Abstract
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 Representative Concentration Pathway (RCP) scenarios. The speed of ecological processes such as leaf physiology, demography or migration can differ from the speed of changes in environmental conditions. Such mismatches imply lags between the actual vegetation state and the vegetation state expected under prevailing environmental conditions. Here, we used a dynamic vegetation model, the adaptive Dynamic Global Vegetation Model (aDGVM), to study lags between actual and expected vegetation in Africa under a changing atmospheric CO2 mixing ratio. We hypothesized that lag size increases with a more rapidly changing CO2 mixing ratio as opposed to slower changes in CO2 and that disturbance by fire further increases lag size. 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 CO2 under recent and near-future levels. When averaged across all biomes and simulations with and without fire, times to reach an equilibrium vegetation state increase from approximately 242 years for 200 ppm to 898 years for 1000 ppm. These results have important implications for vegetation modellers and for policy making. Lag effects imply 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 lag effects in models and in data used for model testing. Policy makers need to consider lagged responses and committed changes in the biosphere when developing adaptation and mitigation strategies.
Highlights
Climate and the composition of the atmosphere have been subject to substantial changes during Earth’s history (Beerling and Royer, 2011)
We explore the consequences of these predictions for projections of climate change impacts on African vegetation under rates of CO2 change as predicted in RCP2.6, 4.5, 6.0 and 8.5, examining the difference between transient and equilibrium vegetation states as the CO2 mixing ratio changes
Equilibrium simulations for fixed CO2 mixing ratios show that the cover of C4-dominated vegetation (C4 grasslands and savannas) in Africa decreases with increasing CO2, whereas the area covered by C3-dominated woody vegetation increases (Figs. 1, 2)
Summary
Climate and the composition of the atmosphere have been subject to substantial changes during Earth’s history (Beerling and Royer, 2011). Paleo-records indicate that the expansion of forest vegetation during the Devonian (419.2–358.9 Ma) dramatically reduced the atmospheric CO2 mixing ratio (Le Hir et al, 2011), and Milankovitch cycles cause periodic changes in the climate system and the atmosphere on millennial timescales (Milankovic, 1941; Hays et al, 1976). Current carbon emission rates are unprecedented and higher than during the Paleocene–Eocene Thermal Maximum (PETM), a period with high carbon emissions some 56 million years ago (Zeebe et al, 2016). During the PETM, temperature increased by approximately 5–8 K due to massive carbon release likely caused by volcanic activity. As temperature increased by 6 K within a 20 kyr period, the PETM is often considered the best analogue for current and future climate change (Zeebe et al, 2016)
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