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Africa in the Balance
- Africa and the global carbon cycle

Africa is second only to Eurasia in continental surface area. It has large areas of moist tropical forest, seasonal and semi-arid woodland, savanna, grassland and desert, as well as smaller regions of Mediterranean and montane vegetation in extra-tropical and high elevation areas (Figure 1).

Initial estimates [21] of carbon stocks and the various flux pathways (Table 1, Figure 2) suggest that the continent plays a significant role in atmospheric CO2 dynamics at time scales ranging from sub-seasonal to decadal and longer. The balance of terms in Figure 2 should not be interpreted as identifying a large net biotic source for the continent but rather that independent studies which estimate the magnitude of fluxes associated with individual pathways can not be used in a budget calculation without careful consideration of the processes represented in each estimate and the associated uncertainties. For example, biomass burning emissions are not modeled explicitly in many biogeochemical or biophysical models and may thus be effectively lumped into heterotrophic respiration.

Patterns of soil and vegetation carbon stocks and net primary production (NPP) are highly correlated with annual rainfall (Figure 1). Africa's fraction of global annual NPP is estimated to be similar to the fractional terrestrial area of the continent (Table 1 and Figure 1); the large unproductive arid regions are compensated by high productivity in forests and woodlands. Carbon stocks and NPP per unit land area center on the equator and decline to the north and south toward increasingly arid environments. However, greater land area in Africa's northern hemisphere cause latitudinally summed C stocks and NPP to peak north of the equator (Figure 1).

African fossil fuel emissions are a tiny fraction of global totals, even when normalized by land area or human population (Table 1), while renewable energy sources (wood, charcoal) are a substantial component of domestic emissions. With low fossil emissions, Africa's current continental scale carbon fluxes are dominated by biogenic uptake and release from terrestrial ecosystems as well as pyrogenic emissions in savanna and forest fires. As is generally true globally, the continent's large carbon uptake from photosynthesis is offset by an equivalently large respiration flux, leading to near-zero net biotic flux at multi-year or longer timescales. In spite of these broad patterns, estimates can differ widely between studies (Table 1) and temporal variability is large.

Bottom-up simulation models [22-24] indicate large interannual variation in Africa's net ecosystem carbon exchange (NEE), with an interannual variability (expressed as the standard deviation of annual NPP) that is approximately 50% of the variability estimated for the global land mass (Figure 3), primarily induced by climate fluctuations [24]. Particularly large between-year coefficients of variation in NPP are found for Africa's woodlands, savannas, and grasslands, according to one model incorporating satellite measurements of vegetation activity [25,26].

Africa plays a global role in C emissions through land use and fire (Table 1), though lack of information from the limited number of studies on the continent [e.g. [27-31]] restricts confidence in their magnitudes. Deforestation is the largest term in current assessments of tropical land use emissions [32], with Africa contributing 25% to 35% of total tropical land clearing from deforestation, and as much as 0.37 Pg C y-1, in the last decades [32,33]. Carbon losses through deforestation tend to be 'permanent' in Africa, as afforestation and reforestation rates are modest, at less than 5% of annual deforestation [32]. The associated net release of carbon from land use in sub-Saharan Africa is estimated to be 0.4 Pg C y-1, or 20% of the tropical total, nearly all attributed to deforestation [32]. Annual net C emissions from conversion to agriculture and cultivation practices alone are estimated [24] to be about 0.8 Pg C y-1 for tropical land masses, but only 0.1 Pg C y-1 from Africa [24,31], where shifting cultivation is prevalent [31].

Lack of information prohibits even the best land use change C emissions assessments from including all of the terms anticipated to be important for Africa. Pastoralism, shifting cultivation, and domestic wood harvest are widespread across the continent, but are often assumed to be inconsequential or are not considered [e.g. [32]], such that land use and land use change emissions from Africa are likely to be underestimated. Recent work [31] explicitly simulates aspects of these practices, though still focuses exclusively on forest and cropland conversions, missing land use change C emissions in Africa's vast savannas and grasslands which are home to much of the continent's livestock and the center of Africa's cereal and grain production. Furthermore, net C fluxes associated with changes in land use practices but not involving land conversion, such as management of tillage, slash, crop residues, and crop rotation, are refinements currently missing from continental scale land use change assessments. Finally, much of Africa, particularly in the semi-arid regions, is vulnerable to degradation, that may be the result of periodic drought or caused by agricultural and pastoral activities, releasing presumably large but unknown amounts of CO2 from cleared and dead vegetation [34] as well as possibly triggering strong biophysical feedbacks to the climate system [35] that may accelerate warming and prolong droughts [36-38].

Fire and land use emissions of carbon are entwined, especially in the humid and subhumid forest areas where fire is a primary tool for land transformation. Fire emissions associated with deforestation, shifting cultivation, burning of agricultural residues, and fuelwood may be as large as 2 Pg C y-1 globally and 0.4 Pg C y-1 for Africa, each of similar magnitude to estimates of total land use-related C emissions from those regions (Table 1). Consequently, estimates of land use change and deforestation C emissions already include, at least in theory, the associated fire emissions. New methods to estimate fire emissions using satellite sensors and atmospheric carbon monoxide measurements [39,40] will improve our ability to diagnose C emissions in fires.

Fire is also a common dry season occurrence in the seasonal savannas that encircle the humid forest zone. Carbon emissions in savanna fires represent a much shorter-term C loss than forest fires, since the main fuel is dead herbaceous vegetation, representing just one or two years of growth [27,41]. Thus savanna fires may only lead to faster cycling of biomass carbon rather than a net emission. Even if carbon emissions from savanna fires are roughly balanced over the long-term by growth in subsequent years, fires provide intense and localized injections of carbon into the atmosphere potentially shifting the seasonal or interannual distribution of CO2 releases [27,41]. Given the large magnitude of these fluxes in Africa, even fairly small (e.g. 20%) variation in year to year total fluxes could translate into annual variation in pyrogenic fluxes of 300 Tg of C or more. Correspondingly, recent results suggest that biomass burning is the largest source of interannual variability in land-atmosphere carbon fluxes [42].

Unlike respiration, fires return carbon to the atmosphere as a wide range of compounds, some of which are chemically or radiatively active (e.g. methane, carbon monoxide and aerosols), or are precursors to radiatively active gases (e.g. ozone precursors). Methane and other hydrocarbons, carbon monoxide, and black carbon releases in Africa are almost entirely of pyrogenic origin, and are thus included in the biomass burning term (Table 1) [27,28,41]. Methane consumption in upland soils is small, and available estimates of methane release from African wetlands suggest that they are globally insignificant [43]. However, given that there is no reliable map of wetland extent in Africa, and virtually no direct emission estimates, the true size of this flux is unknown. Recent work suggests the possibility of a large methane source of unknown magnitude from living plants [44,45]. Emissions of volatile organic compounds (VOCs) such as isoprene and monoterpenes have been studied in some detail in southern and central Africa and are estimated to return as much as 0.08 Pg C y-1 to the atmosphere [46]. At the scale of the continent, industrial emissions of carbon dioxide, carbon monoxide and hydrocarbons from Africa are small, but can be locally very significant in the industrial areas of South Africa, the oilfields of the Gulf of Guinea, Angola, and Libya, and around major cities elsewhere in Africa.

The export of dissolved organic and inorganic carbon (DOC and DIC) in river water discharged to oceans is, by and large, offset by DOC and DIC delivered in precipitation (Table 1). Africa is also a minor net global source of biomass carbon through international exchange, mainly from export of wood products [47].

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