Environmental Science Critical Zone
Tim White, Sarah Sharkey
  • LAST REVIEWED: 26 May 2016
  • LAST MODIFIED: 26 May 2016
  • DOI: 10.1093/obo/9780199363445-0055


The critical zone (CZ), a term coined by the US National Research Council in 2001, encompasses the thin outer veneer of Earth’s surface, extending from the top of the vegetation canopy down to the subsurface depths of fresh groundwater, the zone “where rock meets life.” The CZ involves coupled physical, biological, and chemical processes, and scientific expertise from an array of disciplines is needed to understand the zone and its processes: geology, soil science, biology, ecology, geochemistry, hydrology, geomorphology, atmospheric science, and many more. The complex biogeochemical-physical processes combine in the CZ to transform rock and biomass into soil, the central component of the CZ, which in turn supports much of the terrestrial biosphere, including humanity. The structure and functioning of the CZ have evolved in response to climatic and tectonic perturbations throughout Earth’s history, with the processes driving change more recently accelerated by human activities. CZ science offers enormous potential to integrate basic knowledge of Earth’s surface with sustainable adaptation to ongoing rapid and intensive land use and climate change.

A Brief History of CZ Science

Critical zone observatories (CZOs) are a new scientific endeavor that evolved from the recognition that similar questions were being asked by diverse groups of Earth surface scientists who did not typically collaborate. For example, while the hydrological-science community was considering issues regarding human pressures and climate trends changing the water cycle, geobiologists were considering questions such as how will increasing global temperatures change carbon (C) losses and weathering fluxes, and how might these changes compare to impacts from land use change? At this time, the Weathering System Science Consortium was created by the geochemistry community and then transformed to the Critical Zone Exploration Network to engage the many disciplines needed to study the CZ. These initial interactions led to the development of four driving questions of CZ science: (1) What processes control fluxes of C, particulates, and reactive gases over different timescales? (2) How do variations in and perturbations to chemical and physical weathering processes affect the CZ? (3) How do biogeochemical processes govern long-term sustainability of water and soil resources? and (4) How do processes that nourish ecosystems change over human and geologic timescales? The concepts of CZ science have captured the imagination of scientists worldwide—thus, a brief description of funded networks of sites follows, acknowledging that other CZOs exist globally. The interdisciplinary and integrative science of the CZ was formalized through US National Science Foundation (NSF) funding of the CZO program. The US program began in 2007 with support of the Susquehanna–Shale Hills Observatory, Pennsylvania; Southern Sierra Observatory, California; and Boulder Creek, Colorado. Shortly thereafter, the Soil Transformation in European Catchments (SoilTrEC) program was funded by the European Community, establishing four observatories: Lysina, Czech Republic; Koiliaris, Crete; Damma Glacier, Switzerland; and Fuchsenbigl, Austria. The challenge for the SoilTrEC project has been to understand the rates of processes that dictate soil mass stocks and their function within the CZ. In 2009, three additional observatories were added in the United States: Luquillo Mountains, Puerto Rico; Christina River Basin, Delaware and Pennsylvania; and Jemez River Basin / Santa Catalina Mountains, Arizona and New Mexico. More recently, the Réseau des Bassins Versants developed in France as a multidisciplinary research network to unite the efforts of national organizations that perform fundamental and applied watershed science. Similarly, Terrestrial Environmental Observatories in Germany has embarked on an interdisciplinary and long-term research program involving six observatories extending from the northern lowlands to the Bavarian Alps. In 2014, four new US observatories were selected for funding: Eel River CZO, California; Reynolds Creek, Idaho; Intensively Managed Landscape, Illinois, Iowa, and Minnesota; and Calhoun Forest, South Carolina. Details of the US CZO program may be found on their website.

  • Critical Zone Exploration Network.

    Critical Zone Exploration Network is a diverse community of researchers and educators and a network of field sites investigating the physical, chemical, and biological processes shaping and transforming the CZ.

  • Critical Zone Observatories.

    Nine US observatories study the zone at Earth’s surface, where the atmosphere, hydrosphere, lithosphere, biosphere, and pedosphere interact. The network seeks to answer fundamental questions and to help predict how the zone will change in the future.

  • Réseau des Bassins Versants.

    The Réseau des Bassins Verdants (RBV), or Network of Drainage Basins, consists of fifteen observatories located in France and also within developing countries, where they are the focus of collaborations encouraging a holistic scientific approach to the study of the CZ.

  • Terrestrial Environmental Observatories.

    Terrestrial Environmental Observatories (TERENO) aims to catalogue the long-term ecological, social, and economic impact of global change in Germany and to use the findings to show how humanity can best respond to environmental changes.

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