The Critical Zone

Introduction to Critical Zone Science


The Critical Zone encompasses the external or near-surface Earth extending from the top of the vegetation canopy down to and including the zone of freely circulating fresh groundwater. Complex biogeochemical processes combine here to transform rock and biomass into the central component of the Critical Zone—soil. The Zone sustains nearly all terrestrial life including humanity, nonetheless, ever-increasing negative impacts of human society on the Critical Zone continue. Will we allow these impacts to continue unabated, and if so how will our impacts affect humankind and the rest of nature?

The degraded state of Earth's surface has been well documented for example in the United Nations Environment Programme's Millenium Ecosystem Assessment report (2005), and One Planet Many People: Atlas of Our Changing Environment (2005); the Intergovernmental Panel on Climate Change Fifth Assessment Report (2013), and more recent versions; The Penguin State of the World Atlas: Ninth Edition (2012); and, The Atlas of Global Conservation: Changes, Challenges, and Opportunities to Make a Difference (ed., J.L. Molnar, 2010). Hooke et al (Land Transformations by Humans: A review; GSA Today, 2012, v. 22, no. 12, 4-10) summed up some of the topics and tone of these reports concluding that humans have modified more than half of Earth's land surface, that the current rate of land transformation is unsustainable, and that "changes that human activities have wrought on Earth's life support system have worried many people". To many scientists and citizens, these threats to an essential component, i.e. the Critical Zone, of our life support system, have reached an acute level, yet the science of understanding and managing these threats mostly remains embedded within individual disciplines, and the science has largely remained qualitative - there has never been a more important time for a truly international and interdisciplinary approach to accelerate our understanding of Critical Zone processes and how to intervene positively to mitigate threats and sustain and enhance Critical Zone function. All life on Earth relies on humanity embracing balance and sustainability on a global scale.

In this lesson, you will be introduced to Critical Zone science. To accomplish this you will read "The Critical Zone: Earth's Near-surface Environment" and "Frontiers in Exploration of the Critical Zone: Report of a workshop sponsored by the National Science Foundation (NSF)." I stress that this is an introduction. While I want you to grasp key concepts from the readings, I do not want you to fret over every detail — we have plenty of time for that throughout the remainder of the semester. For now, I want you to understand that Critical Zone science truly is multi- and inter-disciplinary — Critical Zone processes are represented by coupled physical, biological and chemical processes — and that an array of scientific expertise is needed to understand the Critical Zone: geology, soil science, biology, ecology, geochemistry, geomorphology, hydrology, to name a few.

Reading Assignment

Read the following selections:

  1. Committee on Basic Research Opportunities in the Earth Sciences. (2001). The Critical Zone: Earth's Near-surface Environment. In Basic Research Opportunities in Earth Science (pp. 35–45). Washington, D.C.: National Academy Press.
    • Please pay particular attention to Figure 2.1.
  2. Brantley, S., White, T., White, A., Sparks, D., Richter, D., Pregitzer, K., et al. (2006). Frontiers in exploration of the Critical Zone. In Frontiers in exploration of the Critical Zone: Report of a workshop sponsored by the National Science Foundation (NSF) (pp. 1–30). Newark, DE.
    • Please pay particular attention to Figures 4–7, 10, 11 and 13, and Table 1.


These readings are also available through Library Reserves.


  1. The U.S. Critical Zone Observatory Web site
  2. The Critical Zone Exploration Network Web site
    • Read the "About CZEN" section.
    • Check out and browse the final report from a 2011 meeting of international CZ scientists if you care to learn about the evolution of scientific inquiry at CZOs.
  3. The National Science Foundation Web site
    • Click on "About" and read through this brief introduction to the NSF, following the MORE links for greater detail.
    • Please pay particular attention to the "Classroom Resources" section ( I want you to understand how basic scientific research is funded in the United States and how this research is made available for teachers like yourselves.
  4. The National Research Council Web site
    • Browse the section on the Earth and Life Studies Division (DELS)
    • Under DELS, read the "About us" section, including "Who we are and what we do" Also view "Our Boards". You may find information that is useful to you during this class and in your classrooms.