The anti-poverty group Oxfam says global warming is altering the human food supply and threatening some of the world's poorest people with hunger. At the United Nations Conference on Climate Change in Bali, the group argued developed countries should pay to address the problem, and costs could top $50 billion a year. Trish Anderton reports from Bali.

Climate change, which many scientists believe is fueled by human activities, alters rain cycles and causes increased droughts and flooding.

Rising temperatures mean some traditional crops do not grow as well as they used to. The United Nations predicts that in some African countries, crop yields could fall by half.

Oxfam researcher Kate Raworth says coping with these issues will be expensive.

"Oxfam has estimated that for all developing countries to adapt to climate change will cost at least $50 billion a year," she said. "And for that to be done with justice we believe that the rich and the most polluting countries should pay the vast share of the money to make possible for those countries most vulnerable and least responsible for causing the problem to cope with the new realities they have to deal with."

A report released by Oxfam at the conference on Tuesday says some countries are already feeling the effects of a warming planet. In South Africa, farmers are planting faster-maturing crops to adapt to unpredictable rainfall.

Bangladeshis are creating floating vegetable gardens that can thrive in spite of floods. But Mozaharul Alam, who heads Bangladesh's climate adaptation effort, says overall improvements to the food system still leave many families vulnerable.

"Most of the agriculture is subsistent in nature," said Alam. "So if [there is] any failure of the crop on the ground, even if there is food available in the market, unless the people has the buying capacity, that availability of the food in the market has no meaning to the vulnerable family."

Oxfam argues the United States, the European Union, Japan, Canada and Australia should be among the major sources of funds to help developing countries solve their climate change problems.

These countries have agreed in principle, but one task of the Bali Conference is to find out how much individual countries are willing to spend on climate-related issues.

It also aims to begin deciding how to divide those resources between prevention of further climate change, and adaptation to the changes that are already taking place.

The conference involves thousands of scientists, government officials and development group representatives, and runs for another 10 days.