On the biogeochemical effects of nature-based solutions in Earth System modelling

Main objective and motivation This PhD work aims to broaden the assessment of how changes in land use and land cover impacts the climate. Altering land surface properties, such as reforesting or expanding croplands and grazing areas, can lead to multiple, complex, and sometimes opposing climatic consequences. For instance, planting a new forest can absorb CO2 from the atmosphere, benefiting the climate. However, it can also reduce surface reflectance—e.g. forests are darker than grasslands—resulting in more energy being absorbed by the Earth system and potentially warming the climate. Since current climate mitigation strategies often involve significant and widespread changes in land use and land cover, it is crucial to adopt a holistic perspective when evaluating the benefits or drawbacks of these actions to avoid the risk of counterproductive outcomes. Background and further look Land use, land cover changes, and land management practices are often termed "nature-based solutions" when aimed at mitigating and adapting to climate change. Some examples include restoring wetlands and peatlands for flood control and carbon storage, or agroforestry for sustainable farming. In international policy frameworks, these strategies have gained attention primarily due to the ability of the vegetation to absorb carbon dioxide from the atmosphere. In climate science, this perspective has expanded to include biogeophysical effects, such as changes in surface reflectivity (“albedo”) or water vapor emissions by plants (“evapotranspiration”). However, limited attention has been given to how changes in land use and land cover impacts plant chemical emissions, which can also alter the atmospheric chemical composition, influencing particulate matter (“aerosol”) and cloud properties, and ultimately the Earth's energy balance. This work focuses on understanding the processes initiated by changes in the production of BVOCs (Biogenic Volatile Organic Compounds, a dominant class of plant chemical emissions) that result from land use change, and their impacts on the Earth's energy balance, comparing the importance of these changes with other, better-known impacts (e.g. carbon sequestration and surface reflectance). Methodology To investigate the complex interactions between land and climate, the Norwegian Earth System Model (NorESM2) is chosen as the primary tool for this study. Earth System Models (ESMs) are powerful tools that integrate the best knowledge of the dynamics of Earth's components and their interactions on a global scale. Most global climate targets are evaluated based on the outputs of these models. In this study, NorESM2 is run in a targeted set of different configurations aimed at understanding and isolating the cascade of processes connecting land use and cover changes with atmospheric impacts. Results The first part of the project investigates the climatic impacts of a northward expansion of the boreal forest towards the Arctic in response to rising global temperatures. Preliminary results indicate that changes in albedo dominate the impacts, as dark coniferous forests replace grasslands that are covered by snow for many months of the year, resulting in a pronounced "black and white" effect. The study focuses on investigating the impact of this vegetation shift on BVOC emissions and their influence on aerosol and cloud properties. The radiative forcing related to this chain of effects, known as the vegetation-BVOC-SOA-cloud feedback, shows a much weaker signal compared to albedo changes, despite some studies suggesting it could be of a similar magnitude. The simulation setup has been refined to capture a more significant signal and to extend the analysis to future climate scenarios that are consistent with the vegetation shift—the first set of simulations was run with only land cover changes, without modifying the climate.