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Considering Cumulative Effects
Under the National Environmental Policy Act
Council on Environmental Quality
This handbook presents the results of research and consultations by the Council on Environmental
Quality (CEQ) concerning the consideration of cumulative effects in analyses prepared under the National
Environmental Policy Act (NEPA). It introduces the NEPA practitioner and other interested parties to
the complex issue of cumulative effects, outlines general principles, presents useful steps, and provides
information on methods of cumulative effects analysis and data sources. The handbook does not establish
new requirements for such analyses. It is not and should not be viewed as formal CEQ guidance on this
matter, nor are the recommendations in the handbook intended to be legally binding.
The Council on Environmental Quality’s action on the environment. Analyzing cumula-
(CEQ) regulations (40 CFR $$ 1500 - 1508) tive effects is more challenging, primarily be-
implementing the procedural provisions of the cause of the difficulty of defining the geographic
National Environmental Policy Act (NEPA) of (spatial) and time (temporal) boundaries. For
1969, as amended (42 U.S.C. $$ 4321 et seq.), example, if the boundaries are defined too
define cumulative effects as broadly, the analysis becomes unwieldy; if they
are defined too narrowly, significant issues may
the impact on the environment which results
be missed, and decision makers will be incom-
from the incremental impact of the action
pletely informed about the consequences of their
when added to other past, present, and
reasonably foreseeable future actions
regardless of what agency (Federal or non- The process of analyzing cumulative effects
Federal) or person undertakes such other can be thought of as enhancing the traditional
actions (40 CFR ~ 1508.7). components of an environmental impact assess-
Although the regulations touch on every aspect ment: (1) scoping, (2) describing the affected
of environmental impact analysis, very little has environment, and (3) determining the environ-
been said about cumulative effects. As a result, mental consequences. Generally it is also critical
federal agencies have independently developed to incorporate cumulative effects analysis into
procedures and methods to analyze the cumula- the development of alternatives for an EA or EIS.
tive effects of their actions on environmental Only by reevaluating and modifying alternatives
resources, with mixed results. in light of the projected cumulative effects can
adverse consequences be effectively avoided or
The CEQ’S “Considering Cumulative Effects minimized. Considering cumulative effects is
Under the National Environmental Policy Act” also essential to developing appropriate mitiga-
provides a framework for advancing envir- tion and monitoring its effectiveness.
onmental impact analysis by addressing cumu-
lative effects in either an environmental assess- In many ways, scoping is the key to analyzing
ment (EA) or an environmental impact statement cumulative effects; it provides the best oppor-
(EIS). The handbook presents practical methods tunity for identi&ing important cumulative
for addressing coincident effects (adverse or effects issues, setting appropriate boundaries for
beneficial) on specific resources, ecosystems, and analysis, and identifying relevant past, present,
human communities of all related activities, not and future actions. Scoping allows the NEPA
just the proposed project or alternatives that practitioner to “count what counts.” By evalu-
initiate the assessment process. ating resource impact zones and the life cycle of
effects rather than projects, the analyst can pro-
In their environmental analyses, federal
perly bound the cumulative effects analysis.
agencies routinely address the direct and (to a Scoping can also facilitate the interagency coop-
lesser extent) indirect effects of the proposed eration needed to identi& agency plans and other
actions whose effects might overlap those of the the success of mitigation measures is critical.
proposed action. Adaptive management provides the opportunity
to combine monitoring and decision making in a
When the analyst describes the affected en-
way that will better ensure protection of the
vironment, he or she is setting the environmental
environment and attainment of societal goals.
baseline and thresholds of environmental change
that are important for analyzing cumulative Successfully analyzing cumulative effects
effects. Recently developed indicators of ecolog- ultimately depends on the careful application of
ical integrity (e.g., index of biotic integrity for individual methods, techniques, and tools to the
fish) and landscape condition (e.g., fragmentation environmental impact assessment at hand.
