# 1.3 Systems Approach
The term ***Earth system science*** is typically used to describe the science (especially quantitative modeling) of the interactions between the atmosphere, hydrosphere, and cryosphere, and biosphere---the addition of lithosphere to that list provides all of the main generalized components ("spheres") of the CZ.
In this lesson, we will consider basic concepts of system science (studying complex systems), specifically as it can be applied to CZ science. Students will engage in developing a qualitative systems model graphic of the CZ. The knowledge gained here will be applied later in the semester to more in-depth systems thinking of the CZ.
As you learn about the systems approach, consider the following learning goals:
- Define the term "system" as it pertains to the natural world, and describe the difference between quantitative and qualitative system modeling.
- Design a qualitative Critical Zone system model.
- Explain how (and when) humans have altered global erosion rates.
- Recognize some of the consequences of human domination of ecosystems.
- Discuss how human-induced climate change is expected to alter the hydrologic cycle.
- Describe what global-scale, human-induced changes can be observed in soils, the role of agriculture in these changes, and some of the consequences of changes to our soils.
- Formulate and evaluate any adaptive actions humanity can take to lessen negative impacts to the Critical Zone and soils.
## Earth Systems
We often use the word *system* to talk about Earth as an integrated whole. The system concept allows scientists to break down a large, complex problem into smaller, more easily studied pieces.
>A system is any portion of the universe that can be isolated from the rest of the universe for the purpose of observing and measuring changes.
By saying that a system is any portion of the universe, we mean that the system can be whatever the observer defines it to be. That is why a system is only a concept; you choose its limits for the convenience of your study. It can be large or small, simple or complex. This system can be in the scale of the entire universe (for an astronomer), an aquifer (for a hydrogeologist), or a petri dish (for a microbiologist) The choice depends on the kind of problem of interest. For example, in a shore environment there are different processes occurring and defining the bounds of the system become very important. See {numref}`earth-systems` for an example of how various processes on Earth can be studied using a systems approach.
```{figure} https://mynasadata.larc.nasa.gov/sites/default/files/2019-04/earth_system_diagram_print.jpg
---
name: earth-systems
figclass: margin-caption
---
Systems can be considered at many scales. In this picture we can see how energy and matter (elements, water, etc) is cycled across the entire Earth system at many scales. Image source: [Earth System: Matter and Energy Cycles | MyNASAData](https://mynasadata.larc.nasa.gov/basic-page/earth-system-matter-and-energy-cycles)
```
The fact that a system is isolated from the rest of the universe means that it must have a boundary that sets it apart from its surroundings. The nature of the boundary is one of the most important defining features of a system, allowing us to establish three basic kinds of systems—isolated, closed, and open—with different types of boundaries.
## Systems Feedback
As a system operates, it generates outputs that influence its own operations. These outputs function as “information” that returns to various points in the system. Feedback information can guide, and sometimes control, further system operations, often forming pathways called feedback loops.
If the feedback information discourages change in the system, it is ***negative feedback***. Stated another way, negative feedback happens when performing an action leads to fewer performances of that action. For example, if you e-mail your instructor with questions about this course, and your instructor e-mails back with clear answers, then you will have fewer questions and less need to write e-mails for clarification. In this case, negative feedback leads to stability in the classroom system and reduces the need for extra help.
If feedback information encourages change in the system, it is ***positive feedback***. For example, if you e-mail your course instructor with questions and receive confusing answers that create more questions, then you will continue to send more e-mails as the number of questions increases. In this case, positive feedback causes instability in the classroom system as your confusion increases; if unchecked, this feedback loop could result in you dropping the class.
Note that negative feedback is not “bad,” and positive feedback is not “good.” Each type of feedback relates to system stability: Negative feedback opposes system changes and leads to stability; positive feedback leads to instability and if unchecked can create a runaway (“snowballing”) condition. In natural systems, such unchecked system changes can reach a critical limit, leading to system disruption or death of organisms.
Systems in the CZ consist of components that are not isolated, but instead 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
2. Interactions among the components within a system
## System Equilibrium
Most systems maintain structure and character over time. An energy and material system that remains balanced over time, in which conditions are constant, is considered to be in a ***steady-state*** condition. When the rates of inputs and outputs in the system are equal and the amounts of energy and matter in storage within the system fluctuate around a stable average, the system is in steady-state equilibrium. For example, river channels commonly adjust their form in response to inputs of water and sediment; these inputs may change in amount from year to year, but the channel form represents a stable average—a steady-state condition.
