Despite existing guidance and research-driven methodological advancements, there is a dearth of research on how Cumulative Effects Assessment (CEA) is currently executed in practice.
As a result, there is a lack of understanding of what the enduring CEA practice challenges are. Limited comparison across research, assessment practice and jurisdictions makes it difficult to identify persistent challenges. Uncertainty around how CEA is implemented also obscures which socio-environmental issues are prioritised or overlooked.
A better understanding of these considerations is needed to improve CEA effectiveness and develop comprehensive assessments in the offshore wind energy sector, particularly in light of the growing number of marine spatial plans and the proliferation of offshore wind energy projects across the world.
Enduring sectoral challenges: cumulative effects of offshore wind
Summary and Conclusions
Table of contents
1Purpose and scope
This report presents a comprehensive qualitative register of prevailing practical challenges in Cumulative Effects Assessment practice, as well as key (cumulative) socio-environmental risks within the offshore wind energy sector.
By outlining prevailing CEA challenges and socio-environmental risks, the objective is twofold:
- to establish a shared understanding of current practice issues that need to be addressed to improve the assessment of cumulative effects; and
- to identify critical pressures/receptors that need to be prioritised in cumulative (and trans-boundary) assessments.
It is anticipated that the outcomes of these objectives will provide a robust foundation for developing a CEA framework and inform the development of decision-support tools under SUSTAINOW.
2Current practice challenges
Despite significant advancements, the implementation of CEA continues to face significant challenges. These are characterised by procedural shortcomings, analytical complexities, data limitations and lack of knowledge. They coincide in the following procedural shortcomings:
- Assessment type: There is a predominance of qualitative CEA approaches, largely based in expert judgements, which can entail a degree of professional subjectivity or bias and influence the replicability of assessments and transferability of outputs. Quantitative approaches are only applied at project level and for certain pressures/receptors (e.g. noise and marine mammals, physical presence of turbines and bird collision).
- Methods: Significant variations are observed in CEA methods (e.g. spatial analysis, modelling, expert judgement, field observations), with particular differences between assessment levels (SEA versus EIA). Such inconsistencies lead to non-comparable CEA outcomes and recommendations.
- Spatial extent: The spatial extent or geographic area of CEA varies across assessments: from the plan area boundary, to project ‘zones of influence’, to entire sea basins. The ‘zones of influence’ also vary across and within receptors (e.g. from 300 meters through to 750 meters from the development site, to the entire Economic Exclusion Zone for noise-sensitive receptors). These disparities also apply to monitoring, affecting coherent and coordinated ecosystem-wide assessments.
- Temporal scope: CEAs are typically bounded by plan/project timelines (e.g. 25 to 30 years), which are not necessarily aligned with the long-term, delayed, and often non-linear ecological responses characteristic of marine ecosystems. Importantly, CEAs capture a snapshot in time, and they do not meaningfully capture effects over time due to shifting baselines.
Further challenges include inconsistent assessment scopes and interpretations of CEA, limited consideration of synergistic and antagonistic effects, insufficient guidance, and a lack of thresholds for acceptable cumulative social and environmental change.
Related analytical challenges include:
- Aggregation: The absence of agreed approaches for aggregating multiple assessment indicators, pressure-receptor relationships and model outputs into a single, transparent, and scientifically defensible cumulative effect index. This can affect comparability of assessment as well as communicating assessment outcomes, and thus the capacity of decision-makers to understand overall cumulative effect implications.
- Dynamic nature: The dynamic and highly complex nature of marine ecosystems creates substantial uncertainty in assessing cumulative effects, as well as a risk that findings become rapidly outdated in evolving marine environments.
- Uncertainties: Uncertainties arise when accurate and complete socio-environmental information is missing. This can also result from a limited understanding of how cumulative effects vary across geographic contexts, ecosystem conditions, and exposure pathways. This affects the accuracy and reliability of assessment outcomes.
- Significance: Effect significance or impact judgements are complex and highly dependent on the conceptual interpretation of cumulative effects, assessment scope, thresholds agreed upon, and the spatial and temporal extents considered. They are also influenced by planning phases considered and, importantly, by data and knowledge gaps.
