Cumulative effects occur when two or more activities affect an environmental resource, ecosystem, or human community, or when an activity causes effects on two or more occasions. For example, marine mammals in the Arctic can be adversely affected by hunting, oil spills, loss of habitat, and commercial fishing pressure on prey species. The cumulative effect is the result of all impact-causing activities that affect a resource while the impacts of the proposed action are occurring or remain in effect.
A particular action may cause only minor adverse (or beneficial) effects on the environment. However, when added to the effects of other activities, the overall (cumulative) effect may be significant. In some cases, the effects of a project, when combined with those of other activities, cause synergistic effects, which are different than those of the projects individually and could be significant. Additive and synergistic effects are identified by a cumulative analysis. It is important to describe and analyze cumulative effects in EIAs because they can be significant, even if the effects of the proposal are minor.
In January 1997, the Council of Environmental Quality (CEQ) issued a handbook entitled, Considering Cumulative Effects Under the National Environmental Policy Act. The handbook provides principles and procedures for cumulative effects analysis.