The meanings of the Critical Zone

The original meaning of the Critical Zone (CZ) was spatial and pointed to one physical referent: the terrestrial surface of the entire Earth. As usage increased among researchers in the geosciences, social sciences, and humanities, new meanings led to the concept pointing to different places and ideas. Emerging trends have expanded the CZ further: CZs are mapped in computational spacetime and on distant planets and asteroids. The polysemous character of the CZ can be confounding for a field-based science, but Earth scientists and technologists have collaborated to collect and harmonize Big Data sets into a sizable library of CZ research in a short time (around 20 years). In this review, we map the semantic range of the CZ and explore how CZ science has remained coherent even as researchers diversified the concept by developing distinguishable but loosely overlapping meanings. We organize extant meanings into three tiers: (1) Earth’s spatial interface of the geochemical and biological; (2) scientific knowledge of geophysical functionality of the CZ, as represented in an ever-growing library of data or by a single feature as proxy (e.g. soil); (3) a planetary home vulnerable to human disruption. In a time of immense human influence on the CZ, we underscore the latent meaning of planetary home, which marshals motivations of care and protection. These three tiers—the ontological, epistemic, and anthropocenic—build on each other to make the CZ a uniquely valuable concept for navigating the socio-ecological challenges of the Anthropocene.