The ceremony is called Kuomboka, meaning "moving out of the water". Every year the king of the Lozi people journeys from the flooded plains to higher ground. Thousands gather to dance, feast and watch the royal barge rowed by dozens of oarsmen beneath a giant replica elephant.

The Kuomboka is traditionally the cue for local people to follow the king in escaping the rising waters, but the reality of climate change is catching up with this colourful ritual. The most recent flood came too soon and too strong, killing at least 31 people in Zambia's impoverished western province. The devastating aftermath has left people starving and homeless.

"Flooding here is an annual event, but it came earlier than expected and people were caught off guard," said Raphael Mutiku, a public health engineer for Oxfam in Mongu.

The Red Cross recently warned that global warming will lead to more disasters and suffering along the entire Zambezi river basin, where floods have increased dramatically in recent years.

The Zambezi once flooded the plains as predictably as the changing seasons, in late March or early April. But now the great river is less regular and more extreme. The volatile climate – annual rainfall has risen in recent years from 900mm to 1,300mm – is disrupting rhythms that have sustained generations. Crops that should have been harvested in January or February this year were destroyed by flooding that began in November. Even on higher ground, cassava crops were no longer safe.

Thousands of people have been forced to move further inland than ever before without food or sanitation. They have become refugees in their own country, camping in informal settlements accessible only by boat. They cannot grow crops as the land is infertile, they are exposed to malarial mosquitoes and respiratory infections, and are cut off from hospitals and schools.

Lutangu Mulambwa, 25, and his wife Sandra, 17, had to flee their home in a canoe with their 10-month-old daughter, Mulima, and found refuge 15km away. The maize crop on which they depend is lost. "It's totally gone," said Lutangu, sitting outside a shelter improvised from dry reeds. "There is nothing at all we can do for food here to sustain our lives. We are dying of hunger."

Elsewhere in the Kaama settlement, a patch of scrubland where children in torn clothes play in the dirt, Nasilele Sapilo, 70, wondered how she is going to feed her five grandchildren. "We planted maize and pumpkin to sustain us for the whole year but we've lost over three-quarters to the floods," she said. "I move from the low ground every year, but this time the rain was heavy and the houses submerged to roof level."

The family home is 17km away. Nasilele's grandson, Liyiungu, nine, wearing a ragged green jacket and a filthy vest, said: "I don't have soap or schoolbooks – they were swept away by water. I miss my socks and school shoes."

In another village, Liyoyelo, the floods have receded and people were starting to rebuild their lives. The waterline was visible on the wall of a wooden shack. In one corner, a film of brown soil clung to an old vinyl record player.

The village of more than 200 people was now a sprawl of ruined homes and fetid cesspools. Before, people braved the floods and stayed at home, but not this time. "It came in early December and in 12 hours the water filled the yard," recalled Mukelabai Ilishebo. "Our maize was lost and our home destroyed. The blankets and clothes are gone."

Mukelabai, 25, and her family packed all the belongings they could into a canoe and paddled 24km to safety. After four months they came back to find the roof of their home fallen in and the mudbrick walls crumbling away. She added: "We are having to start again. There is no food so we are not eating anything. My husband has no job. I worry about the children."

Elsewhere, at Soola, the settlement resembled even more closely a desolate refugee camp, with shelters fabricated from thatch and reeds and draped with dirty clothes and blankets. Remnants of sweet potato tubers were scattered on the ground. An area where homes used to be was now a muddy wasteland save for a single door, standing like the freak survivor of a shockwave that vaporised everything else.

Masela Kababa, 30, a mother of three young children, said: "There isn't enough food to feed the children. They all have aching joints and eye infections. There's nothing we can do."

She was pessimistic. "This problem is here to stay. I think it will keep happening to the end of time."

Today's report from Oxfam on the human cost of climate change calls on world leaders attending this week's G8 meeting to act now on behalf of those already suffering its consequences.

Raphael Mutiku of Oxfam said: "We have types of catastrophe such as volcanoes and tsunamis, but now our focus is shifting towards climatically induced matters. The question is, how do you respond so you don't see the same crisis next year, and the year after?"