Asymmetry and uncertainties in biogeophysical climate–vegetation feedback over a range of CO2 forcings

[摘要] Climate–vegetation feedback has the potential to significantly contribute to climate change, butlittle is known about its range of uncertainties. Here, using an Earth system model ofintermediate complexity we address possible uncertainties in the strength of the biogeophysicalclimate–vegetation feedback using a single-model multi-physics ensemble.Equilibrium experiments with halving (140 ppm) and doubling (560 ppm)of CO₂ give a contribution of the vegetation–climate feedback to globaltemperature change in the range −0.3 to −0.1 °C and −0.1 to0.2 °C, respectively. There is an asymmetry between warming andcooling, with a larger, positive vegetation–climate feedback in the lowerCO₂ climate. Hotspots of climate–vegetation feedback are the borealzone, the Amazon rainforest and the Sahara. Albedo parameterization is thedominant source of uncertainty in the subtropics and at high northernlatitudes, while uncertainties in evapotranspiration are more relevant in thetropics. We analyse the separate impact of changes in stomatal conductance,leaf area index and vegetation dynamics on climate and we find that differentprocesses are dominant in lower and higher CO₂ worlds. The reduction instomatal conductance gives the main contribution to temperature increase fora doubling of CO₂, while dynamic vegetation is the dominant process inthe CO₂ halving experiments. Globally the climate–vegetation feedback israther small compared to the sum of the fast climate feedbacks. However, itis comparable to the amplitude of the fast feedbacks at high northernlatitudes where it can contribute considerably to polar amplification. Theuncertainties in the climate–vegetation feedback are comparable to themulti-model spread of the fast climate feedbacks.

[学科分类] 地球化学与岩石