International Association for Impact Assessment

Cumulative Effects Assessment and Management

Edited By: Bridget

Overview & History

Cumulative Effects Assessment 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 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 the:

“impact on the environment which results from the incremental impact of the action when added to other past, present, and reasonably foreseeable future actions regardless of what agency (federal or non-federal) or person undertake such other actions. Cumulative impacts can result from individually minor, but collectively significant, actions taking place over a period of time”.

In the 20-year period encompassing the 1980s and 1990s, environmental impact studies in both the USA and Canada began to routinely incorporate cumulative effects considerations in study documents. Further, other definitions appeared. For example, the Canadian Environmental Assessment Agency (CEAA) has suggested a simple definition:

“cumulative effects are changes to the environment that are caused by an action in combination with other past, present, and future human actions.”

During the latter part of the 1990s, the Annual Meetings of the International Association for Impact Assessment (IAIA) began to include papers and topical sessions on CEAM, and resultant and related papers have been published in Project AppraisalImpact Assessment and Project AppraisalEnvironmental Impact Assessment ReviewEnvironmental Practice, and Environmental Assessment Policy and Management. Litigation related to the adequacy of CEAM within impact study documents also began to appear in the USA in both the 1980s and 1990s. The absence of acceptable frameworks or multi-step processes for addressing CEAM seemed to be fundamental to many plaintiffs’ challenges. As a result, in the late 1990s, both the CEQ in the USA and CEAA in Canada published practitioner guidance. Both processes were conceptually similar, and they both included 11 steps which were generally matchable. These steps are consistent with the six step process described below.

The first decade in the 21st century has experienced continuing improvements in CEAM practice, particularly as related to proposed projects. Strategic environmental assessments (SEAs), also referred to as Programmatic EISs in the USA, also have given greater attention to cumulative effects issues. In fact, CEAM should be a central feature of these strategic studies. Further, methods and tools used in EIA practice began to be modified to incorporate consideration of the combined effects of multiple actions on selected VECs (Valued Ecosystem Components). New topics such as environmental sustainability have also been recognized as integrators of concerns related to cumulative effects. Finally, the potential benefits of cumulative effects thinking relative to global issues such as climate change are being explored along with the development of appropriate procedures for such evaluations. 

Process Context of CEAM

As noted above, CEAM frameworks have been promulgated in the USA and Canada. Further, such frameworks (step-wise processes or procedures) have also been developed for usage in the European Union countries, Australia, New Zealand, and elsewhere. The 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 environs of the project location. The VECs can be selected based on information related to current or anticipated future degraded or stressed conditions, the occurrence of protected species or habitats, and the presence or anticipated presence of other human activities that would (adversely) affect the same VEC. It should be noted that information on incremental effects is also needed to address the direct and indirect effects from the proposed project. Further, once the VECs have been selected, they should be subject to each of the following five steps.


  • Step 2 – Identify other past, present, and reasonably foreseeable future actions within the space and time boundaries that have been, are, or could contribute to cumulative effects (stresses) on the VECs or their indicators. Based on this knowledge, identify appropriate spatial and temporal study boundaries for each VEC. Guidance related to establishing the scope of the CEA relative to space, time, and other actions is available elsewhere. Such guidance is not limited in application to the USA or Canada; it can be used internationally, as can pertinent guidance from other countries as noted above.


  • Step 3 – For the selected VECs, assemble appropriate information on their indicators, and describe and assess their historical to current conditions. The historical information should coincide with the selected past temporal boundary (or historical reference point). Further, and depending upon the availability of information, any identified trends in the conditions of the VECs and their indicators should be determined and analyzed. Further, comparisons to numerical standards or policies, or to identified thresholds of significance, should also be presented for each VEC.


  • Step 4 – “Connect” the proposed project (or plan, program or policy) and other actions in the CEA study area to the selected VECs and their indicators. Numerous types of tools could be used to establish either descriptive or quantitative connections. Examples of such tools will be mentioned in the next section. Predictions related to future VEC effects resulting from multiple actions may be problematic due to the absence of detailed information; however, identification of “up-or-down” changes in the VECs and their indicators can be useful. Again, several models and other tools will be mentioned later. Finally, emphasis should be given to the aggregation of effects (that is, to the anticipated cumulative effects on each VEC).


  • Step 5 – Assess the significance of the cumulative effects on each VEC over the time horizon for the study. Such significance determinations should begin with the incremental effects (the direct and indirect effects) of the proposed action on specific VECs. The focus is on the VEC, not on the action. Criteria for such determinations of significance already exist within the EIA systems in numerous countries; as well as development banks and aid agencies. For example, the USA has a structured definition for significance based on considering the location; compliance with environmental media, natural resources and cultural resources laws and policies; and other factors such as risk, controversy, human values, etc. The same definition can and should be applied to significance determinations for the composited cumulative effects on the VECs. Further, the concept of environmental sustainability (including social and economic sustainability) could be considered both in relation to incremental effects and cumulative effects. This concept was used in a CEA study on waterway navigation improvements on the Ohio River in the USA.


  • Step 6 – For VECs or their indicators that are expected to be subject to negative incremental impacts from the proposed project and for which the cumulative effects are significant, develop appropriate action-specific “mitigation measures” for such impacts. Further, if significant cumulative effects are anticipated on any VEC or its indicators, consideration should be given to multi-stakeholder collaboration to develop joint cumulative effects management measures, either locally or regionally, or both. Recent reports from Canada provide information on both mitigation measures for incremental impacts, and management measures when there are multiple contributors to regional cumulative effects. An emerging topic of growing relevance to cumulative effects management, either locally or regionally, is adaptive management (AM). AM is envisioned as a follow-up practice to traditional impact studies, particularly when there are uncertainties. A foundational element for AM is a carefully planned monitoring program, with the results used to inform subsequent operational practices and decision-making. Finally, multi-stakeholder collaboration in follow-up activities can be both cost-efficient and an aid in local and regional planning.


Methods and Tools

The first decade of EIA practice occurred in the 1970s, and one topic which received considerable attention was related to appropriate methods and tools which could be used by practitioners. Over the ensuing three decades, the types of EIA-related methods and tools have continued to expand. Since the practice of CEA is now into its second decade, interest is continuing on appropriate methods and tools which could be used to improve professional practice. Based upon the review of various CEA informational sources, it can be concluded that many of the current and developing methods and tools are similar to those used for EIA practice. The primary difference is related to the need to incorporate other actions and their contributions to cumulative effects on specific VECs. Such incorporation is often done by simple modifications to existing EIA methods and tools. Brief examples of such modifications include: 

(1) adding “other actions” questions to questionnaire checklists focused on identifying direct and indirects of proposed actions;

(2) modifying interaction matrices to include columns related to past, present, and future actions; and

(3) modifying network diagrams to include other actions.


Special thanks to Larry Canter and William Ross for providing initial content for this IAIA Wiki topic.

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