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The periodic warming of the Pacific Ocean known as El Niño can reduce African crops yields, in some years endangering food supplies for about 20 million people, say researchers.

They warn that if — as widely predicted — climate change increases the number of years with El Niño-like conditions, southern Africa in particular could face declines in food production. 

But they say their findings should allow nations to prepare for food shortages by advising farmers on how to select and store their crops.

The study, led by Hans Herren of the US-based Millennium Institute, is published online this week by Proceedings of the National Academy of Sciences.

El Niño occurs every 3–7 years when water off the Pacific coast of South America becomes abnormally warm, triggering changes in wind patterns with far- reaching effects.

The team looked at how El Niño and a similar phenomenon called the North Atlantic Oscillation affect food production, by analysing 40 years of national crop and livestock records and 20 years of satellite data that showed changes in vegetation.

They found strong links between El Niño and declining yields of sorghum, millet, groundnuts, and especially maize. The trends were strongest for southern Africa, where crop productivity could drop by 20-50 per cent in extreme El Niño years, they say.

The study found no effect on wheat production, possibly because of the more frequent use of irrigation with this crop, says co-author Nils Stenseth of the University of Oslo, Norway.

The researchers say their approach means predictions can be made months in advance, allowing governments to advise farmers to plant more resilient crops in anticipation of El Niño events.

This could mean switching from maize to sorghum in northern Africa, and to cassava in southern Africa, they suggest.

"The results are of direct value to planners, as they can see how much the production of different crops in different regions of the continent could change during an extreme El Niño year", Stenseth told SciDev.Net. 

Read more about climate change and Africa in SciDev.Net's news focus, Climate change and Africa.