Sammanfattning: Plastics are a ubiquitously used material in modern life. People worldwide encounter plastics and their associated chemicals, such as chemical additives and non-intentionally added substances daily, whether voluntarily or involuntarily. Recently, there has been a significant increase in research into the ecological impacts of plastic pollution. Plastics pollution is often misrepresented in popular narratives as a waste problem caused by consumer behavior and poor littering habits. While better solid waste management, including increased recycling and clean-up efforts, and changes in consumer behavior may contribute addressing the issue, it is important to urgently recognize the complexity of the problem beyond this end-of-life approach. This oversimplification ignores the impact of plastics pollution on on the processes that regulate Earth system conditions the extent of which is currently unknown. This Licentiate aims to enhance comprehension of the impact of plastic pollution on biophysical and biogeochemical processes in the Earth system. The study is guided by the Planetary Boundaries framework and seeks to answer the following questions: i) has plastics pollution contributed to exceeding the novel entities' planetary boundary and departing from the safe operating space for humanity?; and ii) are there Earth system impacts of plastic pollution? Paper I sets out control variables that could track the planetary pressures in the planetary boundaries framework. Paper II explores Earth system impacts via linkages from plastics pollution to the other processes in the planetary boundaries framework. The research is guided by pluralism and reflexivity, and the methods used include expert deliberation and synthesis. In Paper III, I begin to link the social and ecological dimensions of plastic pollution, in a scientific commentary that uses normative analysis to integrate social-ecological systems. This thesis argues that plastics pollution, as a human-made novel entity, has exceeded the safe operating space for humanity. It explores the challenges of establishing a quantiTiable limit for novel entities and suggests using several control variables to capture the complexity of this boundary. It provides speciTic examples for each variable and discusses the complexity of plastics pollution at the planetary level. Moreover, it highlights the potential negative consequences of setting up a single quantiTiable boundary. Additionally, it demonstrates how plastics pollution disrupts Earth system processes across all planetary boundaries, and how changes in one boundary can have knock-on effects on others. It shows that the impacts of plastics pollution can exacerbate the impacts of other planetary boundaries. Furthermore, this thesis presents a synthesis of complex information from multiple fields of natural science. This research has demonstrated the ability to engage negotiators and has the potential to facilitate scientific knowledge for the science-policy interface in the ongoing UN- led Plastics Treaty multilateral agreement. By proposing ways to translate these assessments into clear and understandable language, it enables country delegates and civil society to better use science-based knowledge to inform policy. The next phase of the research will investigate the global problem of plastic pollution from a social-ecological systems perspective.