Model validation is another key analytical challenge. Predictive models are essential for forecasting cumulative effects but often lack long-term, post-construction validation, risking compounded uncertainties and inaccurate predictions.
Literature, environmental reports and experts identify four major data limitations and knowledge gaps:
- Centralisation: The lack of accessible, interoperable data systems makes baseline data difficult to find and limits consistent, comprehensive assessments.
- Data gaps: Missing, outdated, inconsistent or unreliable data reduces the accuracy of cumulative effect identification, prediction, monitoring and management.
- Planning phases: Interactions between construction, operation and decommissioning remain poorly understood. Decommissioning is rarely assessed due to limited empirical data.
- Social effects: Assessments generally focus on fisheries and tourism, while broader effects on employment, culture, health and well-being are rarely examined.
Differences in data quality, formats, collection methods and monitoring frameworks also limit comparison across projects, sectors and jurisdictions. Restricted access to proprietary or sensitive data creates further gaps. Limited understanding of how pressures affect ecological and social receptors, and how these receptors recover over time, can result in incomplete or inaccurate assessments.
3Environmental risks
Cumulative environmental effects of wind farms vary across their planning stages. They are characterised by a temporal split: intense, short-term disturbances during construction and long-term, chronic shifts during operational phases. Decommissioning effects remain understudied.
Risks can be positive or negative, and largely relate to species-specific responses based on sensitivity and/or tolerance. In all cases, such risks should be jointly considered, as appropriate to the marine area and plan/project context. Existing challenges with regard to data and knowledge limitations may affect the extent to which some of these risks may be effectively considered.
The main risks during construction are those derived from underwater noise pressures. It affects bats, birds, fish and marine mammals, creating permanent or temporary disturbance and displacement (e.g. from habitats and/or migratory routes). For marine mammals, it can lead to temporary or permanent auditory injury and behavioural changes which can lead to potential starvation. Habitat change pressures during construction (e.g. removal of natural habitat; introduction of hard surfaces and artificial reefs) can also affect fish communities, and alter predator-prey dynamics.
During operation, physical structures and barriers associated with the presence of wind turbines and foundations can increase collision and mortality risk for birds as well as displacement from their habitats and migratory routes. For fish and other marine species, physical structures can lead to changes in species interactions through food supply. Turbine foundations can also create changes in marine physical processes and hydrodynamics (e.g. waves and currents around foundations) and subsequently influence changes in habitats and food supply.
The positive effect of the existence of wind farms is the potential for biodiversity conservation as a result of the removal of fishing and shipping activities (i.e. wind farms acting as marine protected areas).
Additional environmental pressures during operation include the introduction of non-native species resulting in ecosystem changes. There is also the positive or antagonistic reef effect (i.e. hard substrate from foundations and scour protection) leading to changes in species composition, benefiting fish in particular through increased food supply and spawning connectivity.
Decommissioning is understudied, but the partial or full removal of physical structures can pose the risk of detrimental effects on habitats and species richness.
4Social risks
Cumulative social risks are understudied compared to environmental risks, despite often being the most contentious. The interaction between fisheries, conservation and offshore wind is a major source of cumulative social friction. It involves competing values, subjective data and cultural, identity-related and economic risks for local communities.
The presence of physical structures and barriers represents the main pressure, affecting significantly the fisheries sector by creating the risk of:
- marine use conflicts (e.g. spatial squeeze that forces fishermen to less favourable conditions, and increased competition),
- navigation safety risks for the shipping industry.
- visual intrusion and seascape changes reducing the value of coastal tourism destinations.
- negative wellbeing of coastal communities and local public opposition to renewable energy development if local communities are not engaged in planning and decision-making.
- A potential positive ‘risk’ is that of conservation (by removing fishing from wind farm sites and the creation of reef effects) with the subsequent spillover of adult fish into grounds adjacent to wind farm sites, increasing catch for fishermen.
5Basis for further work
The CEA challenges and socio-environmental risks identified in this report will guide the project’s next activities. They will inform the development of a practical CEA framework and a tool for more informed and sustainable offshore wind farm siting. The identified risks will also shape data collection and the project’s shared data space.