of habitat patches) can be used as benchmarks of There is a close relationship between impact
accumulated change over time. In addition, assessment and environmental planning, and
remote sensing and geographic information many of the methods developed for each are
system (GIS) technologies provide improved applicable to cumulative effects analysis. The
means to analyze historical change in indicators unique requirements of cumulative effects anal-
of the condition of resources, ecosystems, and ysis (i.e., the focus on resource sustainability and
human communities, as well as the relevant the expanded geographic and time boundaries)
stress factors. Many dispersed local information must be addressed by developing an appropriate
sources and emerging regional data collection conceptual model. To do this, a suite of primary
programs are now available to describe the cum- methods can be used: questionnaires, interviews,
ulative effects of a proposed action. and panels; checklists; matrices; networks and
system diagrams; modeling; trends analysis; and
Determining the cumulative environmental
overlay mapping and GIS. As with project-
consequences of an action requires delineating
specific effects, tables and matrices can be used
the cause-and-effect relationships between the
to evaluate cumulative effects (and have been
multiple actions and the resources, ecosystems,
modified specifically to do so). Special methods
and human communities of concern. Analysts
are also available to address the unique aspects
must tease from the complex networks of possible
of cumulative effects, including carrying capacity
interactions those that substantially affect the
analysis, ecosystem analysis, economic impact
resources. Then, they must describe the re-
analysis, and social impact analysis.
sponse of the resource to this environmental
change using modeling, trends analysis, and This handbook was developed by reviewing
scenario building when uncertainties are great. the literature and interviewing practitioners of
The significance of cumulative effects depend on environmental impact assessment. Most agen-
how they compare with the environmental base- cies that have recently developed their own
line and relevant resource thresholds (such as guidelines for analyzing cumulative effects recog-
regulatory standards). Most often, the historical nize cumulative effects analysis as an integral
context surrounding the resource is critical to part of the NEPA process, not a separate effort.
developing these baselines and thresholds and to This handbook is not formal guidance nor is it
supporting both imminent and future decision- exhaustive or definitive; it should assist practi-
making, tioners in developing their own study-specific
approaches. CEQ expects that the handbook
Undoubtedly, the consequences of human
(and similar agency guidelines) will be updated
activities will vary from those that were pre-
periodically to reflect additional experience and
dicted and mitigated. This will be even more
new methods, thereby, constantly improving the
problematic because of cumulative effects; there-
state of cumulative effects analysis.
fore, monitoring the accuracy of predictions and
new methods, thereby, constantly improving the designing mitigation, Table E-1 illustrates how
state of cumulative effects analysis. the principles of cumulative effects analysis can
be the focus of each component of the NEPA
The handbook begins with an introduction to
process. Chapter 5 discusses the methods, tech-
the cumulative effects problem and its relevance
niques, and tnols needed to develop a study-
to the NEPA process. The introduction defines
specific methodology and actually implement
eight general principles of cumulative effects
cumulative effects analysis. Appendix A provides
analysis and lays out ten specific steps that the
summaries of 11 of these methods.
NEPA practitioner can use tQanalyze cumulative
effects. The next three chapters parallel the Cumulative effects analysis is an emerging
environmental impact assessment process and discipline in which the NEPA practitioner can be
discuss analyzing cumulative effects while (1) overwhelmed by the details of the scoping and
scoping, (2) describing the affected environment, analytical phases. The continuing challenge of
and (3) determining environmental conse- cumulative effects analysis is to focus on impor-
quences. Each component in the NEPA process tant cumulative issues, recognizing that a better
is the logical place to complete necessary steps in decision, rather than a perfect cumulative effects
cumulative effects analysis, but practitioners analysis, is the goal of NEPA and environmental
should remember that analyzing for cumulative impact assessment professionals.
effects is an iterative process. Specifically, the
results of cumulative effects analysis can and
should contribute to refining alternatives and
Table E-1. Incorporating pdnclples of cumulative effects analysis (CEA) into the components of
environmental Impact assessment (EIA)
EIA Components CEA Principles
jcoping q Include pad, present, and future actions.
q include all federal, nonfederal, and private actions.
q Focus on each affected resource, ecosystem, and human
q Focus on truly meaningful effects.
Describing the Affected Environment q Focus on each affected resource, ecosystem, and human
q Use natural boundaries.
determining the Environmental Consequences q Address additive, countervailing, and synergistic effects.
q Look beyond the life of the action.
q Address the sustainability of resources, ecosystems, and human