SINGAPORE (Reuters) - If farmers think they have a tough time producing enough rice, wheat and other grain crops, global warming is going to present a whole new world of challenges in the race to produce more food, scientists say.
In a warmer world beset by greater extremes of droughts and floods, farmers will have to change crop management practices, grow tougher plant varieties and be prepared for constant change in the way they operate, scientists say.
"There certainly are going to be lots of challenges in the future. Temperature is one of them, water is another," said Lisa Ainsworth, a molecular biologist with the United States Department of Agriculture.
Spiraling grain prices in recent months have startled governments long used to affordable rice, wheat, soy and maize.
But rising demand and likely greater climate variability and more fluctuations in crop output could mean even more uncertainty for prices.
Current estimates suggest demand for cereals will jump by more than 50 percent by 2050 as the world's population rises from 6.6 billion to about 9 billion.
The world has already warmed by about 0.7 degrees Celsius on average since the 1800s and studies show higher temperatures can cut yields, particularly in the tropics where a lot of rice is grown.
"In Northeastern China, low temperatures, a short growing season and lack of water limit production, so rising temperatures in the future may have beneficial impacts there," said Ainsworth.
"However, in the southern parts of the country, higher temperatures will likely cause yield losses," she told Reuters.
Pollution is another threat. Ozone, which is produced at ground-level by sunlight interacting with pollution from burning fossil fuels, can cut plant productivity. The higher the ozone levels, the worse the damage.
In the northern hemisphere, ozone is a growing problem and is estimated to cost farmers billions of dollars in lost production.
"In the major rice-growing regions, which are India and China, ground-level ozone concentrations even today are very high and certainly exceed the threshold for damage. Ozone is already decreasing yield potential in many areas," Ainsworth said.
Rice is grown in more than 100 countries. More than half the world's population rely on it as their main food source.
A major U.N. climate panel report last year said in low-latitude regions, even moderate temperature rises of between 1 and 2 degrees Celsius could trim crop yields.
In cooler mid to high-latitude regions, localized increases up to 3 degrees Celsius plus higher amounts of carbon dioxide in the air could be beneficial to yields, provided there was enough water.
But anything beyond 3 degrees locally would be bad for crop yields, said the panel's Fourth Assessment Report on the impacts of climate change, which drew on the expertise of hundreds of scientists from around the world.
Studies show that higher carbon dioxide levels, on their own, can boost rice and wheat production but any benefit is negated by the stress caused by higher temperatures, lack of water from changes in rainfall patterns and pollution.
Plants breathe in CO2 to make sugars and other complex compounds grow. Carbon dioxide levels from burning fossil fuels are rising rapidly and are now the highest than at any time in the past 650,000 years.
Last year, they reached near 390 parts per million from about 280 ppm at the start of the Industrial Revolution. Unless nations drastically rein in their carbon emissions, the level could reach 550 ppm by 2050, the climate panel says, leading to sharp increases in temperatures, particularly at higher latitudes.
The trick for farmers is to adapt, and scientists say the crops of the coming decades will have to be very different.
"We will see a suite of new traits that are integrated into the varieties that are planted in the future," said Ainsworth.
"This is something we do need to start soon because anything that we hope to develop today probably won't be in the field until 2018 and the climate then will be very different."
In the Philippines, the International Rice Research Institute has a long history of developing rice varieties. But the pace of climate change in the future is a big worry.
"There is still a lot of uncertainty in the climate modeling when it comes to the regional level," said Reiner Wassmann of IRRI. But it was clear temperatures would rise.
"The other mega trend we see is that we will have more climate extremes. In some places there might be more drought, in others it may be submergence, from floods, in some places it might be both," said Wassmann, coordinator of the Rice and Climate Change Consortium at IRRI.
"That is really a new challenge for development of cropping systems and I don't want to limit it to only plant breeding. We have to be clear that this is no silver bullet and that if we speed-up plant breeding everything will be fine. Certainly not.
"We also have to improve crop management and water saving techniques have come into the picture to cope with drought," he said.
In major wheat grower Australia, higher temperatures will threaten crop yields, depending on the location. But it will be changes in rainfall that are likely to have the biggest impact.
"A rise of up to 2 degrees Celsius, that by itself could be coped with. The much bigger problem is what happens to rainfall," said Mark Howden, a senior scientist with Australia's state-backed research body the CSIRO.
"Once we get above a 10-15 percent reduction in rainfall we start to get into negative conditions for everything," he said.
"Under those circumstances we would see major declines in wheat production even allowing for the technological improvements that we've got under our belts that we could use," said Howden, theme leader of the CSIRO's climate adaptation program.
If the world keeps on emitting greenhouse gases at the present rate, computer models suggest southern Australia will become much drier.
"We will need to think about smart ways of feeding the world," said Howden, who called for more investment for research by government and industry.
"We are looking at a requirement to double agricultural production by 2050, so this isn't the time when we have the luxury of cutting back our agriculture. We're on a treadmill (and) we need to keeping running faster and faster."
Editing by Megan Goldin