Climate models are based on well-established physical principles and have been demonstrated to reproduce observed features of recent climate and past climate changes. There is considerable confidence that Atmosphere-Ocean General Circulation Models (AOGCMs) provide credible quantitative estimates of future climate change, particularly at continental and larger scales. This chapter assesses the capacity of the global climate models used for projecting future climate change. Confidence in model estimates of future climate evolution has been enhanced via a range of advances since the IPCC Third Assessment Report (TAR). This chapter highlights the following areas of progress since the TAR:

enhanced scrutiny of models and expanded diagnostic analysis of model behaviour have been increasingly facilitated by internationally coordinated efforts to collect and disseminate output from model experiments performed under common conditions
climate models are being subjected to more comprehensive tests which increase confidence in the fidelity with which models represent processes that affect climate projections
substantial progress has been made in understanding the inter-model differences in equilibrium climate sensitivity
there have been ongoing improvements to resolution, computational methods and parametrizations, and additional processes have been included in more of the climate models
most AOGCMs no longer use flux adjustments, which were previously required to maintain a stable climate
progress in the simulation of important modes of climate variability has increased the overall confidence in the models’ representation of important climate processes
the ability of AOGCMs to simulate extreme events has improved
simulation of extratropical cyclones has improved
systematic biases have been found in most models’ simulation of the Southern Ocean which is important for ocean heat uptake
the possibility that metrics based on observations might be used to constrain model projections of climate change has been explored for the first time, through the analysis of ensembles of model simulations
to explore the potential importance of carbon cycle feedbacks in the climate system, explicit treatment of the carbon cycle has been introduced in a few climate AOGCMs and some Earth System Models of Intermediate Complexity (EMICs)
EMICs have been evaluated in greater depth than previously – demonstrating that these models are useful in addressing questions involving long time scales or requiring a large number of ensemble simulations or sensitivity experiments.