Why Do We Need Climate Scenarios?
The specific climate scenario needs vary, depending on the geographic region considered, the type of impact, and the purpose of the study. For example, scenario needs for research in climate scenarios development and in the methods of conducting impact assessment differ from scenario needs for direct application in policy relevant impact and integrated assessments.
Variables
The types of climate variables needed for quantitative impacts studies vary widely. The six variables most commonly requested are:
- maximum and minimum temperature
- precipitation
- incident solar radiation
- relative humidity, and
- wind speed.
This list is far from exhaustive. Other important climate or climate-related variables include CO2 concentration, sea-ice extent, mean sea level pressure, sea level, and storm surge frequencies.
Scale
A central issue is determining at what spatial and temporal scales the variable in question can sensibly be provided, compared to the scales most desired by the impacts assessors. From an impacts perspective, it is usually desirable to have regional detail about future climate and a sense of how its variability (from short to long time-scales) might change. However, the availability of the output from climate models and the advisability of using climate model results at particular scales, from the point of view of the climate modelers, ultimately determines what scales can and should be used. Important considerations for the baseline include the time period adopted as well as the spatial and temporal resolution of the baseline data.
Uncertainty
Scenarios also should provide quantitative measures of uncertainty. The many sources of uncertainty include the trajectory of greenhouse gas emissions in the future, their conversion into atmospheric concentrations, the range of responses of various climate models to a given radiative forcing and the method of constructing high resolution information from global climate model outputs (see Figure 1). Simply defining a single climate future is insufficient and unsatisfactory for many purposes. Multiple climate scenarios that address one or several sources of uncertainty allow these uncertainties to be quantified and explicitly accounted for in impact assessments.
Another important requirement for impact assessments is to ensure consistency among different scenario components, such as between climate change, sea level rise and the concentration of actual (as opposed to equivalent) CO2 implied by a particular emissions scenario.
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