

Hunger may become the defining human tragedy of the century as the climate changes and hundreds of millions of farmers already struggling to grow enough food are forced to adapt to drought and different rainfall patterns, a report warns.

Oxfam International, in a comprehensive look at the expected effects on people of climate change, says some of the world's staple crops will be hit and the implications for millions could be disastrous .

"Climate change's most savage impact on humanity in the near future is likely to be in the increase in hunger … the countries with existing problems in feeding their people are those most at risk from climate change," the report warns.

"Millions of farmers will have to give up traditional crops as they experience changes in the seasons that they and their ancestors have depended on. Climate-related hunger [may become] the defining human tragedy of this century."

The report, published as world leaders prepare to meet for the G8 summit in Italy, says that farmers around the world are already seeing changes in weather patterns which are leading to increased ill-health, hunger and poverty. Oxfam staff in 15 countries collected records from communities and observed that:

• Seasons appear to have shrunk in number and variety.

• Rainfall is more unpredictable, tending to be shorter in duration.

• Winds and storms are felt to have increased in strength.

• Unseasonal events such as storms, dense fogs and heavier rains are more common.

"Once-distinct seasons are shifting and the rains are disappearing. Poor farmers from Bangladesh to Uganda and Nicaragua, no longer able to rely on centuries of farming experience, are facing failed harvest after failed harvest," it says.

The evidence of changing weather patterns is anecdotal but the results are striking because of the extraordinary consistency they show across the world, said Oxfam programme researcher John Magrath.

"Farmers are all saying very similar things: the seasons are changing. Moderate, temperate seasons are shrinking and vanishing. Seasons are becoming hotter and drier, rainy seasons shorter and more violent," said Magrath.

The report, released before the G8 meeting in Italy this week, where Barack Obama will chair a session on climate change, warns that without immediate action on climate all the development gains made in 50 years are under threat.

Rice and maize, two of the world's most important crops, on which hundreds of millions of people depend, face significant drops in yields. Maize yields are forecast to drop by 15% or more by 2020 in much of sub-Saharan Africa and in most of India.

The report also documents how rising temperatures are affecting productivity in factories, with manual workers needing longer siesta times and outdoor workers experiencing dehydration. Cities in the tropics are becoming some of the most dangerous places in the world as heat stress increases, it says.

The "heat island effect", where heat retention in concrete and air conditioning combines to raise night temperatures in tropical cities by as much as 10C, can devastate vulnerable populations.

"Projections suggest a sixfold increase in heat-related deaths in Lisbon by 2050, and a fivefold increase in Greater London, two to seven times more deaths in California and a 75% increase in deaths among old people in Australian cities."

In Delhi, mortality rates rise by up to 4% with every 1C of temperature rise. The figure is 6% in Bangkok.

It also says many diseases are already migrating as temperatures rise. Malaria, dengue fever, river blindness and yellow fever are all considered highly likely to increase their distribution, it says.