Rising sea levels and deadly floods are already putting tens of millions of lives at risk in Bangladesh, but they bring another problem that threatens the entire nation: waterlogged land and high salinity in streams and soil are killing crops.

Bangladesh ranks seventh for countries most affected by extreme weather in the past two decades, according to the Global Climate Risk Index.

Farmers are desperately trying to adjust to these ever more destructive and unpredictable conditions caused by global warming – from using floating seedbeds to developing salt-resistant rice.

“Even 25 years ago, we could grow crops throughout the year … but then water started to stay here seven months. We were clueless how to survive,” said Altaf Mahmud.

“Most of the farmers here are poor and the land is scarce. But if we can’t grow anything during the seven months, we would starve,” neighbour Mohammad Mostofa added.

So they and other local farmers in Mugarjhor, a region 200km (120 miles) south of Dhaka, revived a century-old technique of using seedbeds that sit atop the water.

They stack layers of water hyacinth and bamboo tied together by their roots to create a raft, 2-4 feet (0.6-1.2 metres) high, on which to plant seeds – often using wood chippings and coconut coir as a fertiliser.

This forms a light, floating vegetable garden – bitter gourds, spinach and okra can all be grown this way – able to rise and fall with the water levels.

The floating farms have become community initiatives, and in some villages, women spend months preparing the beds before boatmen take them across waterlogged fields.

Ever-more frequent cyclones, rising sea levels, floods, erosion, drought and unreliable rains have already displaced millions, either into city slums or abroad.

Those that stay have no choice but to find new ways of working.

Some farmers have stopped growing crops, opting instead to grow shrimp in the brackish water, or crab-fattening – capturing wild crabs and feeding them up to then sell – as well as rearing ducks, which fetch a high price in Dhaka eateries.

Bangladesh Rice Research Institute (BRRI) has created new salt-resistant varieties of the staple crop.

“Normal rice does not grow in saline water. Salinity saps the energy of rice stalks,” explained scientist Alamgir Hossain.

BRRI has now created a strain that can grow in water with triple the saline levels that normal rice can cope with, he said.

This has offered “new hope” to farmers in coastal regions, where seawater is increasingly encroaching the land, he added.

But Saiful Islam, a climate expert at Bangladesh University of Engineering and Technology, said such efforts are a drop in the ocean.

“We need to spend billions to raise and strengthen embankments along our big coastline. We need to create mangrove forests along the coastal belt to work as natural barriers to cyclones, subsidence and sea level rise.”

“We need to build new roads, preserve rainwater and create alternate livelihoods for millions of people. Just inventing crops won’t do. Bangladesh alone can’t do it,” said Islam.

He added that Western nations were “responsible for emitting most of the greenhouse gases” and so needed to help.

Islam said Bangladesh received “barely any” of proposed $100bn set aside by developed nations for adaptation to and mitigation of climate change.