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close this bookConducting Environmental Impact Assessment in Developing Countries (United Nations University, 1999, 375 p.)
close this folder9. Emerging developments in EIA
close this folder9.2 Cumulative effects assessment
View the document(introduction...)
Open this folder and view contents9.2.1 Concepts and principles relevant to CEA
Open this folder and view contents9.2.2 Conceptual framework
View the document9.2.3 Conclusion


Cumulative effects refer to the accumulation of changes in environmental systems over time and across space in an additive or interactive manner. Changes may originate from actions that are single or multiple, and similar or different in kind. A unit of environmental change attributable to an individual action may be considered insignificant because of confined spatial and temporal scales. However, environmental changes originating from repeated or multiple human actions can accumulate over time and across space, resulting in cumulative effects deemed significant.

CEA is the process of systematically analysing and assessing cumulative environmental change. The practice of CEA is complex because of the need to consider multiple sources of change, alternative pathways of accumulation, and temporally and spatially variable effects. CEA can be guided by an approach that recognizes the components of sources, pathways, and effects and distinguishes attributes specific to each component. Such guidance is particularly relevant in Canada where enactment of the Canadian Environmental Assessment Act in 1992 has simulated inquiry into the theoretical and methodological bases of CEA.

Some countries have incorporated an explicit requirement to address cumulative environmental effects in their EA legislation, for example, Canada and the United States. The requirements to analyse and assess cumulative effects reflects a broadened perspective on the nature of environmental change. This perspective acknowledges multiple perturbations, complex causation, interactive processes, expanded and permeable spatial boundaries, and extended time horizons and time lags. These attributes characterize cumulative effects, or cumulative environmental change.

CEA literature generally concentrates on pervasive, regional environmental problems. Examples include acid rain, agricultural land loss, and watershed management. Clearly there is a need for regional planning and management initiatives to address such matters, but a CEA perspective can also be incorporated into individual project EIAs. Indeed, it is essential because EIA requirements usually focus on individual projects. Such requirement, and the desirability of placing project EIAs within a broader environmental management perspective, contribute to an urgent need for practical, project-level CEA approaches.

Table 9.1 highlights the major differences between conventional EIA and CEA. The distinctions listed in the table create something of a false dichotomy; in practice, it is more a question of emphasis. Conventional EIA can be applied at the policy and programme levels in ways that mirror CEA characteristics. Similarly, project-level planning can apply many CEA properties. Thus, there is considerable fertile ground within the overlap between these two related fields. A careful attention to this middle ground will both renew EIA and ground the largely conceptual field of CEA.

Table 9.1 Characteristics of conventional EIA and CEA


Conventional EIA



Project evaluation

Management of pervasive environmental problems


Single proponent

Multiple projects and/or no proponents


Individual projects with high potential for adverse environmental impacts

Multiple projects and/or activities

Disciplinary perspective

Disciplinary and, to a lesser extent, interdisciplinary

Transdisciplinary and, to a lesser extent, interdisciplinary

Temporal perspective

• Short to medium term
• Continuous dispersion over time
• Proposed activity

• Medium to long term
• Discontinuous dispersion over time (e.g., time lags)
• Past, present, and future activities

Spatial perspective

• Site-specific
• Focus direct on-site and off-site impacts
• Continuous dispersion over space

• Broad spatial patterns
• Wide geographic areas (e.g., cross-boundary impacts)

Systems perspective

• Tendency - single ecological system
• Tendency - single socio-economic system

• Multiple ecological system
• Multiple socio-economic systems


• Interactions among project components
• Interactions among components of environment
• Interactions between project and environment
• Primarily major, direct interactions
• Assumption that interactions are additive

• Also interactions among projects and other activities
• Also interactions among environmental systems
• Also interactions between activities and environmental systems
• Major and minor, direct and indirect interactions
• Expectation that some interactions are non-additive (e.g., synergistic, antagonistic)

Significance of interpretations

• Significance of individual effects interpreted
• Assumption that if individual impacts are insignificant, combined impacts are also insignificant

• Significance of multiple activities interpreted
• Expectation that combined impacts may be significant even though individual impacts may be insignificant

Organizational level

• Intraorganizational

• Interorganizational

Relationship to planning

• Weak links to comprehensive environmental objectives
• Project-level planning
• Incremental project evaluation

• Explicit links to comprehensive environmental objectives
• Programme and policy-level planning
• Middle ground project evaluation and comprehensive planning

Relationship to decision-making

Reactive; after initial decision to initiate activity

Proactive; anticipates future actions

Impact management

Monitoring and management of major, direct impacts

Comprehensive impact monitoring and management system