African farmers will soon face growing seasons hotter than any in their experience. To cope with this rapid climate change, they – and the plant breeders who supply their crops – will need to make big changes, and soon. Agricultural experts have predicted for some time that farmers are likely to face problems as climates become hotter and drier than they are today. Indeed, some farmers in South Africa are already reporting difficulties (pdf).

To see how fast, and how broadly, this will strike, Marshall Burke, an agricultural economist at Stanford University, and colleagues, averaged the results from 18 global climate models to forecast likely temperature and rainfall conditions in 2025, 2050 and 2075 in regions of Africa where maize, millet and sorghum are grown today. Then, assuming that year-to-year variability would remain the same as today – perhaps a conservative assumption – they asked how much these future climates would overlap with existing climates.

They found that farmers in Africa will face average temperatures outside the current range of experience in their locality in 42% of years by 2025 – and 97% by 2075. Since temperature strongly affects crop yields, farmers will need to find new varieties adapted to these higher temperatures, Burke says. Future rainfall showed more overlap with current conditions, largely because rainfall already varies more from year to year.
Maize trap

The researchers then looked to see whether the warmer temperatures forecast for 2050 can be found anywhere in Africa today. If so, they reasoned, these analogous conditions might yield crop varieties already adapted for the future conditions. A few lucky countries, such as Tanzania, Ethiopia and South Africa, have diverse enough climates today that they can find climates analogous to the potential conditions of 2050 within their own borders today, Burke's team found. At the opposite extreme, Sahelian countries such as Chad, Mali and Niger may have nowhere to turn. "By 2050, they're going to be hotter than any current growing season in any maize country in the world," says Burke. Most countries, however, will be able to find analogous climates in other countries today.

That would be good news, except that plant breeders have done very little collecting of locally adapted varieties from some of the most likely analog countries, such as Cameroon, Sudan and Nigeria, Burke's team found. To cope with future climates, genetic prospectors must sample much more of the genetic diversity of crops in these countries – and those nations must then do a better job of sharing these genetic resources, says Burke. "We've got to do something serious about agriculture and we've got to start now," agrees Gerald Nelson, an agricultural economist who heads research on agriculture and climate change at the International Food Policy Research Institute in Washington DC.

(Global Environmental Change / New Scientist, 17/06/2009)