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Link between temperature increase and crop declines in West Africa

The average temperature increase across the West African region, linked with rising atmospheric carbon pollution, is in line with the global trend: 1°C, relative to pre-industrial times. New crop modelling now confirms that this rise in regional temperature could account for significant cereal losses in West Africa in recent years.

Crop yields can go up or down for many reasons, argues climatologist Dr Benjamin Sultan from the French Institute of Research for Development (IRD). Political instability, land management, changes in the local economy, drought, or temperature changes could all explain recent decreases in key cereal crops like millet and sorghum in West Africa.

How can scientists tease out the role of changing climatic conditions on crop yield losses, when there is so much background ‘noise’ in the data?

Sultan and a team working with the AMMA-2050 initiative recently ran a series of computerised crop models to get a clearer picture of the likely role of temperature increases on millet and sorghum yields in West Africa.

Their findings show that the measured rise in temperature of 1°C is likely to account for a drop in millet yields of 10% to 20%, and a decline in sorghum of 5% to 15% across the region. This loss of production adds up to a total of USD 2.33 to USD 4.02 billion for millet, and USD 0.73 to USD 2.17 billion for sorghum.

Working with the team, Sultan ran two computerised crop models. The first computer run used the real-world conditions which have caused an increase in global temperatures of 1°C since the start of the industrial revolution, where the concentration of carbon dioxide pollution in the atmosphere is now at 415 parts per million (ppm).

The second model run used hypothetical conditions, which kept the carbon dioxide levels at what they were before the mass burning of fossil fuels began. This model assumed that there was no increase in the greenhouse effect and no global heating.

Comparing the outcomes of these two model runs, the researchers are able to show that with a 1°C rise in temperature, the heat stress on two regionally important cereal crops reduces yield significantly. Of the five most important cereal farming countries in the region, the highest losses were for sorghum in Nigeria, and millet yields in Senegal and Mali. This analysis shows that Benin and Gambia are also vulnerable to production losses.

‘Our study demonstrates that human activities have already had a significant impact on the regional climate in the Sahel with negative consequences on agricultural production,’ Sultan and colleagues Dr Dimitri Defrance and Dr Toshichika Iizumi write in a recent paper in the journal Scientific Reports.

These crop losses are the result of what the researchers describe as relatively ‘moderate’ levels of regional warming. Future warming is expected to further reduce farming yields by 8% across the whole of Africa, and by another 11% in West Africa.

Teasing out the findings, Sultan and his team say that the model runs confirm temperature increases account for crop declines, rather than changes in rainfall or drought conditions in the Sahel.

The take-home message for policy makers and those in the agricultural sector in West Africa is that as regional temperature continues to rise, crop yields are expected to drop further, highlighting the urgency for proactive adaptation responses from the farming sector across the Sahel.

One solution to supporting agricultural resilience is for farmers to move towards long-duration strains of millet and sorghum, says Sultan. Long-duration crops usually take four months to reach harvest maturity, compared with shorter-duration strains, which need three months to complete their growth cycle.

‘The long-duration crops are more resilient to temperature increases since there is still enough time for the plant to grow and produce biomass,’ Sultan explains.

The next step for this research group is to consider how best to use rainfall and runoff in complementary irrigation as an adaptation measure. By combining crop and hydrology models, the team hopes to be able to show what the current need is for additional irrigation to offset the crop stress caused by global heating, and what future needs might be. This will help the agriculture sector in the region to begin putting plans in place that can build resilience for farmers in the region, and offset the threat to food security owing to crop losses.

The work reported on in this story is part of the AMMA-2050 (African Monsoon Multi-disciplinary Analysis 2050) research project, which aims to support decision-makers in West Africa to integrate this knowledge into climate-sensitive decisions. 

This article was written by Leonie Joubert.