Terrestrial carbon source and sink estimates for Africa, tropical and global lands. (a) Net carbon dioxide flux totals, and (b) net carbon dioxide flux per unit area. Positive values indicate a surface source. Boxes show the range of +/- 1 standard deviation from the IPCC report  for global and tropical land during the 1980s (dark) and 1990s (light), whereas symbols report results from inverse analyses cited in Appendix I. Triangles and error bars indicate mean flux estimates from individual inversion studies and associated posterior uncertainties. Squares indicate the average, and pluses indicate the standard deviation, of mean flux estimates from a group of inverse solutions. Circles indicate the average uncertainty estimates among the group of inverse solutions. Atmospheric inversion results for Africa are taken from Bousquet  (B99), Ciais  (C00), Rödenbeck  (R03), Gurney  (G02), and Gurney  (G04), with years spanned in each analysis shown below literature source abbreviations of (a).
Williams et al. Carbon Balance and Management 2007 2:3 doi:10.1186/1750-0680-2-3