The Critical Zone

Lesson 12 Introduction


At this stage in your career, you should have encountered the term Earth system science, primarily used to describe the science (especially quantitative modeling) of the interactions between the atmosphere, hydrosphere and cryosphere, and biosphere. It should not require a huge leap in logic to see that the addition of lithosphere to that list provides all the components ("spheres") of the Critical Zone that we've studied this semester.

Check this out . . .

For a reminder and useful classroom video, see "Earth as a System," a Quicktime video on the Teachers' Domain Web site.

Systems (in our case, the Critical Zone) consist of components (the "spheres"). The components are not isolated. Instead, they typically interact in complex ways; systems may also interact with other systems. These interactions, or linkages, are called couplings in Earth system science vocabulary. Positive couplings mean a change in one component, whether positive or negative, causes a change in the same direction in a linked component, whereas negative couplings mean the linked component undergoes change in the opposite direction. Feedback loops are circuits of change and response to change: negative feedback loops typically diminish the effects of change, whereas positive feedback loops usually amplify the change.

The state of a system is described using the characteristics of the system at a particular time. Changes to the state of a system are caused by (1) interactions between other systems and (2) interactions among the components within a system. An equilibrium state will not change unless the system is disturbed. Temporary disturbances to a system are called perturbations, whereas persistent disturbance is called forcing. When slight disturbances carry a system further from equilibrium it is said to be an unstable system.

Reading assignment

  1. Humanity now dominates the Earth and Critical Zone systems. Before you move on to the body of this lesson, return to your reading from Lesson 10, page 5 (Campbell, Ch. 55) and review pages 1236–1242 to remind yourself how humans affect nutrient cycles, atmosphere, and precipitation chemistry, and the introduction of toxins. You may also want to study the two figures on p. 1243, since you are about to create your own qualitative Critical Zone system model.
  2. Read the short one-page statement about how human actions modify the physical environment, recognizing that our actions can be adaptive or excessive.