The fragile state of the world's food supplies has become increasingly apparent in recent years, as extreme weather events have been linked to spikes in commodity prices and food riots, bringing into sharp focus the threat of climate change.
Now, researchers say they have a much better quantitative understanding of the role of extreme weather in disruptions in food supply — and their conclusions are raising eyebrows.
Looking at cereals such as wheat, rice, maize, and barley between 1964 and 2007, researchers from the University of British Columbia (UBC), University of Sussex, and McGill University found that droughts and heat waves reduced global production up to 10 percent. In contrast, the researchers found little or no impact on production from either cold weather spells or floods.
The worst effects of these weather disasters were in the developed world with the hardest hit regions in Australia, North America, and Europe. Production levels in North America, Europe, and the islands of Australia, New Zealand, and New Guinea dropped by an average of 19.9 percent because of droughts — roughly double the global average.
"We have always known that extreme weather causes crop production losses," said UBC's Navin Ramankutty. "But until now we did not know exactly how much global production was lost to such extreme weather events, and how they varied by different regions of the world."
Ramankutty and his colleagues concluded that heat and drought had a greater impact than floods and cold mostly because of timing. A deep freeze or a snowstorm, they suggest, typically occurs outside the growing season, while floods are most common in the spring just as crops are being planted.
A drought or heat wave, though, strikes when farmers are ready to harvest their crops and are often "large-scale events."
"Our hypothesis is that extreme heat and droughts happen at the peak of your growing season," Ramankutty said. "Think about a temperate growing season. We plant in the spring. The plant grows. It matures. Right when it matures and you have full grain and you're in mid-summer, you have extreme heat or a drought."
The developed world seems more susceptible, they said, partly because rich countries are more inclined to produce a narrow number of commodities on an industrial scale, compared to a wide range of crops planted by a larger number of individual farmers in the developing world.
"Across the breadbaskets of North America, the crops and methods of farming are very uniform across huge areas. So if a drought hits in a way that is damaging to those crops, they will all suffer," said McGill University's Corey Lesk. "By contrast, in much of the developing world, crop systems are a patchwork of small fields with diverse crops. If a drought hits, some of those crops may be damaged, but others may survive."
Another factor, the researchers said, might be that farmers in the developing world are inclined to avoid risk compared to their richer counterparts in the West.
"Farmers in developed world have access to crop insurance and price support schemes. They don't have an incentive to worry about climate shocks," Ramankutty said. "Developing countries farmers typically don't have these price support schemes. They tend to choose risk-minimization strategies. They are much more interested in having stable yields rather than high yields … so they are more resilient in that sense."
Pedram Rowhan of the University of Sussex in Brighton, UK worked with Ramankutty and Lesk on the study. Their findings were published in the journal Nature.
But Stephan Baas, an advisor to the Food and Agriculture Organization of the United Nations, took issue with the study's conclusion that floods had only a minor effect on production and that rich nations suffered the most.
"We think this is fundamentally the opposite," he said. "It's wrong."
"Our perspective is clearly that it's developing countries that suffer most, not because their losses are the highest," he said. "The losses may be higher in developed countries but the overall economic damage as compared to other sectors is much lower."
In other words, in Africa or Asia, where agriculture might account for 30 percent of GDP, the impact might be greater than a country like Germany, where the dollar amount of damages to the sector might be higher, but account for a much smaller share of the country's overall economy.
While current hits to production are up to 10 percent, Ramankutty worries that a few years down the road that might rise to 12 or 15 percent, which he called "serious."
"It might not affect total production capacity globally in the sense that we have grain reserves and so on," he said. "But what it might do is, what has happened previously, which is that when some of these extreme events happen and they happen in multiple places simultaneously, food prices might go up and that then effects food security."
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