Note that systems in equilibrium tend to maintain their functional operations and resist abrupt change. However, a system may reach a threshold, or tipping point, where it can no longer maintain its character, so it lurches to a new operational level. A large flood in a river system may push the river channel to a threshold where it abruptly shifts, carving a new channel. A landslide may send a hillside or coastal bluff toward a new equilibrium form, balanced among slope, materials, and energy over time. Plant and animal communities can reach thresholds. For example, frogs are reaching a tipping point in response to a deadly fungus that has spread worldwide; extinctions of over 200 known species (about 3% of the total) have occurred since 1970. Scientists estimate that 10% of all frog species will be gone by 2100.
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.
## System Models
A model is a simplified, idealized representation of part of the real world that helps us understand complex processes. Scientists design models with varying degrees of specificity. A conceptual model is usually the most generalized and focuses on how processes interact within a system. A numerical model is more specific and is usually based on data collected from field or laboratory work. The simplicity of a model makes a system easier to understand and to simulate in experiments. An example is a model of the hydrologic system, which represents Earth’s entire water system; its related energy flows; and the atmosphere, surface, and subsurface environments through which water moves.
The storage and movement of material and energy in a group of interacting systems is often represented as a ***box model***. A box model can be used to show the following essential features of a system:
1. The processes by which matter (or energy) enters and leaves the system, and the rates at which they do so.
2. The processes by which matter (or energy) moves among the various parts of the system internally, and the rates at which this happens.
3. The amount of matter (or energy) in the system at a given time and its distribution within the system.
As shown in {numref}`hydro-box-model`, the “boxes” in a box model represent the places where water (or energy, or whatever might be the material of interest) is stored for a period of time within the system. These storage places are called ***reservoirs***. When the flux of matter into a reservoir matches the flux out of that reservoir, we say that the reservoir is at ***steady state***. The average length of time water spends in any of these reservoirs is called its ***residence time***. The residence time of any material in any particular reservoir is deter- mined by the interaction of many factors, including the physical, chemical, and biologic properties of the material itself, the properties of the reservoir, and any external forces or processes acting on either the material or the reservoir.
```{figure} https://www.science.org/cms/10.1126/science.1128845/asset/4b82afe0-53e4-4cb1-b20c-16d1ac9644ff/assets/graphic/313_1068_f1.jpeg
---
name: hydro-box-model
figclass: margin-caption
---
The global hydrological cycle is shown as a box model in this figure. Fluxes ($\pu{e3 km3 yr-1}$) and reservoirs ($\pu{e3 km3}$) with natural and anthropogenic cycles are included. Big vertical arrows show total annual precipitation and evapotranspiration over land and ocean ($\pu{e3 km3 yr-1}$), which include annual precipitation and evapotranspiration in major landscapes ($\pu{e3 km3 yr-1}$) presented by small vertical arrows; parentheses indicate area ($\pu{e6 km2}$). The direct groundwater discharge, which is estimated to be about $\pu{10 \%}$ of total river discharge globally, is included in river discharge. Image source: [Global Hydrological Cycles and World Water Resources | Science](https://www.science.org/doi/10.1126/science.1128845)
```
Under steady state conditions, the residence time can be calculated as shown in eq {eq}`residence-time`.
```{math}
:label: residence-time
\text{Residence time} = \dfrac{\text{Reservoir Amount}}{\text{Rate of addition/removal}}
```
## Readings and resources for this section
1. Syvitski, J., _et al._ Extraordinary human energy consumption and resultant geological impacts beginning around 1950 CE initiated the proposed Anthropocene Epoch. _Commun Earth Environ_ **1**, 32 (2020). https://doi.org/10.1038/s43247-020-00029-y
2. Wilkinson, B.H., Humans as geologic agents: A deep-time perspective. _Geology_ 2005; 33 (3): 161–164. doi: [https://doi.org/10.1130/G21108.1](https://doi.org/10.1130/G21108.1)
3. Vitousek, P.M., *et al.* Human Domination of Earth's Ecosystems. *Science* , 277, No. 5325 (Jul. 25, 1997), pp. 494-499. https://www.jstor.org/stable/2892536
4. McNeill, J. R., & Winiwarter, V. (2004). Breaking the sod: Humankind, history, and soil. _Science_, 304 (5677), 1627–1629. https://doi.org/10.1126/science.1099893
5. Wardle, D. A., *et al.* (2004). Ecological linkages between aboveground and belowground biota. _Science_, 304 (5677), 1629–1633. https://doi.org/10.1126/science.1094875
6. Jouquet, et al. (2006) Soil invertebrates as ecosystem engineers: Intended and accidental effects on soil and feedback loops. *Applied Soil Ecology*, 32 (2), 153-164. https://doi.org/10.1016/j.apsoil.2005.07.004