General Information
Cumulative Effects Assessment and Management
Definition
Cumulative effects and cumulative effects assessment are defined in various ways. A common definition of cumulative effects, around since the 1980s, is:
“Cumulative effects are changes to the environment that are caused by an action in combination with other past, present, and future human actions.”
Overview and History
Cumulative Effects Assessment and Management (CEAM) began in the early 1970s when it was realized that proposed projects needed to be analyzed in relation to their location and surrounding land uses. Further, agencies that processed multiple concurrent permit approvals for similar types of projects also realized that such approvals needed to incorporate consideration of all applications in close spatial and temporal proximity to each other, as such actions often contribute to cumulative effects.
In the practice of Environmental Impact Assessment (EIA) in the USA, the term “cumulative effects” was first mentioned in guidelines of the Council on Environmental Quality (CEQ) in 1973. In mid-1979, CEQ’s first EIA-related regulations defined a cumulative impact (effect) as:
“An impact on the environment that results from the incremental impact of an action when added to other past, present, and reasonably foreseeable future actions, regardless of what agency (federal or non-federal) or person undertakes such other actions. Cumulative impacts can result from individually minor but collectively significant actions taking place over time.”
During the 1980s and 1990s, environmental impact studies in both the USA and Canada began to regularly incorporate cumulative effects considerations. Further, other definitions appeared. For example, the Canadian Environmental Assessment Agency (CEAA) provided the definition provided above.
During the late 1990s, the Annual Meetings of the International Association for Impact Assessment (IAIA) began to include papers and topical sessions on Cumulative Effects Assessment (CEA), and related papers have been published in Impact Assessment and Project Appraisal, Environmental Impact Assessment Review, Environmental Practice, and Environmental Assessment Policy and Management. Litigation related to the adequacy of CEA within impact study documents also began to appear in the USA during this period. The absence of established frameworks or multi-step processes for addressing CEA was often a central issue in legal challenges. As a result, in the late 1990s, both the CEQ in the USA and CEAA in Canada published guidance for practitioners. These processes were conceptually similar, each outlining structured assessment steps.
The early 2000s saw continued advancements in CEA practice, particularly for proposed projects. Strategic Environmental Assessments (SEA), known as Programmatic Environmental Impact Statements (EIS) in the USA, increasingly addressed cumulative effects. In fact, CEA should be a central feature of these strategic studies. Further, methods and tools used in EIA practice were adapted to assess the combined effects of multiple actions on Valued Ecosystem Components (VECs). New topics, such as environmental sustainability, were recognized as key to understanding cumulative effects. Additionally, cumulative effects analysis has become essential in addressing global challenges like climate change, with frameworks emerging to integrate climate considerations into assessment methodologies.
Methods and Tools
The first decade of EIA practice (1970s) saw considerable attention given to methods and tools for practitioners. Over subsequent decades, assessment methodologies have evolved, with CEA now an integral part of impact assessment practice. Many CEA methods are adaptations of EIA tools, modified to incorporate the cumulative contributions of multiple actions on specific VECs.
Examples of such modifications include:
- Expanding questionnaire checklists to include considerations of past, present, and foreseeable future activities affecting a resource or system.
- Enhancing interaction matrices to assess cumulative impacts over time and across multiple projects.
- Modifying network diagrams to map relationships between multiple stressors affecting VECs.
CEA frameworks were first developed and refined in the USA and Canada and are now widely used in the European Union, Australia, New Zealand, and elsewhere. While different jurisdictions apply unique methodologies, common principles guide the approach. These frameworks can generally be condensed into the following six steps:
Step 1 – Initiate the CEA process by identifying the incremental effects of the proposed project (or policy, plan, or program) on selected VECs within the project area. VECs can be selected based on existing or anticipated environmental stressors, the presence of protected species or habitats, and potential interactions with other human activities that may affect the same components. It should be noted that information on incremental effects is also needed to address the direct and indirect effects of the proposed project.
Step 2 – Identify other past, present, and reasonably foreseeable future actions within the defined spatial and temporal boundaries that could contribute to cumulative effects on the VECs or their indicators. This step involves defining the study area and timeframe for the analysis. Guidance related to establishing appropriate spatial and temporal boundaries for CEA is available internationally.
Step 3 – For the selected VECs, gather relevant baseline data on their indicators and assess their historical and current conditions. The historical reference point should align with the chosen temporal boundary. Where possible, trends in VEC conditions should be identified and analyzed. Comparisons to established environmental thresholds, regulations, or policy standards should also be included to contextualize impacts.
Step 4 – Analyze how the proposed project (or plan, program, or policy) interacts with other actions in the CEA study area to affect the selected VECs and their indicators. Various qualitative and quantitative tools can be used for this analysis. While data limitations may pose challenges for predictive modeling, identifying directional trends in cumulative effects remains valuable. The emphasis should be on aggregating effects to determine overall cumulative impacts.
Step 5 – Assess the significance of the cumulative effects on each VEC over time. This assessment should begin with the incremental effects of the proposed action, considering location, compliance with environmental laws, resource conservation priorities, and other relevant factors such as risk, controversy, and human values. Many countries, development banks, and aid agencies provide established criteria for evaluating significance. Additionally, integrating sustainability considerations—including social and economic dimensions—can enhance the evaluation of cumulative effects.
Step 6 – For VECs expected to experience significant negative impacts, develop mitigation strategies tailored to both incremental and cumulative effects. Where multiple contributors influence regional cumulative effects, multi-stakeholder collaboration should be considered to implement joint management measures at local and regional levels. Recent research has emphasized adaptive management (AM) as a critical approach for addressing cumulative effects, particularly in situations of uncertainty. AM relies on ongoing monitoring and stakeholder engagement to refine impact mitigation strategies over time.
Resources
For more information refer to IAIA’s FasTips No. 16: Cumulative Effects Assessment and IAIA’s Key Citations: Cumulative Effects, both available in the IAIA Resource Library.
