"The outlook is bleak. If the average global temperatures rise by two degrees or more, then most of Uganda is likely to cease to be suitable for coffee..this may happen in 40 years or perhaps as little as 30," the report said.

The report, "Turning up the heat, Climate Change and Poverty in Uganda," said effects of global warming like increasing temperatures, more intense rains and storms, had led to erratic rainfall patterns in Uganda.

Coffee output in 2007/08 (Oct-Sept) is seen at 2.85 million bags, up from 2.7 million the year before.

"According to the United Nations Environmental Programme, only patches of land on the periphery will still be able to grow coffee...In the meantime, coffee farmers are going to have to adapt to rising temperatures," the report said.

Across much of Uganda, the climate is bimodal, meaning that there are two rainy seasons -- the first from March to June and the second from October/November to December/January.

Rainfall during the rainy seasons has become unreliable, it said, adding that reduced rain during the March to June season was causing drought, reductions in crop yields and plant varieties.

The late season rainfall was coming in more intense and destructive downpours, bringing floods, landslides and soil erosion, it said.

"But, farmers have continued to invest in Uganda's Robusta coffee and export earnings have continued to increase. This has helped protect losses from climatic problems," said Philip Gitao, head of the East African Fine Coffees Association.

Farmers have also adopted good husbandry practices such as using more hardy coffee plants, added Gitao, who was quoted in the Oxfam report. (Writing by Wangui Kanina) (For full Reuters Africa coverage and to have your say on the top issues, visit http://africa.reuters.com/)