(introduction...)

Building on the above concepts and principles, a conceptual framework of cumulative environmental change can be developed. The framework is based on an input-process-output model.

• Input is represented by sources of cumulative environmental change (i.e., human actions). Sources are characterized by time, space, and the nature of the perturbation.

• Process is manifested in pathways of cumulative environmental change which are distinguished as additive or interactive.

• Output is exemplified by the resulting cumulative effects, broadly differentiated as structural or functional.

There are three components of the conceptual framework, that is the source, pathways, and effects. They are interconnected in as much as there is a cause and effect relationship between components, together with feedback mechanisms. The various pathways may stimulate other sources of cumulative environmental change and an effect itself may become a source, or activate other pathways, of cumulative environmental change. The following discussion briefly elaborates on each of the three components of the conceptual framework.

Table 9.2 A typology describing the source of cumulative environmental changes applied to three examples of human-environment interactions

9.2.2.1 Sources of cumulative environmental change

A typology to describe and classify various sources of cumulative environmental change is shown in Table 9.2. The typology differentiates sources according to temporal, spatial, and perturbation attributes. Three examples illustrate various ways in which the typology can be applied to different sources. Construction of a hydroelectric dam is typically viewed as a single, discontinuous event at the local scale. However, dam construction is a potential source of cumulative effects when characterized by multiple perturbations of the same type (e.g., several hydroelectric projects) or different type (e.g., access roads, transmission corridors), expanded spatial scales (e.g., loss of upstream terrestrial habitat as the impoundment fills, altered downstream flow), and extended temporal bounds (e.g., eventual release of methyl mercury and the gradual deposition of sediment in the reservoir).

It could also be argued that clear-cutting of a forest is a single, discontinuous event at the local scale. However, once a particular stand of trees is cut, the operators move elsewhere. Clear-cutting is a potential source of cumulative effects because the action is repeated over time and across space (see Table 9.2).

Finally, CO₂ emissions are also representative of cumulative effects. The increasing accumulation of CO₂ in the atmosphere has occurred over the long term (i.e., since the pre-industrial era) and at a global scale. CO₂ emissions come from diverse and multiple sources (e.g., thermal power plants, transportation, heating) (see Table 9.2).

This typology broadens the consideration of sources (i.e., human actions) beyond the bounded projects typically appraised by environmental assessments to include activities which are repeated over time and dispersed across space.

9.2.2.2 Pathways of cumulative environmental change

Environmental changes accumulate through different processes or pathways. As with sources of change, these pathways vary by number, type, and temporal and spatial attributes. A perturbation may follow single or multiple pathways and involve additive or interactive processes. Additive pathways are summative because one unit of environmental change can be added or subtracted from a previous unit of environmental change. Interactive pathways are multiplicative, or synergistic, in that the nett accumulation is more or less than the sum of all environmental changes. Temporally, pathways may be characterized by instantaneous processes or involve time lags. From a spatial perspective, pathways may function at local, regional, or global scales, and involve cross-boundary movement among systems at the same scale.

Cumulative environmental change generally involves processes that are characterized by a series of incremental changes in environmental components or relationships. These incremental changes are categorized into four types by Sonnatag et al.

1 Linear additive changes are distinguished by a series of small, incremental additions to, or removals of, energy or materials from a fixed large storage (e.g., a lake). Each addition or removal has the same effect as the previous increment. A linear dose-effect relationship between a contaminant and a fish species exemplifies this category of change.

2 Amplifying or exponential changes involve a series of incremental additions to or removal from a seemingly limitless storage (e.g., atmosphere). Each increment of change has a greater effect than the previous one so that system response increases over time. An example is the steady release of CO₂ into the atmosphere and the associated change in global temperature.

3 Discontinuous changes involve incremental additions or removals that are assimilated until a threshold is reached. Each increment of change that exceeds the threshold results in a response. An example is the addition of nutrients into a lake which triggers algae blooms once critical concentrations are attained.

4 Structural surprises refer to a process whereby increments of local and slow environmental changes gradually accumulate so that spatial scales are increased (i.e., local to regional to global) and temporal scales are intensified (i.e., slow to rapid rates). The result is a collection of various effects on system structures. These effects are measured by spatial homogenization of key system variables and a loss of major system functions. The spatial and temporal accumulation of wetland loss, and the subsequent change in wetland functions (e.g., groundwater recharge, biotic diversity, floodwater storage), is an example of structural surprise.

