Drought, heat slash grain crops

Drought and heatwaves depleted grain harvests by 10 percent from 1964 to 2007, with sharper losses in the latter two decades and rich nations, reports a study released Wednesday.

The first global overview of how extreme weather disasters affect grain output comes as climate scientists project even more severe and frequent warming over the next half-century.

At the same time, other research has shown, food production will probably need to double by 2050 to feed a population of more than nine billion people.

The study "highlights the important historical effects of extreme weather disasters on agriculture," the authors note in a study published in the journal Nature.

It also "emphasises the urgency with which the global cereal production system must adapt to extremes in a changing climate."

A trio of researchers led by Corey Lesk of McGill University in Montreal crunched data from 177 countries covering nearly 3,000 heatwaves, drought and floods.

Using average yields as a benchmark, they looked at how extreme weather events affected output of 16 cereals, including wheat, maize and rice.

"Until now we did not know exactly how much global production was lost," said co-author Navin Ramankutty, also of McGill.

During the 43-year period examined, some 1.2 billion tonnes of grain were lost to heatwaves, and 1.8 billion tonnes to drought -- the equivalent of global wheat and maize output for 2013.

As expected, the impact of heatwaves was more short-lived than droughts, which sometimes extended over more than one growing season.

The losses over the period 1985 to 2007 were higher, averaging nearly 14 percent, raising the question of whether climate change is playing an greater role.

The authors pointed to other research suggesting that a jump of one degree Celsius (1.8 degrees Fahrenheit) in seasonal mean temperature can shave six or seven percent off of yields in some regions.

But the additional crop losses at the end of the 20th century could be due to other causes as well, they said.

Global warming has heated Earth by 1C since the start of the Industrial Revolution, mostly over the last 50 years.

Under the umbrella of the United Nations, the world's nations have vowed to hold the increase to "well under 2C".

Somewhat surprisingly, heatwaves and drought claimed twice as much cereal production -- 20 percent -- in the United States, Canada and Europe than in the developing world.

This was probably due to the prevalence of industrial-scale mono-culture -- growing a single crop over vast tracts of land.

"If a drought hits in a way that is damaging to those crops, they will all suffer," Lesk said in a statement.

"By contrast, in much of the developing world, the cropping systems are a patchwork of small fields with diverse crops. If a drought hits, some of thos crops may be damaged, but others may survive."