By Jonathan Amos
BBC News science reporter, Dublin
Climate change threatens to put far more people at risk of hunger over the next 50 years than previously thought, according to new research.
Africa is expected to take the brunt of the changes
Scientists say expected shifts in rain patterns and temperatures over that time could lead to an extra 50 million people struggling to get enough food.
And the situation could be even worse if the important cereal crops do not show the improved yields many expect.
US and UK teams reported this grim assessment at a conference in Dublin.
"We expect climate change to aggravate current problems," Professor Martin Parry told the British Association's Festival of Science.
"If we accept that broadly 500 million people are at risk today, we expect that to increase by about 10% by the middle part of this century."
The researcher is part of a team at the UK Met Office's Hadley Centre for Climate Prediction that has been modelling the future impact of climate change for the past 10 years.
He said sharp reductions in the emissions of the greenhouse gases thought responsible for global warming could counteract the damaging effects expected on agriculture - but this would not be achieved under the current climate treaty, the Kyoto Protocol.
"Climate change as you know has an inertia, so even if we were to chop emissions off at the knees now we've got 40 or 50 years of warming and drying to go," he said.
Most of those extra individuals projected to be put at risk of hunger would be in Africa, according to the research.
Professor Parry said this underlined the need for world governments to meet their development goals and lift people out of poverty. Giving them a higher standard of living would enable them to better adapt to the changes that were coming, he added.
But if the Met Office study seemed depressing in itself, new research from the University of Illinois in the US suggests an even worse scenario.
Scientists there have been growing crops in the field and exposing them to elevated levels of carbon dioxide via a system of pipes and valves controlled by computer.
Previous greenhouse studies had suggested that higher levels of CO2 - which plants take out of the atmosphere to make food for themselves - would actually help boost yields in some places in the coming years.
And some assessments have indicated this may actually offset losses projected to come from changes in precipitation and rising temperatures elsewhere on the planet.
But the Illinois work points to a more pessimistic outcome, the university's Professor Steve Long told the meeting.
"The alarming result is that the yield increase we see due to raised carbon dioxide is only about half that predicted and in the case of maize - and last year there was more maize than any other crop in terms of tonnes globally - we see no increase at all," he said.
These Free Air Concentration Enrichment (Face) experiments are among the first that try to replicate real field conditions. They were run on a range of crops such as rice, wheat, maize and soya, and showed the hoped for gains in a CO2-rich world to be unachievable in all of them.
Professor Parry commented that if the Illinois work was correct then crop losses across Africa currently projected to be 2.5-5% over the next 50 years could actually be as high as 10%.
And Friends of the Earth said the new research highlighted the colossal economic and political challenge ahead.
"Famine is not caused by a shortage of food; it's caused by a shortage of justice," the FoE's Duncan McLaren told the BA festival.
"In a world where there are even greater constraints to the availability of environmental resources, including food, that shortage of justice is going to become even more critical."