Abstract

This paper examines how climate scientists use models to explore possibilities—so called “modal modelling.” I begin by distinguishing several senses of “possible” and considering how they might be relevant to the project of studying climate change. These include epistemic possibility, metaphysical possibility, and possibility according to a set of laws of nature—e.g., physical possibility. Determining whether a claim is possible in the relevant sense is a question of modal epistemology. I contrast this with the task of determining whether a claim is supported by a body of evidence, which is a matter of confirmation theory, a subset of ordinary epistemology. I then examine a debate about the interpretation and use of collections of climate models, for example in the IPCC assessment reports. One party argues that climate models don’t provide the right sort of information to allow us to make probabilistic claims about how the climate will be in future. They argue instead that climate models allow for only claims about possible futures rather than probable futures. I call these critics possibilists, and their more orthodox opponents probabilists. The possibilists think that climate models allow us to distinguish between which possible futures are “nearer” or “more relevant”, and that these distinctions reflect facts about our evidence. I argue that there isn’t a concept of “possibility” that can do the work these critics want: fitting the evidence from climate models, admitting of degrees, supporting decisions, and yet not amounting to probability.