There are signs that productivity growth has stagnated in many areas, falling behind the pace needed to meet future demand, and leaving fewer opportunities for future global gains (Ray et al. 2012; Ray et al. 2013). Poor management of water, fertilizer and pesticide use globally, and specifically during Asia’s Green Revolution, have degraded land and water resources, and have likely played a role in declining yields (Pinstrup-Andersen and Hazell 1985; Pingali and Rosegrant 1994; Godfray et al. 2010a; Stevenson et al. 2013). Especially in Africa, inadequate fertilizer use has depleted the nutrient content of soils (Drechsel et al., 2001; Cobo, Dercon and Cadisch, 2010). Further ahead are the uncertain consequences of climate change on food production and prices (IPCC, 2014). What’s more, the anticipated transition to mechanization in fast-growing countries in Asia, where wages are increasing, may have the unexpected effect of undercutting future land productivity gains (Otsuka et al., forthcoming).

Using projections of food and biofuel demand by Bruinsma (2009) and Fischer (2009), Hall and Richards (2013) calculate that grain yields on current agricultural lands would need to grow by 1.2 to 1.3 percent per year pace (depending on uncertain policy-driven biofuel demand) to keep pace with growing demand through 2050. In their study, the researchers consider known but unexploited opportunities to improve yield potentials for rice, maize and wheat, and speculate about promising new technologies, but conclude that current and expected future rates of improvement in genetic potential will be insufficient to meet future demand.

Against this background, we discuss two alternative paths to global food security especially relevant for Sub-Saharan Africa. The first is to expand production by bringing more land and water resources into agriculture. In general, the potential for bringing more natural resources into agriculture in a sustainable way is limited, especially once competing resource demands and agricultural lands lost to urbanization are accounted for. Nevertheless, many researchers conclude there is scope to bring new lands under sustainable production, especially in Sub-Saharan Africa. An alternative approach is to improve the productivity of existing farms. Worldwide, there are a number of places where there is significant potential to “close yield gaps” – that is, to reduce the difference between obtained productivity and potential productivity, as determined by using the best available genetic materials and technologies. However, some of the largest yield gaps are to be found in Sub-Saharan Africa, where most agricultural resources are managed by smallholder farmers.

# **4. African smallholder resources as a solution to future food needs**

A number of studies suggest that staple crop yields in Africa could be improved by using better seeds, better nutrients, and by improving water management (Neumann et al., 2010; McDermott et al., 2010; Mueller et al, 2012). For example, in their global study, Mueller et al. (2012) note that some of the largest yield gaps are for African staple cereals, including rice and maize.

Alternatively, it is also possible to expand food production in Africa by planting more land to crops in places where agricultural land remains relatively abundant. Deininger et al. (2011) estimate that more than 200 million hectares in Sub-Saharan Africa could be converted to rainfed agriculture, roughly 45 percent of the total area in the world suitable for expansion. Much of the land is isolated; however, they calculate that about 95 million hectares could be accessed without a major investment in infrastructure. In the case of rice, Balasubramanian et al. (2007) estimate that, once double-cropping is taken into account, 5