Transient climate scenarios are now widely used in impact assessmentan improvement on earlier use of equilibrium climate scenarios (even if scaled to temporal projections) (see Chapter 3). A corresponding shift from static to dynamic, process-oriented impact models is apparent (as shown in the sectoral chapters).
Many models applied for predicting climate change effects on the behavior of an exposure unit are derived from equilibrium models. These include many basin watershed models, crop models for potential agricultural productivity, and potential vegetation models. With such models, the change effected on the unit at a fixed point in time is estimated, ignoring potentially relevant processes of change.
Systems often consist of elements with different time responses to climate change. This means that the present equilibrium will not be maintained in the next point of time. The velocity of change is a key factor in deciding this transient pattern.
Some terrestrial biosphere research illustrates that the world's biomes will not shift as homogeneous entities in response to changing climate and land use (see Section 5.2.1). Competition between individuals and species, modified disturbance regimes (e.g., fires, windstorms), and migration of species all lead to significant time lags in biospheric responses. Furthermore, if mortality from increased disturbance occurs faster than regrowth of other vegetation, there will be a net release of carbon to the atmosphere, which will change climate forcing. Responses may be a function of spatial scale as well. Dynamic global vegetation models illustrate the shift to transient, scalable impact models (e.g., Woodward et al., 1995).
To ensure a temporally sensitive assessment, impact models should include the different time responses of the system. For example, impacts of malaria depend on human tolerance to repeated infection (Martens et al., 1999). Alternatively, the value of climate change damages could be related to the rate of change rather than solely to the magnitude of climate change (Tol and Fankhauser, 1996). Understanding of the temporal interactions between climate change, impacts, and responses in a truly transient methodology is still a major methodological challenge.
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