By David Shukman
BBC News, Bangladesh
Flood defences in many parts of the country are not up to job, experts say
Up to 20 million people in low-lying Bangladesh are at risk from rising sea levels in the coming decades, according to new research.
Scientists predict that salty water could reach far inland, making it hard to cultivate staple foods like rice.
The research comes as the government appeals for $5bn (£3bn) over five years to combat climate change.
In May, Cyclone Aila left thousands homeless, killed many and caused widespread flooding and damage.
The predictions come from the Centre for Environmental and Geographic Information Services (Cegis) in Bangladesh.
It suggests a surprisingly small area of land will be permanently lost to the waters, but notes that vast tracts in the south-west could be inundated every monsoon season.
Ahmadul Hassan, a senior scientist at Cegis, told the BBC that the intrusion of salt water would disrupt rice production in one of Bangladesh's poorest regions.
"These are very poor people, and vulnerable. For four months they'll have nowhere to work," he said.
Floating gardens help in vulnerable areas - Video of projects courtesy of DfID
"So people will migrate to the cities for jobs, because of the uncomfortable situation with sea level rise.
"We are talking about 20 million people," he adds.
According to the researchers, data from 11 Bangladeshi monitoring stations shows an average sea-level rise of 5mm per year over the last 30 years, with climate models forecasting further rises.
Of Bangladesh's total rice production, nearly half is so-called "monsoon" rice and much of that is grown in the areas most vulnerable to flooding.
In an interview with BBC News, Bangladesh's Minister of Disaster Management, Dr Muhammed Abdur Razzaque, said he wanted sea defences similar to those in Holland.
"We have to have new designs for embankments and we have to raise their height," he said.
"We are expecting $5bn over the next five years in support from the international community.
"This must be a grant, not a loan with interest," he stipulated.
Bangladesh is among a number of developing countries campaigning for finance to help adapt to the effects of climate change.
There are hopes that the richest nations will agree to massive funding at the UN climate conference in Copenhagen in December.
Staff from the charity Oxfam point to the damage caused by Cyclone Aila last May to highlight why Bangladesh needs help preparing for future sea-level rise.
Abdul Khaleque is managing Oxfam's emergency response in Satkhira region, where more than 20,000 people lost their homes on Gabura Island.
He said: "This place is very near to the sea and we know climate change is causing sea levels to rise.
"If the situation gets worse then these people cannot go back to their villages, so permanent arrangements to improve these embankments need to be made."
Four months after the cyclone, the sea defences are still breached and the island floods with every high tide.
The chairman of the Gabura Island "union" or council, Shofiul Ajam Lenin, is calling for the embankments to be far higher.
"If the current design is not changed then not only my union, but the other unions as well will not exist."
The flooding has ruined the island's freshwater supplies and hygiene in the camp is poor.
Among those living in tents on a narrow strip of high ground is Asma Khatun, a 25-year-old widow, who is now eager to leave.
"I think it is not possible to live in this country any longer. We have to move to other countries.
"We can't live here just by drinking this water. It is not possible to live here."