Increasing intensity of heat and drought as a result of global warming may have caused worldwide cereal harvests to be cut by up to a tenth since the mid-1960s

by Tim Radford, Climate News Network, January 8, 2016

LONDON – Climate change may have already begun to take its toll of agriculture. New research suggests that drought and extreme heat in the last 50 years have reduced cereal production by up to 10%. And, for once, developed nations may have sustained greater losses than developing nations.

Researchers have been warning for years that global warming as a consequence of rising levels of carbon dioxide in the atmosphere – in turn, a pay-off from increased fossil fuel combustion – will result in a greater frequency or intensity in extremes of weather.

They have also warned more recently that weather-related extremes could damage food security in Europe, Africa and India.

Global cost

But a study in Nature journal by Navin Ramankutty, professor in global food security and sustainability at the University of British Columbia in Canada, and colleagues is perhaps the first to identify the global cost of weather-related disasters in the last half of the 20th century.

The researchers looked at 2,800 extreme hydro-meteorological disasters – floods, droughts and extremes of heat and cold – reported between 1964 and 2007 from 177 countries, and matched the data with production figures for 16 cereal crops.

They could detect no significant influence from floods or ice storms, but they found that drought and extreme heat reduced average national cereal production by between 9% and 10%.

Drought reduced both cereal yield and the area under harvest, while heat mainly affected  yield. This is likely to be a consequence of different timescales: droughts can last for years, but heatwaves tend to be counted in no more than weeks.

And the more developed nations reported harvest reductions from 8% to 11% more than the poorer societies. In Australia, North America and Europe, harvest levels dropped by an average of 19.9% because of drought − roughly double the global average.

Significantly, the more recent droughts had a far larger effect on cereal production than the earlier droughts, the implication being that such extreme events are on the increase. [Actually, the implication is that the droughts are more intense.]

Research such as this depends on clever statistical approaches and vast volumes of data − much of it from UN agencies such as the Food and Agriculture Organisation and the Emergency Events Database, which recently confirmed that extreme weather-related events are more frequent.

And, the researchers say, the hazards to harvests will continue, and continue to grow.

Climate projections

“Present climate projections suggest that extreme heat events will be increasingly common and severe in the future,” they report. “Droughts are likely to become more frequent in some regions, although considerable uncertainty persists in the projections.”

Professor Ramankutty says: “We have always known that extreme weather causes crop production losses. 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.”

His co-author, Corey Lesk, a geographer at McGill University in Montreal, Canada, says: “Across the breadbaskets of North America, for example, 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.

“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 those crops may be damaged, but others may survive.”