"Bangladesh set to disappear under the waves by the end of the century"

This spring, I took a month-long road trip across a country that we – you, me and everyone we know – are killing.

One day, not long into my journey, I travelled over tiny ridges and groaning bridges on the back of a motorbike to reach the remote village of Munshigonj. The surviving villagers – gaunt, creased people – were sitting by a stagnant pond. They told me, slowly, what we have done to them.

Ten years ago, the village began to die. First, many of the trees turned a strange brownish-yellow colour and rotted. Then the rice paddies stopped growing and festered in the water. Then the fish floated to the surface of the rivers, gasping. Then many of the animals began to die. Then many of the children began to die.

The waters flowing through Munshigonj – which had once been sweet and clear and teeming with life – had turned salty and dead.

Arita Rani, a 25-year-old, sat looking at the salt water, swaddled in a blue sari and her grief. "We couldn't drink the water from the river, because it was suddenly full of salt and made us sick," she said. "So I had to give my children water from this pond. I knew it was a bad idea. People wash in this pond. It's dirty. So we all got dysentery." She keeps staring at its surface. "I have had it for 10 years now. You feel weak all the time, and you have terrible stomach pains. You need to run to the toilet 10 times a day. My boy Shupria was seven and he had this for his whole life. He was so weak, and kept getting coughs and fevers. And then one morning..."

Her mother interrupted the trailing silence. "He died," she said. Now Arita's surviving three-year-old, Ashik, is sick, too. He is sprawled on his back on the floor. He keeps collapsing; his eyes are watery and distant. His distended stomach feels like a balloon pumped full of water. "Why did this happen?" Arita asked.

It is happening because of us. Every flight, every hamburger, every coal power plant, ends here, with this. Bangladesh is a flat, low-lying land made of silt, squeezed in between the melting mountains of the Himalayas and the rising seas of the Bay of Bengal. As the world warms, the sea is swelling – and wiping Bangladesh off the map.

Deep below the ground of Munshigonj and thousands of villages like it, salt water is swelling up. It is this process – called "saline inundation" – that killed their trees and their fields and contaminated their drinking water. Some farmers have shifted from growing rice to farming shrimp – but that employs less than a quarter of the people, and it makes them dependent on a fickle export market. The scientific evidence shows that unless we change now, this salt water will keep rising and rising, until everything here is ocean....

What happens to a country's mind as it drowns? Professor Philip Jenkins of Pennsylvania State University believes he can glimpse the answer: "The connection between climate change and religious violence is not tenuous," he says. "In fact, there's a historical indicator of how it could unfold: the Little Ice Age."

Between the ninth and 13th centuries, the northern hemisphere went through a natural phase of global warming. The harvests lasted longer – so there were more crops, and more leisure. Universities and the arts began to flower. But then in the late 13th century, the Little Ice Age struck. Crop production fell, and pack ice formed in the oceans, wrecking trade routes. People began to starve.

"In this climate of death and horror, people cast about for scapegoats, even before the Black Death struck," he says. Tolerance withered with the climate shocks: the Church declared witchcraft a heresy; the Jews began to be expelled from Britain. There was, he says, "a very close correlation between the cooling and a region-wide heightening of violent intolerance."

This time, there will be no need for imaginary scapegoats. The people responsible are on every TV screen, revving up their engines....