A full greenhouse gases budget of Africa: synthesis, uncertainties, and vulnerabilities

Abstract

Abstract. This paper, developed under the framework of the RECCAP initiative, aims at providing improved estimates of the carbon and GHG (CO 2 , CH 4 and N 2 O) balance of continental Africa. The various components and processes of the African carbon and GHG budget are considered, existing data reviewed, and new data from different methodologies (inventories, ecosystem flux measurements, models, and atmospheric inversions) presented. Uncertainties are quantified and current gaps and weaknesses in knowledge and monitoring systems described in order to guide future requirements. The majority of results agree that Africa is a small sink of carbon on an annual scale, with an average value of Published by Copernicus Publications on behalf of the European Geosciences Union. R. Valentini et al.: A full greenhouse gases budget of Africa−0.61 ± 0.58 Pg C yr −1 . Nevertheless, the emissions of CH 4 and N 2 O may turn Africa into a net source of radiative forcing in CO 2 equivalent terms. At sub-regional level, there is significant spatial variability in both sources and sinks, due to the diversity of biomes represented and differences in the degree of anthropic impacts. Southern Africa is the main source region; while central Africa, with its evergreen tropical forests, is the main sink. Emissions from land-use change in Africa are significant (around 0.32 ± 0.05 Pg C yr −1 ), even higher than the fossil fuel emissions: this is a unique feature among all the continents. There could be significant carbon losses from forest land even without deforestation, resulting from the impact of selective logging. Fires play a significant role in the African carbon cycle, with 1.03 ± 0.22 Pg C yr −1 of carbon emissions, and 90 % originating in savannas and dry woodlands. A large portion of the wild fire emissions are compensated by CO 2 uptake during the growing season, but an uncertain fraction of the emission from wood harvested for domestic use is not. Most of these fluxes have large interannual variability, on the order of ±0.5 Pg C yr −1 in standard deviation, accounting for around 25 % of the year-toyear variation in the global carbon budget.Despite the high uncertainty, the estimates provided in this paper show the important role that Africa plays in the global carbon cycle, both in terms of absolute contribution, and as a key source of interannual variability.

The global methane budget 2000–2012

“…Fire is the most important disturbance event in terrestrial ecosystems at the global scale (van der Werf et al, 2010) and can be of either natural (typically ∼ 10 %, ignited by lightning strikes or started accidentally) or anthropogenic origin (∼ 90 %, deliberately initiated fires) (USEPA, 2010a, chap. 9.1).…”

“…The Global Fire Emission Database (GFED) is the most widely used global biomass burning emission dataset and provides estimates from 1997. In this review, we use both GFED3 (van der Werf et al, 2010) and GFED4s (Giglio et al, 2013;Randerson et al, 2012). GFED is based on the Carnegie-Ames-Stanford approach (CASA) biogeochemical model and satellite-derived estimates of burned area, fire activity and plant productivity.…”

“…Also methane emission factors vary greatly according to fire type, ranging from 2.2 g CH 4 kg −1 dry matter burned for savannah and grassland fires up to 21 g CH 4 kg −1 dry matter burned for peat fires (van der Werf et al, 2010). Tian et al (2016) estimated CH 4 emissions from biomass burning during the 2000s (top-down, 17 ± 8 Tg C yr −1 ; bottom-up, 15 ± 5 Tg C yr −1 ).…”

“…Some secondary (in magnitude) emission zones are also consistently inferred between models: Scandinavia, continental Europe, eastern Siberia, central USA, and tropical Africa. Using improved regional methane emission datasets (such as studies over North America, Africa, China, and Amazon) can enhance the accuracy of the global budget assessment Xu and Tian, 2012;Ringeval et al, 2014;Valentini et al, 2014).…”

Response of CO2 fluxes and productivity to water availability in two contrasting ecosystems in northern Benin (West Africa)

“…Consequently, most ecosystems (anthropogenic and natural) may be significantly modified in the context of climate change and high population growth in West Africa (IPCC 2013;Ajonina et al 2014). Few recent networks underlined that the knowledge of carbon exchanges between atmosphere and ecosystems remains very limited in West Africa (Bombelli et al 2009;Valentini et al 2014;Grace et al 2014). In this context, the analyses of photosynthesis and respiration of terrestrial ecosystems could be important to better understand vegetation responses to extreme climatic events as severe droughts or delayed rains.…”