Introduction to the Critical Zone

A diverse array of transdisciplinary Earth surface and environmental scientists are considering the Critical Zone (CZ)—the thin outer veneer of the near-surface Earth extending from the top of the vegetation canopy down to subsurface depths at which fresh groundwater freely circulates. The CZ is a major link in the Earth system, defining the mass and energy flux interface connecting the solid Earth (lithosphere) to the atmosphere, biosphere and hydrosphere—initially it forms and evolves through geological processes of the rock cycle: tectonic uplift, subsidence, weathering, erosion and sedimentation. A series of physical, chemical and biological processes transform CZ structure and function and is reflected in a vertical sequence from intact rock at the base of the CZ, to fully formed soils and vegetation cover at the land surface. CZ depth refers to the solid foundation beneath the land surface, but also to a temporal component, deep time, that holds the legacy of past geologic, tectonic, and paleoclimatic events, and even to future events, e.g. ongoing climate and land use change, that will change the form and function of the CZ that we know today. As the physical character of the CZ evolves it affects the storage and flux of water, solutes, sediments, gases, biota and energy—critical services to humanity. Humans have modified more than half of Earth’s land surface and have wrought substantial changes in Earth’s life support system, yet paradoxically the best projections suggest a continued accelerated trajectory of harmful human activities. Due to its vertical connectivity, CZ functions that provide benefits to people are thus vulnerable to the adverse impact of human-induced environmental change. However, due to its proximity to the land surface, the CZ is also accessible to intended human interventions. A primary tenet of CZ science is that to achieve environmental sustainability, society must first understand the CZ system, the natural processes and services of the CZ that are of value to society, and how those processes operate with and without the presence of humanity. Critical Zone Observatories (CZOs) are natural laboratories for investigating Earth surface processes and they can be at the core of collaborative centers comprising field sites, samples, data and models, and people. Most CZOs share a similar approach to answering similar questions: 1) What controls CZ properties and processes? 2) What will be the response of CZ structure, and its stores and fluxes, to climate and land use change? and 3) How can improved understanding of the CZ be used to enhance resiliency and sustainability and restore function of the CZ? The environmental diversity and transdisciplinary approach of CZOs globally promises to more rapidly advance new understanding of key societally important issues in Earth surface and environmental science. Currently, a model of the whole CZ is still missing. Building such a model is not an easy task, not that different from modeling the full Earth System: a large number of dynamical variables and components, a vast range of spatial and temporal scales, and the need for parameterizing unknown or unresolved processes.