Climate and Energy Action Plan (CEAP)

CLIMATE TRENDS & PRO JECTIONS | CITY OF ASHLAND, OREGON | FINAL REPORT | 35 Uncertainty Inherent in GCM projections is uncertainty due to emissions scenario, internal variability, and modeling physics and resolution. Individual GCMs project different magnitudes of warming because the models' "climates" are either more or less sensitive to external radiative forcings (e.g., increasing greenhouse gases). Furthermore, the chaotic nature of the climate system means that ev en a single climate model, if identical simulations were started on a different day, yields a range of outcomes. In addition, e ven at 100 -mile horizontal resolution most GCMs are still unable to resolve key topographical features that influence western US climate. Precipitation projections are generally more uncertain than temperature projections. Temperature projections, while models may vary on the magnitude, are highly robust since all models agree on warming under increasing greenhouse gases. Modeling a ccurate microphysical cloud processes that produce precipitation requires resolutions much finer than current GCMs can attain so most of those processes are estimated (i.e., parameterized), resulting in inherent uncertainty in precipitation projections. Th ere is no consensus among the GCMs on the sign of future precipitation change as some models project increases and othe rs decreases.