|Conducting Environmental Impact Assessment in Developing Countries (UNU, 1999, 375 pages)|
|9. Emerging developments in EIA|
|9.2 Cumulative effects assessment|
|9.2.2 Conceptual framework|
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.