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Lecture 2: Earth system approach
Bold on board:
What is a System?
An entity composed of diverse but interrelated parts that function as a complex whole.
EXAMPLE 1: Good example is human body: the body has a number of systems, like respiratory system w/ O2 in and CO2 out; cardiovascular system that circulates blood, taking the O2 and CO2 around the body; digestive system to process food to fuel body processes; nervous system that senses changes in internal and external environment and controls activities of other systems; and endocrine system that regulates ongoing processes like growth and development, etc. These all are interrelated and work together to maintain life.
Essentials of a system:
Components- individual parts of a system.
E.G., a component can be a reservoir of matter, a reservoir of energy (e.g., Temperature), an attribute of the system (body temp. or pressure) or a subsystem (like the cardiovascular system).
State
Set of important attributes that characterize the system at a particular time. E.g., body temp. level of nutrition, and blood pressure for human body. Components of a system interact so that a change in State of the system is "sensed" throughout. This allows for control of important attributes. E.g., human body’s endocrine system can maintain a nearly constant internal temperature despite changes in surrounding environment. Think about when the temperature outside goes up – the hypothalamus (part of endocrine system) directs sweat glands to increase sweat production which cools. When temp. goes down, circulation to external parts of the body is reduced to keep the essential parts (near the lungs and heart) warm.
Couplings – positive and negative. Feedback loop. (p. 20)
Components of a system, like the human body, are linked, meaning that information can flow from one to another. These links are called couplings.
Positive coupling:
a change in one component (either increase or decrease) leads to a change in another component in the same direction. So when one component increases, the other component responds by increasing. So, with electric blanket, when the blanket temp is increased, the body temperature will also increase. (‡)
Negative coupling:
a change in one component leads to a change of the opposite direction in the linked component. When one component increases, a negatively coupled component responds by decreasing (––o). So with blanket, if body temperature increases, we turn down the blanket temperature.
Feedback Loops:
If we understand couplings, we can move onto feedback loops. A feedback is a self-perpetuating mechanism of change and response to that change. When we receive FB from other people, we are receiving their reaction to an action of ours. Their reaction becomes itself and action, which we react to. So, for example, Benji and Daniel in the morning. When Daniel likes to pounce to within about 1 mm of Ben’s face first thing in the morning. Ben, depending on how awake he is, will react to that pounce by either screaming – which gets Daniel on a time-out and so the pouncing stops; or by pouncing back and the wrestling increases.
A Negative feedback loop tends to diminish the effects of disturbances, so for example, an incr. in body temperature, however caused, stimulates you to turn down the blanket, which causes the blanket to radiate less heat and so your body temperature goes down.
A Positive feedback loop tends to amplify the effects of disturbances. So, for example, imagine that Ben and Daniel are on a fancy, dual gear-system tandem bike, but unbeknownst to either of them, their cousin has switched the gear controls so that Ben’s controls operate Daniel’s gears and Daniel’s controls operate Ben’s gears. As they pedal along, Ben gets tired and so he down-shifts, but he’s really down-shifting Daniel’s gears. Daniel is feeling like the pedaling is too easy and he wants to go faster, so he shifts up – which increases the work Ben has to do, making him feel even more tired. So Ben down-shifts again, which makes Daniel’s work even easier. Daniel is feeling like Superman, so he shifts up again. If the feedback loop continues in the positive fashion, the whole system could crash -- literally!
Equilibrium states:
When the system is in a state where it will not change unless disturbed (perturbed). Going back to the bike – when both boys have their gear controls working, and they are pedaling at a comfortable level – on flat ground they are at stable equilibrium. When the gears work correctly, a slight incline, causing the boys to work harder will be countered by down-shifting, so the equilibrium state is created by a negative feedback loop. It is considered Stable. However, when the gear controls were confused, the equilibrium state was unstable, because the slightest change in topography caused a disturbance that led to system adjustments that carried the system further and further from the original state.
Let’s look at all the possible equilibrium states of a system as a hilly surface, and the present state as a ball that is free to move on that surface. Valleys represent stable equilibrium states, while peaks represent unstable states.
A ball in a valley would be considered in a stable equilibrium state – after slight perturbations, the ball rolls back to its equilibrium state. A large disturbance could cause the ball to roll out of its valley and over an adjacent peak into another valley – or equilibrium state -- as there are limits to the stability of given stable equilibrium states.
In contrast, the peaks represent unstable equilibrium states. Slightest disturbance will push the ball towards a different stable equilibrium state.
What can we conclude about simple systems with single feedback loops ? Are they stable if the FB loop is positive or negative? Earth systems are more complicated and involve many feedback loops which are both positive and negative so that whether the system is stable or not is not so easy to determine. In fact, it isn’t always easy to know if the systems are even at equilibrium.
