Given the need to better understand the spatial and temporal dimensions of the global carbon cycle for prediction and management of future atmospheric CO2 concentrations, a number of research priorities for Africa emerge from this review. Of primary importance is the need for continent-wide observations that support both bottom-up and top-down methods of estimating carbon sources and sinks. Continued and new investment in collection and synthesis of carbon cycle information (measurements of carbon stocks and fluxes within and between the major pools) is needed to advance process-level characterization of seasonal and interannual variations in source/sink strengths. Such data will help to improve the way biophysical and biogeochemical models represent African ecosystems so that they capture the full suite of uniquely African features such as the continent's seasonal fire cycles, pastoralism, fuelwood harvest, cereal/grain production, dryland degradation, and the productivity and isotopic signatures of its extensive C4 vegetation. In particular, carbon flux observations wherever existing need to be used in model development and testing to appropriately represent the sensitivity of production and respiration processes to climate fluctuations. New collaborative research programs and networks are emerging in Africa to address some of the gaps through expanded site-based and regional field measurements and model-based analyses (including, amongst many national and regional activities, the growing network of eddy covariance sites (the "Afriflux" network), the African Monsoon Multidisciplinary Analysis (AMMA), the South African Ecological Observation Network (SAEON), and the Environmental Long-Term Observatories of southern Africa (ELTOSA)).
Support for inventory and monitoring of soil and vegetation carbon stocks by forest and agricultural research stations, long-established in most African countries [61], is critical. The associated national resource inventories [62] provide information invaluable for assessing regionally-specific ecosystem responses to natural and human disturbances and for anchoring regional-scale estimates of land use related carbon sources and sinks. Synthesis of country-level data is of great importance to provide local and regional scale data to underpin regional- and global-scale carbon cycle assessments. These will complement developments in satellite remote sensing of vegetation biomass using, for example, passive and active optical and radar approaches [63,64].
A well-located atmospheric sampling network in Africa is also needed to better constrain inversion estimates of regional carbon sources and sinks and their temporal variability both in Africa and globally. However, improved constraint relies not only on new observations but also improvement of modeled transport and inverse estimation techniques. New transport schemes are needed to represent deep tropical convection, while new data assimilation and computational techniques promise to better resolve the African signal in global atmospheric carbon dynamics by incorporating diurnal variation in surface fluxes, multiple atmospheric tracers, and prior estimates of fire emissions. With regard to additional carbon cycle tracers, Africa is unique in having vast coverage of C4 vegetation [65], associated with a prevalence of semi-arid and hot environments. Seasonal production, respiration, and burning of C4 vegetation alter the carbon stable isotope (13C:12C) composition of the atmosphere because C4 plants discriminate against the heavier isotope less than C3 plants. This imprint provides a tracer for diagnosing Africa's role in global carbon stocks and fluxes [40], presenting a potential opportunity for separation of moist tropical forest exchange from that of the savanna regions. Furthermore, since oceanic and C4 plant discrimination are similar, information on the C4 terrestrial exchange is critical for separation of terrestrial from oceanic fluxes.
An orbiting space-based total column carbon observatory covering the entire globe is anticipated within the next decade [66], but it will still require near-surface and vertical profile measurements of CO2 for calibrating, validating, interpolating and interpreting satellite-derived observations. Satellite-based assessments of local to regional vegetation change from land use practices [e.g. [67]] should be further explored for continental-scale assessment. These and other data could be used to develop land use/land cover transition models that represent Africa's unique human-vegetation-climate settings. Such comprehensive investigations into regionally-specific ecosystem responses to land use in Africa offer needed detail for representing the complex dynamics associated with human-induced disturbances and land use management.
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