|Working Group I: The Scientific Basis|
|Other reports in this collection|
D.4 Overall Assessment of Abilities
Coupled models have evolved and improved significantly since the SAR. In general,
they provide credible simulations of climate, at least down to sub-continental
scales and over temporal scales from seasonal to decadal. Coupled models, as a
class, are considered to be suitable tools to provide useful projections of future
climates. These models cannot yet simulate all aspects of climate (e.g., they
still cannot account fully for the observed trend in the surface-troposphere temperature
differences since 1979). Clouds and humidity also remain sources of significant
uncertainty, but there have been incremental improvements in simulations of these
quantities. No single model can be considered “best”, and it is important
to utilise results from a range of carefully evaluated coupled models to explore
effects of different formulations. The rationale for increased confidence in models
arises from model performance in the following areas.
Flux adjustment The overall confidence in model projections is increased by the improved performance of several models that do not use flux adjustment. These models now maintain stable, multi-century simulations of surface climate that are considered to be of sufficient quality to allow their use for climate change projections. The changes whereby many models can now run without flux adjustment have come from improvements in both the atmospheric and oceanic components. In the model atmosphere, improvements in convection, the boundary layer, clouds, and surface latent heat fluxes are most notable. In the model ocean, the improvements are in resolution, boundary layer mixing, and in the representation of eddies. The results from climate change studies with flux adjusted and non-flux adjusted models are broadly in agreement; nonetheless, the development of stable non-flux adjusted models increases confidence in their ability to simulate future climates.
Climate of the 20th century
Extreme events Analysis of and confidence in extreme events simulated within climate models are still emerging, particularly for storm tracks and storm frequency. “Tropical-cyclone-like” vortices are being simulated in climate models, although enough uncertainty remains over their interpretation to warrant caution in projections of tropical cyclone changes. However, in general, the analysis of extreme events in both observations (see Section B.6) and coupled models is underdeveloped.
Interannual variability The performance of coupled models in simulating ENSO has improved; however, its variability is displaced westward and its strength is generally underestimated. When suitably initialised with surface wind and sub-surface ocean data, some coupled models have had a degree of success in predicting ENSO events.
Model intercomparisons The growth in systematic intercomparisons of models provides the core evidence for the growing capabilities of climate models. For example, the Coupled Model Intercomparison Project (CMIP) is enabling a more comprehensive and systematic evaluation and intercomparison of coupled models run in a standardised configuration and responding to standardised forcing. Some degree of quantification of improvements in coupled model performance has now been demonstrated. The Palaeoclimate Model Intercomparison Project (PMIP) provides intercomparisons of models for the mid-Holocene (6,000 years before present) and for the Last Glacial Maximum (21,000 years before present). The ability of these models to simulate some aspects of palaeoclimates, compared to a range of palaeoclimate proxy data, gives confidence in models (at least the atmospheric component) over a range of difference forcings.
Other reports in this collection