESLR will be increasing more rapidly than in the Pussur. We conclude that the rates we observe in the Pussur are likely to apply elsewhere in the Sundarban and indeed could be exceeded where fresh water inputs are absent.
The results presented here indicate that the average increase in ESLR for the Pussur Estuary of 14.1 mm a-1 will mean that by the year 2100, and without any contribution from global warming, sea level will have risen by 1.24 m with a 95% confidence range from 0.64 m to 1.85 m. The worst case prediction for Khulna in the vulnerable polder area will be an increase in ESLR of 17.2 mm a-1 or a total of 1.51m (±0.42m at P = 95%) by the year 2100 (Table 7), again without any contribution from global warming. This prediction assumes that the increase in tidal range observed over the past 50 years will continue at a linear rate. We suggest that this assumption provides a necessary precautionary approach in view of the potential risk to the large population of the Bangladesh Sundarban. The proposed increase in fresh water input to the Pussur would mean that these rates would increase by an average of 40% increasing to 51% at Khulna.
The existing rate of increase in local ESLR must, however, be in addition to any eustatic increase in sea level due to global warming. As noted above the range of predictions for sea level rise due to global warming is large: ranging from 0.5 m to 2.0 m by the year 2100 (Nicholls et al 2011). Adding this range to our worst case prediction for the Pussur Estuary as a whole, results in a predicted total rise in ESLR of between 1.74m and 3.24m (± 0.6m at P= 95%) by the year 2100 (Table 8). Within the vulnerable SIZ, however, these predictions increase so that, at Khulna, the total predicted sea level rise by 2100 ranges from 2.01m to 3.51m (±0.42m at P = 95%).
Based on long term records from three tide gauges located in the Pussur estuary southwest Bangladesh, it is concluded that sea level rise in the Bangladesh Sundarban is significantly higher than had previously been assumed. This increase is largely a response to anthropogenic processes, principally estuary channel constriction by embankments, resulting in amplification of the tidal range along these channels. We show that, in such an area of tidal range variation, use of the traditional measure of mean sea level can be misleading, and it is proposed instead to use the rate of increase in high water maxima which we refer to as Effective Sea Level Rise (ESLR) as the best estimator of sea-level change for tidally influenced deltas where anthropgenic alterations to channels are present.
The combined impact of land subsidence, eustatic sea level rise, tidal range amplification and a decrease in fresh water input results in an average rate of increase in ESLR in the Pussur Estuary of 14.1 mm a-1 rising to 17.2 mm a-1 in the densely populated SIZ at Khulna. This rate is likely to be equaled, or even exceeded, in the other estuaries of the Bangladesh Sundarban where the mitigating effect of fresh water decrease is absent. Proposals to increase the rate of fresh water flow into the Pussur may result in a significant increase in the rate of ESLR in this estuary.
Although there are some grounds for predicting a gradual reduction in the rate of tidal range amplification, this is far from certain, and we assume a linear trend in the rate of increase over the next century. If the range of predictions for eustatic sea level rise due to global warming, are included, the total rise in ESLR in the Pussur Estuary by the year 2100 could range between 1.74m and 3.24m. This increases to between 2.01 m and 3.51 m at Khulna, in the densely populated area of the Sundarban Impact Zone. Such an outcome would pose almost insuperable obstacles to sustainable flood protection in this densely populated and vulnerable region.
Finally we note that these predictions are based upon the high water values averaged over a month. Flooding is, however, caused not by average events but by extremes. If effective sea level rise is defined by monthly or annual maximum water levels, then the flooding hazard in the Sundarban, and especially the polder area of the Sundarban, becomes of even more serious concern.
JC comments: The IPCC AR5 cites emissions-scenario dependent projections for global sea level rise circa 2081-2100 to be in the range 0.26-0.82 m (note this is higher than the range used by Pethick and Orford). Regardless of the exact value, Pethick and Orford effectively argue that global warming is less than half of the expected sea level rise, and maybe only as small as 10%.
Policies targeted at trying to help this situation through emissions reductions seem futile. But of even greater concern are WorldBank and other adaptation strategies for Bangladesh will not be adequate if they are targeted only at the global warming piece of the problem. This is a very large geo-political issue with regards to the substantial international (UN, WorldBank) funds targeted at climate change adaptation. Bangladesh’s sea level rise problem is not really driven by climate change – the risks here are that UN/WB adaptation solutions will be inadequate to help them deal with their sea level rise problem, or that Bangladesh will find itself ineligible for international climate adaptation funds.
Might Bangladesh become a tragic victim of the UNFCCC/IPCC oversimplification of the climate change problem and its solutions?