This categorization describes various mechanisms involved in the accumulation of incremental environmental changes. The categories increase in complexity as the mechanisms shift from linearity to nonlinearity, continuity to discontinuity, and uniform spatial and temporal scales to hierarchical scales. They also question a common premise that an incremental change in an environmental component or process is similar to the previous unit of change. The above categories recognize that non-linear processes, trigger mechanisms, and surprise events can intensify and amplify the effect of each successive increment. Controlling factors such as assimilative capacity, thresholds, and dynamic variability regulate the accumulation of incremental environmental changes.

9.2.2.3 Cumulative effects

Cumulative effects have been categorized in various ways. Lane et al. characterized four types of cumulative effects by their primary driving force (cause) and their basic spatial pattern (effect).

1 Type A effects are proponent-driven, large, single projects that induce environmental change over a large region (e.g., NATO low-level aircraft flights over Labrador).

2 Type B effects are proponent-driven, multiple projects (related or unrelated) that interact, resulting in spatially diffuse and complex environmental change (e.g., shoreline development along the Great Lakes).

3 Type C effects are ecosystem-driven (no identifiable proponent), catastrophic, or sudden events (natural or anthropogenic origin) with abrupt environmental changes (e.g., eruption of Mount Pinatubo in the Philippines, pollution from oil well fires in Kuwait during Operation Desert Storm).

4 Type D effects are incremental and widespread ecosystem-driven (no identifiable proponent) environmental changes attributed to diverse temporal and spatial processes (e.g., increase in the atmospheric concentration of carbon dioxide).

Table 9.3 A typology of cumulative effects

The emphasis here is on the spatial pattern and minimizes the recognition of temporal attributes. Also, effects are distinguished according to the source of the cumulative environmental change rather than characteristics inherent to different types of cumulative effects. Nevertheless, it makes a useful contribution by discriminating cumulative environmental change according to their cause-effect relationships.

A typology of cumulative effects which incorporates temporal and spatial attributes more explicitly is shown in Table 9.3. These effects can be broadly grouped into two categories. Functional effects refer primarily to the accumulation of time-dependent cumulative environmental changes. Temporal accumulation occurs when the interval between perturbations is less than the time required for an environmental system to recover after each perturbation. An example is harvesting renewable resources, such as forests and fish, at rates that exceed those of replacement. Time lags are exemplified in the continuous exposure to toxins in a food chain which may contribute to intergenerational genetic abnormalities.

Structural effects are primarily spatially oriented (e.g., space crowding, cross-boundary movement, fragmentation). Spatial accumulation is where the spatial proximity between each perturbation is smaller than the distance required to remove or disperse each perturbation. Space crowding is evident in the systematic collection of pesticide residues by farm drainage systems and its movement at a higher concentration into an aquatic system. This movement also demonstrates cross-boundary flow from one environmental system to another (i.e., agroecosystem to aquatic system). Spatial fragmentation is manifested in changes in size, shape, and contiguity of forests and wetlands in intensely farmed rural landscapes.

Other types of cumulative effects such as compounding, indirect effects, and triggers and thresholds are indicative of the manner of accumulation (Table 9.4). These types generally contribute to or manifest themselves as functional or structural effects, or both.

An attempt has been made to relate the types of cumulative effects to specific pathways of accumulation. Although all cumulative effects are potentially associated with each pathway to a certain degree, some effects are potentially more related than others. For example, time crowding, space crowding, and fragmentation are usually dominated by additive pathways. Time lags and cross-boundary movements are likely to also involve interactive processes. Compounding by definition is interactive.

Linking types of cumulative effects to pathways of accumulation enhances the understanding of system response to perturbation in two ways. First, it provides an indicator of potential cumulative effects in the future when unwanted changes in pathways occur. For example, incremental increases (additive processes) in the area and density of agricultural land drainage within a region might signify future cumulative effects such as fragmentation of wetlands. Similarly, drainage can result in space crowding by the systematic gathering of leached residue from widely dispersed farm inputs (e.g., fertilizers, pesticides), and cross-boundary movements by transporting contaminants downstream. Thus, linkage provides the basis for a predictive tool of analysis. Second, when a cumulative effect is observed and the cause is largely unknown (e.g., change in waterfowl migration, loss of ecosystem functions in a regional landscape), the association among effects and pathways can be used to trace and identify sources of cumulative environmental change. In this case, the association provides the basis for a form of hindsight analysis.