Africa stands out among continents for widespread and deeply entrenched poverty, slow economic development, and agricultural systems prone to failure during frequent and persistent droughts [1]. Africa is also home to some rapidly developing economies, tremendous natural resources and remarkable social and ecological diversity. The unique history of Africa, the close dependencies of people on natural resources and a future that will certainly include substantial industrial, agricultural and social development, suggest that Africa will become a key player in the carbon cycle of the 21st century. However, our knowledge about Africa's current role in the global carbon cycle remains remarkably limited. We currently do not know whether Africa is a net sink or source of atmospheric carbon, and have only vague indications of the continent's temporal and spatial patterns of carbon exchange. Given the current development agenda that is intended to elevate Africa's importance in the global economy [2], it is time to focus as well on Africa's role in the global carbon cycle. Here we review what is known about Africa's carbon dynamics from regional and global inventories, and forward and inverse model analyses, and highlight some of the unique features of Africa's contribution to global carbon fluxes.
The diverse elements of the global carbon cycle have been the focus of much recent research [3-5]; research that is vital to our understanding of the missing carbon sink, future atmospheric carbon dioxide concentrations, and future climate [6-8]. Much of that research has concentrated on carbon dynamics of the large ocean basins [9,10] and terrestrial exchange in North America [11,12] and Eurasia [13,14]. Despite representing 20% of the global land mass, Africa has thus far been largely neglected in these studies. Africa contributes a disproportionately small fraction of the global fossil fuel carbon emissions that are responsible for rising atmospheric carbon dioxide concentrations, with 14% of global population [15], but only 3% of fossil emissions [16]. In contrast, Africa plays a globally important role in fire and land use carbon emissions, though the magnitudes of these terms are highly uncertain.
To date, continental assessments of Africa's carbon dynamics are primarily model-based. Plausible estimates of Africa's regional sources and sinks can potentially be supplied by atmospheric inversion using global CO2 concentration measurements and atmospheric transport models. However such 'top-down' solutions have large uncertainties, particularly for Africa and other tropical regions, due to the paucity of appropriately located CO2 concentration measurements [17,18]. Existing data are also insufficient to partition carbon sources and sinks within Africa, and inversion techniques provide little insight regarding mechanisms responsible for net uptake and release of carbon in space and time [17,19,20]. The alternative approach is to perform spatially, temporally and source-differentiated 'bottom-up' estimation using biogeochemical models. However, such regional carbon flux estimates are only weakly supported by the sparse network of place-based observations, and thus are largely founded on models that have been parameterized and evaluated with extra-African observations. The resultant uncertainty reduces our ability to resolve African and global carbon sources and sinks, and hinders wise resource management in Africa for greenhouse gas mitigation.
With the hope of identifying carbon cycle research priorities that may be met through focused research efforts, we synthesize current understanding of carbon stocks and fluxes within Africa, highlight uncertainties in those terms, and diagnose where uncertainty in our knowledge of the African carbon cycle impacts our ability to assess global carbon dynamics. We then outline where new measurements and further research are most likely to contribute understanding of African and global carbon cycling, and discuss implications for African involvement in international climate change agreements.
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