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"The northward movement of a climate zone into an area where crops generally have not been grown does not necessarily mean crops like wheat will do well there," says Dr. Hans Braun, director of the global wheat program at CIMMYT, the Mexico-based crop research institute that conducted the wheat study.
Scientists have made another worrisome discovery, this time about carbon dioxide itself, the main greenhouse gas, which is vital for plant development. It had been assumed in the 1980s, based on greenhouse experiments, that an atmosphere richer in carbon dioxide would stimulate plant growth, raising some crop yields by as much as 30 per cent.
That is part of the reason why, up until now, few people worried much about agriculture and global warming. It was thought that, while climate change might wreak havoc on ice-dependent polar bears and low-lying coastal cities, it held a verdant lining for farmers.
But new research published last year based on experiments in the U.S., Japan, Switzerland and New Zealand found the beneficial effects of carbon dioxide were vastly overrated when crops were grown in the more realistic setting of open farm fields, rather than in greenhouses. Corn yields didn't rise at all, and the rise in wheat and rice yields was less than half previous estimates.
To be sure, not everyone is convinced that crop problems are inevitable.
Donald Coxe, global portfolio strategist for BMO Financial Group, says plant breeders have made remarkable advances in producing crops more tolerant of extreme conditions. "It's quite amazing what they can do," he says.
Mr. Coxe, an investment adviser based in Chicago who follows the commodity markets, where prices would skyrocket if food shortages develop, says last year's corn harvest was a case in point.
Illinois, at the heart of the U.S. corn belt, was sizzled by heat and drought, but many farmers still managed a decent crop thanks to seeds bred to give plants more resistance to drought.
"Illinois was a shocker, frankly, last year, even to ag people. They were amazed," he says.
Researchers affiliated with CGIAR have called for a massive program to develop crops that will be able to cope with global warming, and these developments may well pan out.
But if efforts fail, Mr. Brown, for one, is warning the consequences could be dire, because food supplies are essential for global stability.
Smaller grain harvests will translate into sharply higher food prices. Soaring prices, says Mr. Brown, "could lead to urban food riots in scores of countries around the world, and those food riots could lead to political instability and that political instability could begin to undermine global economic progress."