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Last lesson you met James Lovelock's Gaia hypothesis, which says that the Earth's temperature and the make-up of its atmosphere are held within a range that suits life by feedback control mechanisms. This lesson is about those mechanisms. By the end you should be able to tell a negative feedback loop from a positive one, draw both as diagrams, use the Daisyworld model to explain how life can regulate a planet's temperature when a dead planet cannot, and say what a tipping point is.
A feedback loop forms when part of a system's output comes back round as an input and changes what the system does next. There are two kinds, and their names describe direction, not whether they are good or bad. Negative feedback counteracts a change and pulls the system back towards where it was. Positive feedback amplifies a change and pushes the system further away. Watch the video, then sort its four examples.
[25 August 2014]
1. For each example from the video, describe how the loop works in one or two sentences, then label it positive or negative feedback.
a. Plants dying and enriching the soil
b. Forest removal and erosion
c. Lynx and snowshoe hares
d. Pesticides and insect numbers
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1.2.8 Negative feedback loops occur when the output of a process inhibits or reverses the operation of the same process in such a way as to reduce change. They are stabilizing as they counteract deviation.
Use James Lovelock and Andrew Watson’s Daisyworld model to show how temperature regulation can occur due to the presence of life on a planet in contrast with a dead one. Include other examples.
Application of skills: Use diagrams representing examples of negative feedback.
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In a negative feedback loop, the output of a process works against the process itself. A change sets off a response that reduces or reverses that change, so the system is pulled back towards where it started. That is why negative feedback is stabilising. The lynx and hares in the video are one example: each rise in one population sets off the fall that follows it, so neither runs away.
The guide asks you to study one negative feedback loop in detail. Daisyworld is a model planet published by Andrew Watson and James Lovelock in 1983. It has no atmosphere, no clouds and no animals: only grey ground, a sun, and two kinds of daisy. Black daisies absorb more sunlight than the bare ground and warm their surroundings. White daisies reflect more, because they have a higher albedo (the proportion of incoming sunlight a surface reflects), and cool their surroundings. Both kinds grow fastest at 22.5 °C and cannot grow at all below 5 °C or above 40 °C.
Stars like our Sun slowly brighten over billions of years. Daisyworld asks what happens to a planet's temperature as its sun brightens, with life on it and without.
If the simulation is cramped, open it full screen: revise.alltheway.ing/simulations/daisyworld.
https://revise.alltheway.ing/simulations/embed/daisyworld?v=2
Use the buttons under Try an experiment. Each run takes about 30 seconds; press Fast to speed it up. The dashed line on the temperature graph is the same planet with no life on it, drawn on every run so that you always have the dead planet to compare against. Your last finished run also stays on the graph as a faint line.
Step 1. Run A lifeless planet.