Out of the total land area in Africa, only a fraction is used for arable land. Using soil, land cover and climatic characteristics a FAO study has estimated the potential land area for rainfed crops, excluding built up areas and forests – neither of which would be available for agriculture. According to the study, the potential – if realised – would mean an increase ranging from 150 – 700% percent per region, with a total potential for the whole of Africa in 300 million hectares.
[source: Hugo Ahlenius, UNEP/GRID-Arendal]
However, keep in mind that according to two new reports, climate change is expected to "have dramatically negative effects on African agriculture and food supply by reducing river runoffs and water recharge."