Africa contributes a very small percentage to overall greenhouse gas emissions. Even with South Africa included, overall emissions are small, but of course, the ratio of emissions to economic output in South Africa is one of the highest in the world.
Climate change impacts are felt most by people living in Africa and other developing countries with the least ability to cope. The latest UN Human Development Report 2007/8 made the point that in developing countries, 1 in 19 people suffered from climate-related disasters between 2000 and 2004, compared with only 1 in 1,500 people in wealthy countries (see related article at Scitizen).
Clearly a global externality that need to be adressed by those who are responsible.