Increased soil salinity for sea-level rises is likely to make nearly 0.2 million coastal residents internally displaced in Bangladesh, a study showed.
The climate-induced changes will force them to migrate to inland areas to find alternate livelihoods, it said on Monday.
The study examined, for the first time, a complex link between flooding, soil salinity, rural livelihoods and migration as well as the probable adaptation strategies.
Researchers at International Food Policy Research Institute (IFPRI) and Ohio State University conducted the study titled 'Coastal Climate Change, Soil Salinity and Human Migration in Bangladesh'.
"Many parts of Bangladesh are under severe threat of future sea-level submergence," said Valerie Mueller, an IFPRI senior research fellow.
But studies show migratory response to flooding is likely to be minimal as most farmers in this deltaic region have adapted to the frequency of flooding, she added.
Ms Mueller said, "Our study shows increased soil salinity from rising sea will push nearly 140,000 coastal residents to migrate to another location within their district and nearly 60,000 to alternate districts."
The study shows only a few are likely to migrate to northern areas while most migrants are likely to enter the capital, Dhaka, and their neighbouring coastal districts.
The study is co-authored by Ms Mueller, who is also an assistant professor at Arizona State University, and Joyce Chen of Ohio State University.
It draws on socio-economic data from Bangladesh Bureau of Statistics' sample vital registration system and agricultural production data from household income and expenditure surveys.
To offset the loss from crop production income, coastal households move to aquaculture.
The findings of the study reveal the households engaged in both crop production and aquaculture heavily rely on the latter to buffer adverse environmental conditions.
"As soil salinity increases from low to high levels, we see a nearly 57 per cent rise in share of revenue from aquaculture," said Ms Mueller.
For most households, however, the prohibitive costs associated with converting crop land to fish ponds deter additional diversification.
Increased aquaculture production in the coast, the study pointed out, brings more job opportunities, reducing international migration.
"So, people that would normally migrate abroad might be staying to take jobs that were made available by aquaculture production," said Ms Chen.
"Also, poorer households affected by the changes may have less money available for more expensive international migration," she added.
The increasing soil salinity from the lowest to the highest levels would likely increase internal migration by nearly 25 per cent and decrease international migration by 66 per cent.
The number of migrants who move to a different location within the district is more than double of those who move out of the district.
Ms Chen said, "Financial constraints limit poor households from moving over longer distances, signalling a trapped population dynamic."
This raises concerns that the most vulnerable households may be the least resilient in the face of climate change, she stated.
Only those with greater financial, human or social capital may take the giant step to move internationally, the researcher mentioned.
Due to a rise in soil salinity, Chittagong and Khulna districts are likely to witness highest within-district additional migration, estimated between 15,000 and 30,000 migrants per year.
The districts also contain the second and third largest cities in the country.
Districts without large cities like Bagerhat, Bhola and Feni will expect smaller within-district flows, between 5,000 and 15,000.
"To minimise moving costs and remain close to family, individuals may move inland where the demand for agricultural labour is relatively unaffected by salinity," said Ms Mueller.
However, higher wages and denser labour markets may draw workers instead to urban areas, she added.
With three of the five largest cities in the saline belt, migration may not reduce vulnerability to rising sea in the long run, she added.
The study said salinity reduces total annual crop revenue: households facing moderate saline contamination earn almost Tk 5,000 or 21 per cent less in crop revenue each year.
"Researchers didn't find any significant effect of flooding on crop revenue," it revealed.
Studies also show that in another 120 years, coastal areas, currently home to 1.3 billion people, are projected to be inundated by sea-level rises.
"A two-pronged approach will be necessary to address this growing concern," Ms Mueller suggested.
Government efforts to incentivise individuals that live in key vulnerable areas to relocate may be necessary to detract movement away from Chattagram and Khulna dense areas, she noted.
Expanding the demand for labour in modern industries located in secondary towns can sustain job growth as more and more migrants reach these targeted locations, the study suggested.
Researchers said projects like embankments and polders may have more limited success as salinity raises pressure to increase aquaculture which, in turn, increases the demand for brackish water.
This trend makes households reluctant to maintain such infrastructure, they mentioned.
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