Atmospheric mixing ratios and isotopic compositions measured around the globe [48] can be used to estimate terrestrial and oceanic carbon sources and sinks by inversion with atmospheric transport models [18,49-51]. Inverse solutions for Africa are poorly constrained due to the lack of tropical, especially African, observations (Figure 4) [17,18]. This contributes to larger uncertainty ranges around net CO2 flux estimates for Africa than for global or tropical land areas, in general. Taken together, inversion results demonstrate that Africa's net role in global carbon cycling is highly uncertain. Furthermore, lack of data causes the inverse solution for southern Africa to trade off with solutions for South America and the southern oceans, such that results can vary widely between regions with no net change in overall source/sink strength [e.g. [17,19]]. Solving this problem will require the addition of precise, long-term observations of carbon dioxide in the tropics, located such that they help resolve the longitudinal differences among the southern hemisphere regions [52-54]. Tropical atmospheric dynamics present an additional challenge [55], and a source of uncertainty that is not represented in Figure 4, because deep convection is both poorly represented in transport models and poorly sampled, introducing non-negligible biases in atmospheric inversions.
Recognizing large uncertainties in the inverse solutions, inverse studies to date suggest that Africa as a whole is approximately carbon neutral on an annual to long-term basis. This is so despite significant carbon emissions related to land use change and burning, implying that net plant growth and corresponding sequestration of carbon in vegetation and soils is sufficiently large across the continent to offset the loss terms. If inverse solutions correctly estimate a carbon neutral Africa and assuming a neutral biosphere with a background balance between net primary production and heterotrophic respiration plus natural fires, the remaining biotic uptake or sequestration can be estimated as roughly offsetting Africa's land use (0.4 Pg C y-1) plus fossil fuel (0.2 Pg C yr-1) sources, still noting the large uncertainties.
Despite a near-zero balance, recent time-dependent inverse solutions attribute much of the interannual variability (IAV) in global carbon sources/sinks to the African continent [20,42,51,56]. Estimates of regional IAV are less sensitive to transport and station-selection than seasonal and long-term mean fluxes [20]. Global solutions for the IAV of carbon sources/sinks [20] robustly indicate the strong influence of global lands, particularly those in the tropics, with approximately equal contributions by lands of tropical Asia, Africa, and southern and tropical America (each about 0.5 Pg C y-1) (Figure 5). However, temporal source/sink dynamics are still poorly constrained among tropical regions, especially those of Africa and America [20].
Taken together, large temporal variability of carbon sources and sinks may be Africa's most significant contribution to the global carbon cycle. This is consistent with results from ecosystem models [22-24], which indicate that high interannual variability in rainfall throughout the Sudano-Sahelian and southern African regions [57,58], partly associated with the North Atlantic Oscillation, El Niño Southern Oscillation, and South Pacific circulation [59,60], introduce pronounced multi-year fluctuations in surface-atmosphere C exchanges, which, in turn, appear in atmospheric CO2 concentrations [51]. Inter-annual variability in NPP then translate to variability in fire emissions with a lag of several months to a year. Such departures from long-term average biosphere exchange [51] and fire emissions [42,51] may both be as large as the net exchanges themselves.
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