Perturbations and Forcings
We can learn a lot from observing how systems respond to disturbances. What we know about human physiology has benefited from studying patients. Likewise, what scientists know about Earth’s systems comes from watching how those systems respond to disturbances. For example, the earth’s climate is being modified by natural and anthropogenic (human-caused) factors. One such Perturbation (temporary disturbance to the system) was the eruption of Mt. Pinatubo in the Philippines 1991. EG Figures 1-5 or 4-1, SO2, sulfur dioxide, was injected into the atmosphere and formed sulfate aerosol particles (like what comes from burning fossil fuels), which in turn prevented a small amount of sunlight from reaching the Earth. As a result, the surface temperature of the planet dropped by 0.5°C (1°F) globally. The climate system took 3 years to recover from this perturbation.
More persistent disturbance of a system is called a forcing. An example of forcing of the Earth’s climate system would be the gradual-increase over billions of years of the amount of sunlight the Earth has been receiving. Sun’s luminosity has increased by about 30% over the history of Earth (3.5 by) as a result of the fusion of 4 H ‡ He, releasing energy, but also increasing Temperature, which causes more reactions and so more energy production. Scientists believe that the increased luminosity is balanced by reduced greenhouse gases – the Earth’s atmosphere has less CO2 today than it did billions of years ago.
How does can system without the ability for foresight be self-regulating?
THE DAISYWORLD CLIMATE SYSTEM:
Concepts: albedo (dark is 0, white is 1); graphs (how to interpret – dependent vs. independent variable)
External forcing
Daisyworld has life, but only what appear to be daisies – pure white. They appear to get their nutrients and water directly from the soil, and the atmosphere has no clouds and no greenhouse gases. The surface of the planet is covered either by gray soil or by white daisies.
This means that the amount of sunlight absorbed by the planet depends on the area of darker, bare soil relative to the amount of lighter daisy cover. The more sunlight absorbed means the higher the surface Temperature. The growth and spread of the daisies depends only on the temperature around them. Scientists observing the planet and its sun are alarmed because that sun seems to be increasing its luminosity at a much faster rate than our own sun. They calculate that the planet will quickly become too hot to support daisy growth. But they have forgotten that the daisies themselves are part of the global climate system and must be considered in the calculations. The reflectivity of the planet is affected by changes in the daisy population.
So, the global climate of Daisyworld is a two-component system: 1 is the area of white-daisy coverage, the other is the average surface temperature of the planet. These are interdependent and so form a system. The extent of daisy coverage affects the surface Temp. and the surface temp. affects the growth rate of the daisies, which in turn affects the daisy coverage…
Couplings in the Daisyworld climate system:
Response of surface temperature to changes in Daisy coverage. – Albedo Reflectivity of a surface (expressed as decimal fraction of total incoming energy reflected from the surface so dark surface has low albedo (0.05) and snow has high albedo (.9).
So with what we know about
The relationship between surface temperature (Y-axis) and daisy coverage (X-axis) is negative and can be shown as a graph with a negative slope or as a system diagram.
The graph represents the relationship that as daisy coverage increases, temperature will decrease because of the reflectivity of the daisies.
The graph cannot be interpreted to mean that changes in surface temperature affect daisy coverage. The Independent variable is placed on the X axis in a conventional graph, and the Dependent variable on the Y-axis. Changes in surface temperature affect daisy coverage in a different way than changes in daisy coverage change surface temperature.
Response of Daisy coverage to Changes in Temperature:
With real daisies, there is an upper and a lower temperature limit for survival, with an optimum, or most favorable temperature. A curve drawn through these points is a parabola, with the abundance of the daisies highest near the optimum temperature and down to zero at the upper and lower limits of the temperature range.
Mutual influences of average surface temperature on white daisy coverage (parabola) and white daisy coverage on surface temperature (straight line). Intersection points P1 and P2 can be drawn on. Over lay the graphs.
P1: stable equilibrium: if increase in surface Temp ‡ increase in Daisy coverage ‡ decrease in Temp. SO NEGATIVE FEEDBACK LOOP
P2: Unstable equilibrium: if increase in surface Temp --o decrease in Daisy coverage ‡ increase in Temp. so POSITIVE FEEDBACK LOOP.
Effect of perturbations: Draw on diagram: slight decrease in daisy coverage ‡ slight increase in Temp, leads to slight increase in daisy coverage ‡ decrease in Temp. ‡ decrease in Daisy coverage ‡ increase in Temp. until system adjusted back to P1 state.
If perturbation was too large, like a massive reduction in daisy coverage, the surface Temp. would go up too high and the daisies would go extinct.
Forcing: increased solar luminosity: the parabola which describes coupling of surface temperature to daisy coverage – as the daisies are responding to temperature. As luminosity increases, so will the surface temperature, thought the linkage between albedo and temperature remains the same. So the line itself will migrate to higher values.
Draw new line P1 – P2. At the new equilibrium states are less able to handle changes
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