Assessing biodiversity impacts and public acceptance of offshore wind farm decommissioning

Since the mid-twentieth century, human activities have dramatically reshaped the Earth system, driving climate change and biodiversity loss. In marine environments, habitat degradation has emerged as a major threat to biodiversity. This is evident in many seas, including the North Sea, where centuries of exploitation have transformed the seafloor and largely removed the hard substrate habitats that were once present. Offshore wind plays a vital role in the transition to low-carbon energy systems, as it offers a renewable energy source with a comparatively low climate footprint. While the global capacity of offshore wind is expanding, many of the earliest offshore wind farms are approaching the end of their operational lifetimes and will soon require decommissioning.

Offshore wind farms introduce artificial hard substrates into the marine environment, which can function similarly to natural reefs and offer ecological benefits. At the same time, concerns have been raised about potential downsides, such as the spread of non-indigenous (alien), potentially invasive species. Despite growing interest in these effects, no structured framework currently exists for evaluating them. Life Cycle Assessment (LCA) is a widely used tool to quantify the environmental impacts of offshore wind structures across their life cycle, but it does not currently allow for the inclusion of marine biodiversity impacts, particularly not those associated with marine infrastructure and decommissioning of offshore wind farms. Additionally, little is known about public attitudes toward different decommissioning strategies, despite the fact that social acceptance has proven to be a decisive factor in marine infrastructure governance. Public perceptions are likely to be shaped by how decommissioning strategies affect marine biodiversity, which underscores the importance of integrating ecological considerations into communication and decision-making processes.

This PhD project addresses two key research gaps: the lack of structured approaches for assessing biodiversity impacts of offshore wind farm decommissioning within LCA, and the limited understanding of social acceptance related to different decommissioning scenarios. The project seeks to support more environmentally informed and socially legitimate decision-making for offshore wind farm decommissioning by advancing these areas.

The research presented in this thesis is based on four scientific articles. Article I investigates public attitudes toward decommissioning through a Danish national survey. Article II presents a systematic literature review of biodiversity impacts linked to offshore wind farm decommissioning. Article III introduces a novel Life Cycle Impact Assessment (LCIA) method for quantifying local marine benthic biodiversity impacts from habitat changes in the North Sea. Article IV applies the LCIA method and survey insights in a case study of the Horns Rev 1 wind farm, illustrating environmental and social trade-offs across decommissioning options.

The survey study (Article I) found that public preferences vary depending on how information is framed. Respondents generally supported leaving structures in place when the associated biodiversity benefits were emphasized, but favored full removal when climate benefits from the removal and recycling of wind farm components were also presented. This indicates that public support is shaped by perceived environmental priorities and underscores the importance of transparent and well-designed communication when engaging stakeholders in decommissioning decisions.

Article II identified key biodiversity impact pathways from the literature, with habitat change emerging as the dominating pressure on benthic species. These insights informed the development of the LCIA method in Article III, which uses empirical data from artificial structures in the North Sea to quantify changes in species richness associated with construction and decommissioning. The method enables evaluation of both positive and negative impacts and disaggregates results by eight taxonomic groups and alien species.

The case study in Article IV demonstrates the application of the LCIA method alongside the survey results. It shows that partial removal can preserve a large share of species richness on turbine structures but may be less favorable in terms of limiting the presence of alien species, maximizing material recovery, and aligning with social preferences. These findings reveal trade-offs between environmental and social outcomes and highlight the value of integrated assessments in decommissioning planning.

Altogether, this PhD project contributes new knowledge and methodological tools for assessing marine biodiversity impacts and social acceptance in offshore wind farm decommissioning. It provides the first LCIA method tailored to local benthic biodiversity impacts from habitat change, offers insights into public preferences for decommissioning strategies, and demonstrates how ecological and social dimensions can be considered jointly. The findings are relevant to researchers, policymakers, and industry actors working to balance biodiversity protection, climate goals, and public acceptance in managing aging offshore wind infrastructure.