Clearly, lungs are part of a larger system (the human body)--but their basic function is to take in oxygen and exchange it with our blood, then exhale carbon dioxide. What would happen if the input of oxygen was stopped? First, carbon dioxide would no longer be output. But there are more serious ramifications for the function of other system components! We can diagram this larger system like so:

Earth systems can be diagrammed in the same way. Let's add a simple connection to the human system to understand how we fit into larger life systems:

Now we can see that sunlight is an essential input (among others) to creating food, which is essential to providing our bodies with energy. Waste products are also created as we metabolize our food.

The plus sign indicates a positive relationship that we have all experienced. We can read this two ways: "increased exercise leads to increased body temperature," or also "decreased exercise leads to decreased body temperature." Either way the relationship is positive, as more of one thing leads to more of the other and vice-versa. Fortunately our bodies have a way of responding to increased temperatures, diagrammed below:

The signs get confusing here so remember how we've defined these relationships. Starting with body temperature, we see a positive relationship, meaning "increases in body temperature lead to an increase in feeling hot," and conversely, "decreases in body temperature lead to decreases in feeling hot." The next relationship in the chain shows us that "increases in body temperature lead to decreases in clothing," or that "decreases in body temperature lead to increases in clothing." If we feel hot, we take off some clothes. If we feel "not hot" (cold), we put clothes on. See how increases are met with decreases and vice-versa? That's a negative relationship. Finally, to complete the feedback loop, "increases in clothing lead to increases in body temperature," and "decreases in clothing lead to decreases in body temperature." Let's modify the same example to show positive feedback:

The top feedback is negative. High risk of floods leads people to move out of the flood plain, which in turn puts everyone at lower risk of floods. (Floods will still occur, but people won't be at risk.)

However, the lower feedback is positive. In this example, a high risk of floods leads people to create elevated building pads on the floodplain. That saves the building from a flood, but makes flooding more likely on the rest of the flood plain. The more building pads are built, the higher the floodwaters become until eventually it's nearly the same risk on top the building pads!

Our final example for this section looks at an issue that was quite concerning to many during the second part of the last century. We'll discuss this feedback more fully in our chapter on population, as it turns out this diagram and feedback loop are overly simplified (though they are essentially true if all other factors are held constant):

Here we see that the larger a population, the more babies are born (again, holding all other factors constant); in turn, the more babies are born, the larger the population becomes.