Holism and Entrenchment in Climate Model Validation

Recent work in the domain of the validation of complex computational models reveals that modelers of complex systems, particularly modelers of the earth’s climate, face a deeply entrenched form of confirmation holism. Confirmation holism, as it is traditionally understood, is the thesis that a single hypothesis cannot be tested in isolation, but that such tests always depend on other theories or hypothesis. It is always this collection of theories and hypotheses as a whole, says the thesis, that confront the tribunal of experience. But in contrast to the way the problem of confirmation holism is typically understood in the philosophy of science, the problems faced by climate scientists are not merely logical problems, and nor are they confined to the role of anything that can suitably be called auxiliary hypotheses. Rather, they are deep and entrenched problems that confront the scientist who works with models whose component parts interact in such a complex manner, and have such a complex history, that the scientist is unable to evaluate the worth of the parts in isolation. In what follows, we want to argue for two central claims about complex computational models—with a particular emphasis on models of the earth’s climate. The first claim is about holism. We will argue that recent efforts in the sphere of climate model inter-comparison reveal that modern, state-of-art climate models are what we call “analytically impenetrable.” We will have to spell out this notion with more care in the sequel, but the intuitive idea is that, as a practical matter, it has become impossible for climate scientists to attribute the various sources of relative successes and failures to particular modeling assumptions. The second claim is about entrenchment. In particular, we argue that entrenchment can be identified as one of the principal causes of holism. Here, we want to argue that climate models are, in interesting ways, products of their specific histories. Climate models are developed and