* Bridges, roads could be washed away, hitting harvest transport
* "High degree of confidence" weather extremes linked to climate change
* Enough food, but problem is distribution - professor
By Environment Correspondent Alister Doyle
OSLO, Aug 15 (Reuters) - Downpours and heatwaves caused by climate change could disrupt food supplies from the fields to the supermarkets, raising the risk of more price spikes such as this year's leap triggered by drought in the United States.
Food security experts working on a chapter in a U.N. overview of global warming due in 2014 said governments should take more account of how extremes of heat, droughts or floods could affect food supplies from seeds to consumers' plates.
"It has not been properly recognised yet that we are dealing with a food system here. There is a whole chain that is also going to be affected by climate change," Professor Dr John Porter of the University of Copenhagen said.
"It is more than just the fact that there are droughts in the United States that will reduce yields," he said. Like the other experts, he said was giving personal opinions, not those of the U.N. panel.
After harvest, floods could wash away roads or bridges, for instance, between fields and factories processing the crop. Or warehouses storing food could be damaged by more powerful storms. Such factors were likely to hit poor nations hardest.
"There are reasons to expect more frequent (price) spikes, given that it will be more common to see conditions that are considered extreme," said David Lobell, an assistant professor at Stanford University in California.
Other factors could dampen rises, however, "including responses such as raising grain storage or changing trade policies". He said Stanford was trying to produce models of the likelihood of price spikes to understand the risks.
"It's a distributional problem - there is enough food in the world. But the distribution doesn't work," said Bruce McCarl, a professor at Texas A&M University. Climate extremes could aggravate food price swings, he said.
The worst drought in five decades in the United States has pushed up corn prices by more than 50 percent from late May to record highs above $8 a bushel. Hot, dry weather has also hit crops in southern Europe.
A U.N. report on climate extremes in March said it was "virtually certain" that days with extreme heat would increase. Among other findings, it said it was likely that downpours would increase as a percentage of total rainfall.
Scientists are traditionally wary of linking individual extremes such as the U.S. drought to climate change - weather events from heatwaves to dust storms have happened throughout history.
But James Hansen, director of the NASA Goddard Institute for Space Studies, expressed "a high degree of confidence" this month that the European heatwave of 2003, the Russian heatwave of 2010 and the Texas and Oklahoma droughts of 2011 were "a consequence of climate change" because they were so extreme.
His conclusion was challenged as too definite, even by some experts who say risks of such events are rising with greenhouse gas emissions, led by China and the United States.
On the positive side for food output, a slight rise in temperatures is likely to help plant growth overall.
But long-term net benefits are doubtful, especially because U.N. studies say rising greenhouse gas emissions are on track to push temperatures up by more than 2 Celsius (3.6 Fahrenheit) above pre-industrial times, set by 200 nations as a threshold for dangerous change.
Temperatures have already risen by almost 1 degree (1.6F). Nations such as Australia could lose out more than others benefit.
"In Australia there are huge areas where you can grow wheat. If that goes, I don't think there are northern areas that can take up the huge production lost," said Kaija Hakala, of MTT Agrifood Research Finland.
With more frequent climate extremes, researchers said there would be hard choices with a projected rise in the world population to 9 billion people by 2050 from 7 billion now. They urged more research into drought- or flood-resistant crops.
"We may be hitting a point where it's getting harder to get technological progress" in raising yields, McCarl said. Annual yield growth for U.S. corn had slowed to about 1.5 percent from stellar rates of about 3.5 percent in the early 1970s.
Porter said the world had so far escaped predictions that population growth would outstrip food production, most famously by English writer Thomas Malthus in 1798.
But he said the world now had triple goals of producing food for people, crops for biofuels and feed for animals, often raised for their meat. "In my view we can have two out of those three and not all three," he said.
A shift towards more vegetarian diets would help, he said.