The anthropogenic contribution to the evolution of climate is currently a central topic in scientific studies. The Intergovernmental Panel on Climate Change (IPCC) assessment reports are based on the best science at hand, and summarize the progress in observing, modeling, understanding and predicting the evolution of the climate system. The climate system is forced, dissipative, nonlinear, chaotic, and out of thermodynamic equilibrium. Along with the difficulties that these characteristics come with, the climate system presents some particularities such as, the presence of well-defined subsystems, the continuous variation of the forcings, lack of scale separation, etc., which add more obstacles to the task.