A Stanford University study to be published on Friday estimates that African growing seasons for the continent's staple foods -- maize, millet and sorghum -- will be hotter in nine out of 10 years by 2050. Farmers can adapt by shifting growing times or using new varieties but the pace of change will require extra help, the study in the journal Global Environmental Change concludes. "For a majority of Africa's farmers, warming will rapidly take climate not only beyond the range of their personal experience but also beyond the experience of other farmers within their country," said the study, whose authors were from Stanford and the Global Crop Diversity Trust.

The United States, Britain and the European Union have all recently reviewed the impacts of climate change. The White House published this week a report which forecast that heat, floods, drought and pests would harm food yields and concluded, for example, that cranberry production may no longer be possible by 2050 in its East Coast heartland.

Britain said on Thursday that 2 degrees celsius hotter summers were "inevitable" in southern England by the 2040s and cited a survey of farmers which found half already affected. The EU's executive Commission pointed in April to research which showed a net positive effect for the next 30 years, for example from milder winters, but with increasingly negative effects. "The magnitude of climatic changes may exceed the adaptation capacity of many farmers," it said.

LAG The development of more resistant crops, for example, involves a decade or more to breed or design new varieties, screen these and get them into the hands of farmers. One initiative is the 2001 International Treaty on plant genetic resources for food and agriculture. Its 121 signatories have agreed to give up any patent claims on their native food crops, to speed up the sharing of different strains. But the treaty will have to overcome a lack of funding and poor seed banks in countries in Africa which have rich seams of heat and drought-resistant varieties.

"There was a lack of funds. This is the big challenge," said Shakeel Bhatti, secretary of the treaty's governing body. Earlier this month signatories agreed a target fund of $116 million by 2014 to boost diversity of crop species, especially in developing countries, but have pledges of less than 3 million euros ($4.19 million) so far.

Under the UN-led Kyoto Protocol to fight climate change a global adaptation fund earns a commission from global carbon trading and has so far accrued 66 million euros ($92.11 million). If seed banks were improved, many African countries could switch to crop varieties already grown in hotter parts of the continent. But several nations in the Sahel will have to change food crops altogether, for example from maize to millet, the Stanford study found.