A Few Things Ill Considered

Bangladesh gaining land area

Posted by coby on August 1, 2008
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Via Prometheus I read this article from the AFP that describes some new findings about land mass in the river delta that is Bandladesh. Apparently it is growing and has been for the last 32 years at about 20 km^2/year.

Roger points out, no doubt correctly, that the climate septics will make great hay with this, as supposedly another IPCC prediction turns sour.

But will they be correct?

Regardless of the extent of the Bangladeshi delta, obviously the only thing that will determine if it is submerged or not in future sea level rise scenarios will be its altitude. 640 square km of new land is great, but if it is all below one metre of sea level rise then it may not last til the end of the century. Approximately 50% of existing Bangladesh would be submerged as a direct result of 1 metre of sea level rise, more would be vulnerable to increased erosion. I dare say that most if not all of the new land being created from silt deposits is both very low altitude and very vulnerable to erosion and will join the unfortunate half destined to be continental shelf sooner or later.

So, while it will be an effective talking point for the “lawyer science” of the climate denial industry, I hardly think this makes Bangladesh any less vulnerable to sea level rise.

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Comments

  1. #1 derek
    August 1, 2008

    Well, I’d want to know

    a) how long a length of coast that 20km2 a year is added along. That gives the width of the added strip

    b) what the gradient of the land is on that strip. That gives the sea level rise that would overwhelm the gains.

    If the length is 2000km, the strip is 10m wide. If the gradient is 1:100, the height is 10cm. It wouldn’t take a big sea level rise to top that out. And if the land behind it isn’t growing 10cm taller a year (which I doubt), then that top-out is a once-only event.

    I suspect alluvial growth, like glacier growth, is a feature of recent global warming rather than evidence against it. If more water evaporates and falls on the land, more land is washed down the river. This just says what the new world is like under warming rates so slow denialists can obfuscate it. when the warming rate accelerates, the glaciers won’t grow more from snowing, they’ll grow less from melting. Alluvial fans won’t grow more from wash-down, they’ll grow less from flooding.

    (I know denialists like to crow that this sounds like have-it-both-ways, but it is what it is)

  2. #2 Blind Squirrel FCD
    August 1, 2008

    So where does the new sediment come from? Who loses?

  3. #3 pough
    August 1, 2008

    The Himalayas, I would guess. I think they got enough to last a little while.

    http://maps.google.com/?ie=UTF8&ll=24.527135,90.263672&spn=7.781034,10.953369&t=k&z=7

  4. #4 coby
    August 1, 2008

    checking the website of http://www.cegisbd.com/ where the sutdy came from, gives me the suspicion that they were looking to see where land lost to river bank erosion ended up.

  5. #5 zy
    August 2, 2008

    You know, it’s perfectly possible the IPCC goofed on this detail. It would mean we’re luckier than we should be, not that we have cleverly escaped climate change or its consequences.

    Unfortunately, the article seems to be comparing current land accretion in Bangladesh with a future rise in sea level. Just because land has increased over the last 30 years, of relatively stable sea level, that doesn’t mean it will continue to increase over the next 30 or 100, with rising sea levels. The answer would lie in relative rate of accretion vs. rate of sea level rise. It will be interesting to see how deniers treat this because for them to even engage in that debate, they first have to concede that sea level will rise.

    An even bigger problem with the triumphant proclamation that Bangladesh will gain not lose land is that the land to be gained isn’t that wonderful and can’t accommodate an increasing landless population. Accreted land in Bangladesh is known as char. It can occur as islands or attached to other land. (So derek@#1, it’s not really a strip of land, more like scattered clumps interspersed with water.) The Banglapedia is a good starting point in understanding char, and one of the first things it talks about is the problems.

    The chars are extremely vulnerable to both erosion and flood hazards. Recent analysis of time series satellite images indicates that over 99% of the area within the riverbanks of the jamuna had been char at one time or another during the 27-year period of 1973 to 2000. The same analysis shows that about 75% of the chars persisted between one and nine years, while only about 10% lasted for 18 years or more. In certain areas, however, the chars can be quite stable (for example, in the Upper Meghna area).

    So, pick a spot on the riverbank, look across and imagine a nice grassy island in the middle, perfect to build a little house and raise a few vegetables and cattle. Now, look in ten years, and see no trace of it. It’s migrated, as the river shoves sediment around, or it’s been eroded completely (or maybe it’s joined up with the neighboring island and now you and your neighbor are locked in a bitter conflict over who owns your yard!) It’s a blessing for the poor over there that Bangladesh is currently gaining, but wow, what a precarious livelihood.

    Oh, and though you’d think the Himalayas have enough to last for awhile, from the human welfare and wildlife conservation point of view you’d be looking at the wrong thing. Among the sediments lost are fragile mountain soils, rather thin there to begin with, and formed only slowly. By all accounts, Himalayan erosion is increasing. Whether the cause is natural or anthropogenic, e.g. overgrazing, is a subject of much debate.