Validation of Climate Models

THE CLIMATIC SYSTEM is constituted by four intimately interconnected subsystems—atmosphere, hydrosphere, cryosphere, and biosphere—which evolve under the action of macroscopic driving and modulating agents, such as solar heating, Earth's rotation, and gravitation. The climate system features many degrees of freedom, which make it complicated, and nonlinear interactions taking place on a vast range of time-space scales accompanying sensitive dependence on initial conditions, which makes it complex. The climate is defined as the set of statistical properties of the observable physical quantities of the climatic system.

The evaluation of the accuracy of numerical climate models and the definition of strategies for their improvement are crucial issues in the Earth system scientific community. On one hand, climate models of various degrees of complexity constitute tools of fundamental importance to reconstruct and project in the future the state of the planet and to test theories related to basic geophysical fluid dynamical properties of the atmosphere and of the ocean, as well as of the physical and chemical feedbacks within the various subdomains and between them. On the other hand, the outputs of climate models, and especially future climate projections, are gaining an ever-increasing relevance in several fields, such as ecology, economics, engineering, energy, and architecture, as well as for the process of policymaking at a national and international level. Regarding influences at the societal level of climate-related finding, the effects of the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC4AR) are unprecedented.

The validation or auditing—overall evaluation of accuracy—of a set of climate models is a delicate operation that can be decomposed in two related, albeit distinct, procedures. The first procedure is the inter-comparison, which aims at assessing the consistency of the models in the simulation of certain physical phenomena over a certain time frame. The second procedure is the verification, the goal of which is to compare the models' outputs with corresponding observed or reconstructed quantities. Difficulties emerge because we always have to deal with three different kinds of attractor: the attractor of the real climate system, its reconstruction from observations, and the attractors of the climate models. Depending on the timescale of interest and on the problem under investigation, the relevant active degrees of freedom (mathematically corresponding to the separation between the slow and fast manifolds) needing the most careful representation change dramatically. For relatively short timescales [Page 1054](below 10 years), the atmospheric degrees of freedom are active, whereas the other subsystems can be considered to be essentially frozen. For longer timescales (100–1,000 years), the ocean dominates the dynamics of climate, whereas for even longer timescales (over 5,000 years), the continental ice sheet changes are the most relevant factors of variability. Therefore, the scientific community has produced different families of climate models, spanning a hierarchical ladder of complexity, each formulated and structured for specifically tackling a class of problems.