By Richard Black
Environment correspondent, BBC News website
The global network of agricultural research centres warns that famines lie ahead unless new crop strains adapted to a warmer future are developed.
Experimental biotech rice strains can survive prolonged submergence
The Consultative Group on International Agricultural Research (CGIAR) says yields of existing varieties will fall.
New forecasts say warming will shrink South Asia's wheat area by half.
CGIAR is announcing plans to accelerate efforts aimed at developing new strains of staple crops including maize, wheat, rice and sorghum.
At the network's annual meeting in Washington, scientists will also report on measures to reduce greenhouse gas emissions from farmland.
CGIAR links 15 non-profit research institutes around the world working mainly on agriculture in developing countries and the tropics.
"We're talking about a major challenge here," said Louis Verchot of the World Agroforestry Centre (Icraf) in Kenya, a member institute of CGIAR.
"We're talking about challenges that have to be dealt with at every level, from ideas about social justice to the technology of food production," he told the BBC News website.
"We're talking about large scale human migration and the return to large-scale famines in developing countries, something which we decided 40 or 50 years ago was unacceptable and did something about."
The most significant impact of climate change on agriculture is probably changes in rainfall. Some regions are forecast to receive more rain, others to receive less; above all, it will become more variable.
But increasing temperatures can also affect crops. Photosynthesis slows down as the thermometer rises, which also slows the plants' growth and capacity to reproduce.
The water supply to farms will become more variable in future
Research published two years ago shows rice yields are declining by 10% for every degree Celsius increase in night-time temperature.
A study from the International Maize and Wheat Improvement Center (Cimmyt) in Mexico, yet to be published, projects a major decline in South Asia's wheat yield. The vast Indo-Gangetic plain produces about 15% of the world's wheat - but the area suitable for growing is forecast to shrink by about half over the next 50 years, even as the number of mouths to feed increases.
"The livelihoods of billions of people in developing countries, particularly those in the tropics, will be severely challenged as crop yields decline due to shorter growing seasons," said Robert Zeigler, Director General of the International Rice Research Institute (Irri), a CGIAR affiliate.
Conversely, rising temperatures will open up areas of the world which are currently too cold for crop cultivation, in regions such as Siberia and northern North America. And the same Cimmyt study forecasts that wheat will become viable in parts of Alaska.
But the CGIAR figures suggest that extra yield from these regions will not fill the shortfall in the tropics - added to which there are questions of how poorer tropical countries will afford to buy food from richer temperate states.
All this means, CGIAR says, that research into the technological, social and economic dimensions of future farming needs to accelerate.
Within the CGIAR network, various research initiatives are already under way to develop "climate-proof" varieties.
Scientists at Irri in the Philippines have developed strains which can survive floods of several weeks. Serious flooding is forecast to worsen in some Asian countries, notably Bangladesh.
Conversely, some already arid regions of Africa are forecast to become even drier. With sorghum, the line of research being pursued at the International Crops Research Institute for the Semi-Arid Tropics (Icrisat) is to develop strains which can survive drought.
One of the most exciting initiatives aims to make a fundamental modification to rice so it becomes more efficient at using the Sun's energy.
Rice is a so-called C3 plant. Other crops, including maize, use a better photosynthesis mechanism called C4, and Irri scientists aim to develop rice strains which also use the C4 mechanism.
MODERN PLANT-BREEDING TOOLS
Comparative genomics - analysing genes responsible for key traits
Marker-assisted selection - genetic and genomic analysis is used to select varieties to cross
Embryo rescue - plant-breeder's equivalent of special care baby unit, where embryos from difficult crosses are raised in special conditions
"Boosting the photosynthetic efficiency of rice by changing it from C3 to C4 photosynthesis will be like supercharging a car's engine by fitting a new fuel injection system," said Irri's John Sheehy.
Despite reservations in other parts of the world, notably western Europe, genetic modification is becoming one of the staple tools of researchers aiming to enhance developing world agriculture.
"I can understand the opposition to GM, and I sympathise to a certain extent with it," said Dr Verchot.
"But in developing countries we're dealing with a crisis situation; and whatever tool is available, we need to apply it to that situation."
Away from the field of crop improvements, CGIAR scientists will also be detailing approaches to reducing greenhouse gas emissions from farming.
One simple method which is proven, but which by no means all farmers are aware of, is no-till or minimum-till agriculture, where fields are ploughed and disturbed as little as possible. This keeps carbon in the soil rather than sending it into the air as carbon dioxide.
Nitrous oxide (N2O) is a more potent greenhouse gas than CO2, and is released when fertiliser breaks down.
Nitrous oxide detectors can lower greenhouse gas emissions
Scientists with Cimmyt and the International Center for Tropical Agriculture (Ciat) have developed a hand-held sensor using light and infra-red radiation which can tell farmers whether plants need more fertiliser or not; less fertiliser use means less N2O produced.
All this and more will be discussed at the Washington meeting, along with a key question - is enough money going in to fund the acceleration which the CGIAR believes is needed?
Louis Verchot answers with a simple statistic - CGIAR centres, with a mandate to find solutions for the whole of the developing world, have less to spend on research each year than France, for example, spends on research for its own farms.
"We're seeing awareness coming, we're seeing a shifting of resources, but we're clearly well below where we need to be," he said.
"It's much easier to solve a problem before we get to a crisis. With climate change we're definitely talking about a crisis, and it's coming within our lifetimes."
New research projects a northward shift of wheat-growing in North America. (Map is simplified because existing boundaries are highly complex.)