A hierarchy of comparisons of the model results should be carried out. First-order comparison will be of global mean annual values, at least for temperature, precipitation, top of the atmosphere energy elements, and general model sensitivity to forcing (e.g., CO2 doubling). Second-order comparisons of zonal averages will include comparison of model output fields of temperature (annual and seasonal), precipitation (annual and seasonal), sea ice, snow, zonal wind speed at near and mid troposphere levels, vertical velocity at midtroposphere level, and energy components. Third-order comparisons should include global (map view) distributions of minimum, mean annual, and annual amplitude of surface temperature, mean annual and seasonal precipitation, soil moisture, runoff, and total moisture (precipitation-evaporation). Statistical differences for these fields may prove useful. Other climate parameters that may be of specific relevance to Pliocene climate in general and the PRISM data base in particular should also be examined for these present day cases.
Comparisons between model results and data are an integral part of the long-term goals of PRISM. Comparisons at various stages of the project will serve as the means of evaluating both model results and the synoptic mapping efforts. These comparisons will also be used to plan for future interactions of climate experiments. There are several factors to consider when comparing model results to proxy data interpretations. First, model spatial and temporal resolutions will almost certainly not correspond to the resolutions reflected in the paleoclimate data. This must somehow be taken into account in comparisons. Second, the GCMs parameterize many processes instead of explicitly modeling them, and this may affect the results in relation to data interpretations of climatic factors. Third, careful consideration must be given to the question of what the proxy data have actually recorded; do pollen or macrofaunal assemblages most closely reflect mean annual temperature, growing season temperature, or some other quantity(ies)? This is critical when different types of proxy climate information are related to each other and are compared to climate model results. Last, multiple independent proxy indicators for a given climate parameter are useful to obtain because they yield multiple signals for a given system. This again is useful information for understanding the Pliocene climate system and its possible forcing factors.
Proxy climate data can be compared to model results on the basis of quantitative or qualitative information. Qualitative information includes wet/dry and hot/cold indicators which provide a first-order test of model results. Quantitative information includes minimum temperature, mean annual temperature, mean annual temperature range, mean annual precipitation, and seasonal precipitation, can be used as a second-order test of model results.