Geography and agricultural output
Most Africans live in sub-humid or arid tropics without rivers and the alluvial
plains that permit cheap and efficient irrigation as in much of Asia. Agriculture
in Africa is consequently more dependent on rainfall, but rainfall variability is
higher than in tropical America or Asia. Between 1983 and 1995, 29 countries
in Sub-Saharan Africa experienced at least one year of drought which affected
more than half of the continent’s population, 24 suffered at least two years of
drought and 14 more than three years (Bloom et al., 1998). Rainfall in the
Sahel (the semi-arid region south of the Sahara) has been persistently low since
the 1970s, linked to changing climate and rising surface temperatures in the
Indian Ocean. What rain does fall is subject to high rates of moisture loss
through evaporation and transpiration due to high temperatures (Sachs et al.,
2004). Arguments relating to soil quality are much weaker. Heat and abundant
rain do mean that tropical soils cannot accumulate organic topsoil since
organic matter decomposes rapidly and is quickly lost as rainwater seeps
downward in the soil. Tropical soils are therefore low in plant nutrients and
subject to severe erosion. This does not mean tropical soils are inherently infe-
rior, but that they are different. The higher rate of chemical and physical churn-
ing in humid-tropical conditions means that soil production is more rapid than
in cooler climates, so soils maintain fertility from the dissolution of minerals
and less from the accumulation of organic matter. Overall erosion is more seri-
ous and regeneration more rapid. In tropical climates temperatures tend to
remain high during the year even at night, so plant losses of net photosynthesis
(energy) are high. Many plant species are specifically adapted to these tropical
conditions. These include sweet potato, yam, taro, oil palm, coffee, cocoa, tea,
groundnut, cassava, banana, mango and papaya (Blaut, 1993). Tropical agri-
culture in some parts of the world, such as the Amazon Basin,supports very
low population densities; elsewhere population densities have been extraordi-
narily high (in Java, Bangladesh, Barbados and El Salvador) (Blaut, 1993).
The net impact of these various effects does show a clear negative link
between tropical climates and agricultural productivity. Nordhaus (2006)
found that the optimal temperature for maximizing output density is approxi-
mately 12°C – much lower than the temperature typical of tropical climates.
Overall tropical agriculture suffers a productivity penalty of 30–50 per cent
compared with temperate-zone agriculture (Gallup and Sachs, 1999). Cotton,
for example, is grown in both temperate and tropical environments and yields
are much lower in the latter regions (Bloom et al.,1998).
However, we should not jump to conclusions about causality. Although
climate is relatively fixed agricultural technology is certainly not. The produc-
tivity of agriculture is not only determined by geography. Prior to the Industrial
Revolution, in eighteenth-century East Asia 0.5 acres was enough to support a
family in rice cultivation while 20 acres was required for an equivalent-sized
family in England (Ferguson, 2012:26). This can be linked with the findings of
Acemoglu et al. (2002a) that patterns of population density in the early
Geography and Economic Resources 245