ERTH 535: Planetary Climate Change (Spring 2018) Lab Activity #12: Feedback and Stability Dr. Dave Dempsey Dept. of Earth & Climate Sci., SFSU

(For classes starting Monday, April 16)
Objectives:
• improve understanding of the concept of feedback in a system and its relation to system stability
• learn to apply a symbolic method of representing and evaluating feedback in systems (from our text, The Earth System)

Instructions

For each of the five "systems" described below:
1. draw a systems diagram of the sort described in Chapter 2 of our text, The Earth System (see Reading Assignment #6);
2. evaluate the feedback(s) in the system as positive or negative; and
3. evaluate the stability of the system

There are several common mistakes that people learning to develop system diagrams tend to make. When defining each component of your system, follow this advice:

• Each component should be quantifiable, so that you can at least imagine increasing or decreasing its value (that is, changing it in a measurable way). For example, "baby crying" can't be a component of a baby-parent "system", but some quantifiable aspect of the baby's crying can be a system component.

• Don't confuse a component of a system with a change in the component. For example, if you intend that "A" be a quantifiable component of a system, don't write "increase in A" as the system component. Once you've written "A" as a system component, you are going to imagine increasing or decreasing the value of component "A" to determine whether there is positive or negative feedback in the system, but "A", not "change in A", is the system component.

(1) A baby is hungry and begins to cry. Her parent responds by (a) feeding her; or (b) getting annoyed and yelling at her. (These are two different subsystems.)

(2) When the earth warms, it emits more radiative energy (Stefan-Boltzmann Law).

(3) Humans burn fossil fuels, which releases carbon dioxide into the atmosphere, which enhances the greenhouse effect, which warms the atmosphere, which increases the atmosphere's capacity to hold water vapor, which enhances evaporation from the oceans and increases the amount of water vapor (a greenhouse gas) in the atmosphere.

(4) The sun heats high and low latitudes unequally, creating a temperature difference between lower and higher latitudes in the lower troposphere, which creates a pressure difference between lower and higher latitudes aloft, which drives winds aloft, which lead to the creation of relatively high and low pressure areas near the earth's surface and hence pressure differences near the surface, which drive winds near the surface, which (on the average) move heat from middle to high latitudes; the winds near the surface also drive ocean surface circulations, which move heat from low to midlatitudes.

(5) Humans burn fossil fuels, which releases carbon dioxide into the atmosphere, which enhances the greenhouse effect, which warms the atmosphere, which increases the atmosphere's capacity to hold water vapor, which enhances evaporation from the oceans, which increases the amount of water vapor in the atmosphere available to condense to form clouds, which increases the extent and/or frequency of cloudiness, which both increases the average albedo of the planet and enhances the greenhouse effect.

[Note that high clouds, which consist of relatively low concentrations of ice crystals, don't reflect solar radiation very well. However, they do absorb longwave infrared radiation well, but are relatively cold and hence don't emit radiative energy very intensely. That means that they prevent more longwave infrared radiation emitted below them from reaching space than they radiate to space themselves. As a result of these properties, increases in high clouds have a relatively larger impact on the greenhouse effect than they have on the planet's albedo.

In contrast, low clouds, which consist of relatively high concentrations of water droplets, reflect solar radiation well. Like high clouds, they absorb longwave radiation well, too, but because they are low and hence relatively warm, they emit radiative energy to space with an intensity similar to the intensity of the planet below them. As a result, increases in low clouds have a larger impact on the planet's albedo than they have on the greenhouse effect.

Do you think that this difference is something that the system diagram approach can capture?]

(6) A mountain range grows as two tectonic plates collide. As the mountains grow higher, air must rise farther to pass over them, which makes the air cool further, which causes more water vapor to condense to form clouds, which increases the amount of precipitation falling from the clouds, which increases erosion of the mountains, which removes material from them (tending to make them lower) but also makes the mountains lighter and easier to raise by tectonic forces.

(7) When the temperature in the lower troposphere changes (in polar regions in particular), the extent of ice caps and sea ice can change, which changes the proportion of ocean/rock/sand/soil/vegetation surface on the earth vs. the proportion of ice and snow cover, which changes the albedo in the affected area, which changes the rate at which the earth absorbs solar radiation in that area, which changes the temperature in the lower troposphere in that area (and might have global effects, too).

[Note that this effect is greater where the solar intensity is greater, so the lower the latitutude is the more pronounced the effect is. Do you think that this is something that the system diagram approach can capture?]