African biomes are most sensitive to changes in CO ₂ under recent and near-future CO ₂ conditions

Abstract. Current rates of climate and atmospheric change are likely higher than during the last millions of years. Even higher rates of change are projected in CMIP5 climate model ensemble runs for some RCP scenarios. Mismatches between the speed of ecological processes such as physiological adaptation, demographic shifts or migration, and the speed of changes in en- vironmental conditions imply lags between the transient vegetation state and the vegetation state expected under prevailing environmental conditions. Here, we used a dynamic vegetation model, the aDGVM, to study lags between transient and committed vegetation in Africa under changing atmospheric CO2 mixing ratio. We hypothesized that lag size increases with more rapidly changing CO2 mixing ratio as opposed to slower changes in CO2, and that disturbance by fire further increases these lags. Our model results confirm these hypotheses, revealing lags between vegetation state and environmental conditions and enhanced lags in fire-driven systems. Biome states, carbon stored in vegetation and tree cover in Africa are most sensitive to changes in the CO2 mixing ratio under recent and near-future levels, between approximately 300 and 500 ppm. These results have important implications for vegetation modellers as well as for management and policy making. Lag effects implicate that vegetation will undergo substantial changes in distribution patterns, structure and carbon sequestration even if emissions of fossils fuels and other greenhouse gasses are reduced and the climate system stabilizes. We conclude that modelers need to account for lags in models and data used for model testing and that policy makers need to consider lagged responses and committed changes in the biosphere when developing adaptation and mitigation strategies.