* Food output makes up 19-29 pct of man-made greenhouse gas
* Cuts needed, from fertilisers to transport -CGIAR study
* Climate change may force shifts to alternative crops
By Environment Correspondent Alister Doyle
OSLO, Oct 31 (Reuters) - Food production accounts for up to 29 percent of man-made greenhouse gases, twice the amount the United Nations has estimated comes from farming, a study published on Wednesday said.
Looking at emissions across the food system - including forest clearance, fertiliser production and transport - rather than just farming itself - agriculture research organisation CGIAR said much more work was needed to cut climate change emissions from food.
Its report, "Climate Change and Food Systems", estimated food production was responsible for between 19 and 29 percent of mankind's total greenhouse emissions, far above U.N. estimates of 14 percent based on a narrower definition of farming.
"From a food point of view (the U.N. approach) doesn't make sense," said Bruce Campbell, who heads the CGIAR research programme on climate change, agriculture and food security.
Many countries could make big cost savings by cutting emissions, he said. "There are good economic reasons to improve efficiency in agriculture, not just to cut greenhouse gas emissions."
China, for instance, could sharply reduce emissions with more efficient manufacture of fertilisers. Britain could cut emissions by consuming lamb transported from more efficient farms in New Zealand rather than raising its own sheep.
Global changes in diet, shifting towards vegetarianism from meat, would also help. Growing crops to feed to cows, pigs or sheep takes up far more land and emits more greenhouse gases than producing crops for human consumption.
MAIZE, WHEAT, RICE
A separate report by the CGIAR climate programme indicated that climate change is likely to reduce yields of the three biggest crops judged by calorie production - maize, wheat and rice - in developing nations in coming decades.
That could force some farmers to make radical shifts to growing more heat-, flood- or drought-tolerant crops, according to the report, "Recalibrating Food Production in the Developing World".
More resilient crops including yam, barley, cowpea, millet, lentils, cassava and bananas could fill in the gaps caused by declining harvests of more sensitive crops, it said.
"The world's agricultural systems face an uphill struggle in feeding a projected nine to ten billion people by 2050. Climate change introduces a significant hurdle in this struggle," it said. The world population is now just above seven billion.
The study also said that global warming, blamed by a U.N. panel of climate experts mainly on the burning of fossil fuels, meant risks to food production far beyond fields.
"Every step of the food chain - from the seed to the farm to the cooking pot - is at risk," it said. Higher temperatures or floods could make it harder to store and transport food, for instance, meaning more outbreaks of food-borne illnesses. (Reporting By Alister Doyle, Environment Correspondent; Editing by Robin Pomeroy)