Regional vegetation tipping in a numerical Earth system Model of Intermediate Complexity

Vegetation plays a fundamental role in the climate system, influencing energy exchange, the hydrological cycle and atmospheric composition. At the same time, vegetation dynamics are strongly influenced by climatic conditions, creating complex feedback mechanisms that regulate the Earth’s climate. Representing these interactions between climate and vegetation is essential in climate models, which must realistically simulate vegetation to improve the representation of the simulated climate. This thesis focuses on the Planet Simulator (PlaSim), an Earth system Model of Intermediate Complexity, and on the Simulator for Biospheric Aspects (SimBA), its embedded simple dynamic global vegetation model. SimBA can operate in two modes: in the non-interactive vegetation mode, simulated vegetation does not influence the climate, while in the interactive mode, vegetation influences the climate through four land surface variables: surface albedo, surface roughness, surface conductance and soil water holding capacity. The work begins with an introduction on the role of vegetation in the climate system, with a focus on Leaf Area Index (LAI), followed by a detailed description of the parametrizations used in SimBA. A significant part of the research focused on the tuning of SimBA parametrizations used in the interactive vegetation mode, specifically the snow-free albedo and soil water holding capacity, to minimize biases between modelled climate and observational datasets, obtained from the ERA5 reanalysis. Subsequently, an analysis of the simulated climate was carried out to evaluate the changes induced by an interactive vegetation compared to a non-interactive one and to assess biases with observations. An analysis was conducted to evaluate how well vegetation is simulated in PlaSim at both global and regional scales. The LAI simulated by PlaSim was compared to observations (GIMMS LAI4g dataset and ERA5 reanalysis dataset) and to four Earth System Models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), with the aim of contextualizing PlaSim’s performance within the framework of more complex models. The analysis revealed that PlaSim struggles to represent seasonal cycles both in the mid-latitudes and the tropics, while it performed relatively well in replicating the global spatial distribution and global mean, remaining within the range of CMIP6 models. Lastly, a simulation with increasing atmospheric CO2 concentration was run using PlaSim in the tuned interactive vegetation mode. The goal was to investigate how vegetation responded to rising CO2 levels, and how vegetation changes, in turn, influenced the climate. Specifically, vegetation tipping was analysed in Europe and the Amazon rainforest. The simulation revealed instances of regional vegetation tipping, occurring both in mid-latitudes and the tropics, driven by non-linear feedbacks between changes in hydrological cycle and shifts in vegetation patterns.