Executive Summary
This chapter evaluates the suitability of models (in particular coupled atmosphere-ocean
general circulation models) for use in climate change projection and in detection
and attribution studies. We concentrate on the variables and time-scales that
are important for this task. Models are evaluated against observations and differences
between models are explored using information from a number of systematic model
intercomparisons. Even if a model is assessed as performing credibly when simulating
the present climate, this does not necessarily guarantee that the response to
a perturbation remains credible. Therefore, we also assess the performance of
the models in simulating the climate over the 20th century and for selected
palaeoclimates. Incremental improvements in the performance of coupled models
have occurred since the IPCC WGI Second Assessment Report (IPCC, 1996) (hereafter
SAR) resulting from advances in the modelling of the atmosphere, ocean, sea
ice and land surface as well as improvements in the coupling of these components.
Highlights include:
- Coupled models can provide credible simulations of both the present annual
mean climate and the climatological seasonal cycle over broad continental
scales for most variables of interest for climate change. Clouds and humidity
remain sources of significant uncertainty but there have been incremental
improvements in simulations of these quantities.
- 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.
- There is no systematic difference between flux adjusted and non-flux adjusted
models in the simulation of internal climate variability. This supports the
use of both types of model in detection and attribution of climate change.
- Confidence in the ability of models to project future climates is increased
by the ability of several models to reproduce the warming trend in 20th century
surface air temperature when driven by radiative forcing due to increasing
greenhouse gases and sulphate aerosols. However, only idealised scenarios
of only sulphate aerosols have been used.
- Some modelling studies suggest that inclusion of additional forcings such
as solar variability and volcanic aerosols may improve some aspects of the
simulated climate variability of the 20th century.
- Confidence in simulating future climates has been enhanced following a systematic
evaluation of models under a limited number of past climates.
- The performance of coupled models in simulating the El Niño-Southern
Oscillation (ENSO) has improved; however, the region of maximum sea surface
temperature variability associated with El Niño events is displaced
westward and its strength is generally underestimated. When suitably initialised
with an ocean data assimilation system, some coupled models have had a degree
of success in predicting El Niño events.
- Other phenomena previously not well simulated in coupled models are now
handled reasonably well, including monsoons and the North Atlantic Oscillation.
- Some palaeoclimate modelling studies, and some land-surface experiments
(including deforestation, desertification and land cover change), have revealed
the importance of vegetation feedbacks at sub-continental scales. Whether
or not vegetation changes are important for future climate projections should
be investigated.
- Analysis of, and confidence in, extreme events simulated within climate
models is 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.
Final Assessment
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. The varying sets of
strengths and weaknesses that models display lead us to conclude that no single
model can be considered “best” and it is important to utilise results
from a range of coupled models. We consider coupled models, as a class, to be
suitable tools to provide useful projections of future climates.
