From the Executive Summary (here):
There is considerable debate over the fidelity and utility of global climate models (GCMs). This debate occurs within the community of climate scientists, who disagree about the amount of weight to give to climate models relative to observational analyses. GCM outputs are also used by economists, regulatory agencies and policy makers, so GCMs have received considerable scrutiny from a broader community of scientists, engineers, software experts, and philosophers of science. This report attempts to describe the debate surrounding GCMs to an educated but nontechnical audience.
Key summary points
• GCMs have not been subject to the rigorous verification and validation that is the norm for engineering and regulatory science.
• There are valid concerns about a fundamental lack of predictability in the complex nonlinear climate system.
• There are numerous arguments supporting the conclusion that climate models are not fit for the purpose of identifying with high confidence the proportion of the 20th century warming that was human-caused as opposed to natural.
• There is growing evidence that climate models predict too much warming from increased atmospheric carbon dioxide.
• The climate model simulation results for the 21st century reported by the Intergovernmental Panel on Climate Change (IPCC) do not include key elements of climate variability, and hence are not useful as projections for how the 21st century climate will actually evolve.
Climate models are useful tools for conducting scientific research to understand the climate system. However, the above points support the conclusion that current GCMs are not fit for the purpose of attributing the causes of 20th century warming or for predicting global or regional climate change on timescales of decades to centuries, with any high level of confidence. By extension, GCMs are not fit for the purpose of justifying political policies to fundamentally alter world social, economic and energy systems. It is this application of climate model results that fuels the vociferousness of the debate surrounding climate models.