How do emissions in Abidjan affect the price of chocolate in the UK?

By Peter Hill

The majority of the cocoa beans required to supply the world’s ever increasing demand for chocolate come from southern West Africa. Unfortunately, the volume produced, and consequently the cost of cocoa beans, is heavily dependent on the regional rainfall, which is highly variable.

West African precipitation depends on the West African monsoon (WAM); south-westerly winds during the northern hemisphere summer bring moist air from the Gulf of Guinea, which results in the wet season when most of the annual rainfall occurs (Figure 1). The timing and magnitude of monsoon rainfall is very variable and difficult to predict. Many weather and climate models have problems related to representation of the WAM and climate models show little consensus on the monsoon response to CO2 increases.

2016 03 10 Peter Hill Fig 1

Figure 1: Zonal (10°W – 10°E) mean precipitation for 2000-2014. Note how north of 7°N the precipitation increases from May to August as the monsoon winds  bring increasing moisture further north. Precipitation rates are taken from the Global Precipitation Climatology Centre (GPCC) analysis that is based on rain gauges. The thin light red lines show July mean precipitation from individual years and demonstrate the highly variable nature of the monsoon precipitation.

So how does this relate to emissions in Abidjan? Population growth, economic growth and urbanisation are leading to large increases in anthropogenic emissions in Abidjan and the other cities of southern West Africa. Evidence suggests that anthropogenic aerosol emissions in other monsoon regions affect the strength of the monsoon. For example, reflection of sunlight by sulphate aerosols in India has been linked to a decrease in Indian monsoon rainfall (as explained by Andrew Turner). However, the effect of aerosols on the WAM is unknown, not least because aerosol emissions and concentrations in southern West Africa are not currently well measured or understood.

The EU-funded DACCIWA project (Dynamics-Aerosols-Chemistry-Cloud Interaction in West Africa) brings together scientists from across Europe (including the University of Reading) and southern West Africa to study the effects of anthropogenic emissions in southern West Africa on human health and the regional climate. Measurements of emissions are underway in Abidjan, Ivory Coast and Cotonou, Benin (Figure 2). An extensive field campaign using aircraft and three extensively instrumented ‘supersites’ will be conducted in June-July 2016 in order to improve our understanding of the clouds, precipitation and radiation in this region and how they interact with aerosols. Numerical modelling studies will be used to link these findings to the monsoon dynamics.  Ultimately we aim to understand how emissions affect the West African monsoon (and the production of cocoa!)

2016 03 10 Peter Hill Fig 2

Figure 2: Location of the DACCIWA field sites. The area highlighted by the dotted box shows the broader DACCIWA domain that will be used in modelling studies.

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