Introduction

1. Africa and the global carbon cycle Abstract The African continent has a large and growing role in the global carbon cycle, with potentially important climate change implications. However, the sparse observation network in and around the African continent means that Africa is one of the weakest links in our understanding of the global carbon cycle. Here, we combine data from regional and global inventories as well as forward and inverse model analyses to appraise what is known about Africa's continental-scale carbon dynamics. With low fossil emissions and productivity that largely compensates respiration, land conversion is Africa's primary net carbon release, much of it through burning of forests. Savanna fire emissions, though large, represent a short-term source that is offset by ensuing regrowth. While current data suggest a near zero decadal-scale carbon balance, inter-annual climate fluctuations (especially in rainfall) induce sizeable variability in net ecosystem productivity, and savanna fire emissions, such that Africa is a major source of inter-annual variability in global atmospheric CO2. Considering the continent's sizeable carbon stocks, their seemingly high vulnerability to anticipated climate and land use change, as well as growing populations and industrialization, Africa's carbon emissions, and inter-annual variability in emissions, are likely to increase through the 21st century. New sampling sites! FORWARD MODELS Figure 5. Standard deviation of annual net ecosystem carbon exchange (NEE) estimated with 3 ecosystem models, High Resolution Biosphere Model (HRBM), Terrestrial Ecosystem Model (TEM), and Lund-Potsdam-Jena model (LPJ) (McGuire et al., 2001). Niall P. Hanan1, Christopher A. Williams1, Joseph Berry2, Robert Scholes3 A. Scott Denning1, Jason Neff4, and Jeffrey Privette5 1. Colorado State University, Fort Collins, CO, 2. Carnegie Institution of Washington, Stanford, CA, 3. CSIR, Pretoria, South Africa, 4. University of Colorado, Boulder, CO, 5. NASA-GSFC, Greenbelt, MD CO2 Inverse Analysis Introduction A new initiative seeks to improve our understanding of carbon exchange and land-atmosphere interactions in African ecosystems at hourly to multi-annual time scales and at landscape to continental spatial scales. The African Carbon Exchange project (ACE, jointly funded by NOAA Office of Global Programs and NASA Terrestrial Ecology Program) is designed to improve our understanding of regional and global carbon dynamics by merging point observations, biogeochemical forward process modeling and inverse modeling of atmospheric CO2 sources and sinks. This poster reports results from a review of published literature from regional and continental scale carbon inventories, biogeochemical models driven by climate and, in some cases, by remote sensing data, and inverse analyses of atmospheric [CO2] measurements. Atmospheric CO2 concentrations can be inverted using atmospheric transport models to infer the global spatial and temporal distribution of CO2 sources and sinks. Terrestrial fluxes are least well constrained for Africa where we have few observations and where the role of fire and land use are important. Figure 4. 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 (2001a) for global and tropical land during the 1980s (dark) and 1990s (light), whereas symbols report results from inverse analyses. 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 30 inverse solutions. Atmospheric inversion results for Africa are taken from Bousquet (1999a) (B99), Ciais (2000) (C00), Rödenbeck (2003a) (R03), Gurney (2002) (G02), and Gurney (2004) (G04), with years spanned in each analysis shown below literature source abbreviations of (a). Variability is Highest for Africa Figure 2. Elements of the African carbon cycle estimated as annual averages. Annual fluxes and pools (shown in parentheses) are in units of 1015 g C, where NPP is net primary production, and Rh is heterotrophic respiration. Data (drawn from Table 1) were compiled from published inventory and model simulations. Sources and data reported in full in Williams, Hanan, Neff. Scholes, Berry, Denning and Baker 2006 (in review) Figure 6. Root mean square of annual NEE from time-dependent atmospheric inversions (Baker et al. 2006) for the period 1990 - 2001. NH (Northern Hemisphere land) includes temperate and boreal Asia, temperate and boreal North America, and Europe, SH (Southern Hemisphere land) includes temperate South America, Australia and New Zealand, Tropical includes tropical America, Africa and tropical Asia, and Amer. abbreviates America. INVERSE MODELS Figure 3. Though well developed in much of the northern hemisphere, the global network of [CO2] sampling has gaps in the tropics and southern land masses. ACE has added new precision sampling sites and flux observations in Africa to better constrain tropical sources and sinks. The new African [CO2] measurements will contribute to inversions for 2005 onwards. Novel combinations of forward and inverse approaches within ACE will lead to improved estimates of the spatial and temporal dynamics of carbon and water exchange in Africa, and enhance understanding of the impacts of climate, climate variability, land use and fire on Africa’s carbon exchange and it’s contributions to the global carbon cycle.