On the role of terrestrial ecosystems in a changing Arctic

Abstract: The global temperature rise is proportional to the cumulative amount of CO2 emissions to the atmosphere. This observation is consistent between climate models and historical observations. It has also given rise to the concept of a carbon budget, which sets a threshold for the amount of CO2 that can be emitted into the atmosphere while still fulfilling political goals such as the Paris Agreement.Terrestrial ecosystems are major regulators of greenhouse gases, not least CO2. Since ecosystems may either buffer or add to the anthropogenic emissions depending on if the ecosystem act as a source or a sink of carbon, their functioning is vital to estimating the ‘budget space’ of allowable CO2 emissions for humanity to stay away from dangerous climate change. The Arctic not only contains vast amounts of carbon, but it also warms at a double rate compared to the globe as a whole. The warming will both mobilise carbon that is currently stored in frozen soils, but also induce vegetation shifts such as treeline advance and increased abundance of shrubs. These changes will both affect the biogeochemical cycling of Arctic ecosystems, but also interact with regional climate through changed albedo and partitioning of net radiation. The magnitude and scale of these changes are however uncertain. In this thesis, I use the dynamic vegetation model LPJ-GUESS and a version that has been coupled to a regional climate model – RCA-GUESS – to quantify these complex and interacting processes. The thesis finds that the boreal forests will continue to be of large importance for future regulation of both carbon sequestration, nitrous oxide emissions and land-surface feedbacks. The forests acted as a large and persistent sink of CO2 under a range of climate change scenarios. The forests will expand northward, however, simulations of local treelines revealed that the advance of treelines may be modulated by the soil nitrogen availability. The forest advance also resulted in the greatest climate warming through decreased albedo, although this effect was mostly local. Furthermore, the northward migration of needle-leaved forests and increased abundance of shrubs contributed to indirect climate cooling through increased emissions of biogenic volatile organic compounds (BVOCs). Tundra regions also acted as a sink of CO2 throughout the 21st century, albeit weaker than boreal forests. Furthermore, the model generally performed worse in these areas, making reliable estimates difficult.This thesis investigates a broad range of interactions between vegetation and climate and quantifies the feedbacks from biogeochemical cycling, land-surface change and atmospheric chemistry.