Last lesson you met the tree in the video, learned what a system is, and drew one as a diagram. This lesson you build that tree yourself, and then take the same idea all the way out to the whole planet. By the end you should be able to tell an open system from a closed one, name the six spheres that make up the Earth system, and apply the idea of a system at any scale from a rock pool to the atmosphere.
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1.2.1 Systems are sets of interacting or interdependent components.
System components are organized to create a functional whole.
1.2.2 A systems approach is a holistic way of visualizing a complex set of interactions, and it can be applied to ecological or societal situations.
A system has storages and flows, with flows providing inputs and outputs of energy and matter.
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These two statements were taught last lesson. The simulation below is retrieval practice, not new content.
Tree as a system
Open revise.alltheway.ing/simulations/tree-as-a-system and work through both steps. Allow twenty minutes.
Step 1. Sort the thirty-nine components of the tree into inputs, stores, outputs, flows and transfers, and processes and transformations. Check your sort before you move on.
Step 2. Arrange the components on the whiteboard so that inputs arrive, outputs leave and stores sit inside, then draw arrows to show what affects what.
The sort will make you decide, for each flow, whether the thing moving stays the same or changes. That is the transfer and transformation distinction from last lesson, applied thirty-nine times.
1. Name one transfer and one transformation from your tree, and say which is which.
2. Where did you draw the tree's system boundary? Name one input that crosses it.
3. Which component was hardest to classify, and why?
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1.2.5 Systems can be open or closed.
An open system exchanges both energy and matter across its boundary, while a closed system exchanges only energy across its boundary. Almost all systems are open; only the global geochemical cycles approximate to closed systems. Biosphere 2 is an example of a closed system, and a local ecosystem is an example of an open system.
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An open system exchanges both energy and matter across its boundary. A closed system exchanges only energy. Almost every system you will meet in this course is open. The tree you have just built is open: carbon dioxide, water and mineral ions cross its boundary inwards, oxygen and leaf litter cross it outwards, and sunlight and heat cross it as energy.
Closed systems are rare. Only the global geochemical cycles come close. Take the global water cycle. Sunlight drives evaporation and heat is radiated back to space, so energy moves freely across the boundary, but the water itself is neither gained nor lost in any significant quantity. It is moved between the oceans, the atmosphere, the ice, the ground and living things. The word to hold on to is approximate: a little hydrogen escapes to space and meteorites add material, so the Earth is not perfectly closed.
Now compare that with a drainage basin, such as the Rhône above Geneva. Water enters as precipitation and leaves as river discharge, as evaporation and as groundwater flow. Matter crosses the boundary constantly, so the basin is open. The same water, looked at globally, sits inside a system that is almost closed.
Whether a system is open or closed therefore depends on where you draw the boundary, not on what the system is made of.
1. State the difference between an open system and a closed system. [2]
2. Explain why a drainage basin is an open system while the global water cycle approximates to a closed one. [3]
3. Suggest why the guide says the global geochemical cycles only approximate to closed systems. [2]
https://revise.alltheway.ing/simulations/embed/open-and-closed-systems
[16 June 2009]
Biosphere 2, in Oracle, Arizona, is a sealed 1.27 hectare glass structure built to hold a miniature Earth: rainforest, ocean, savannah, desert, farmland and living quarters. Between September 1991 and September 1993 eight people lived inside it, sealed in. It is the example the IB guide names for a closed system, because matter could not cross the glass while sunlight and heat could.
It did not hold. Oxygen fell from about 20.9 per cent to about 14.2 per cent over sixteen months, a level equivalent to living at roughly 4,000 metres, and oxygen had to be pumped in. The cause was traced to the soil. It had been made unusually rich in compost, so decomposition ran fast, consuming oxygen and releasing carbon dioxide. The carbon dioxide was then absorbed by the curing concrete of the structure, which is why the missing oxygen did not simply show up as extra carbon dioxide in the air.
The crew still grew most of their own food, and the closure produced a great deal of usable science about how tightly coupled these cycles are.
1. Outline what is meant by saying that Biosphere 2 was materially closed but energetically open. [2]
2. Identify two ways in which Biosphere 2 differed from the Earth's biosphere. [2]
3. Explain why the fall in oxygen does not mean that Biosphere 2 failed as a model of a closed system. [3]
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1.2.6 The Earth is a single integrated system encompassing the biosphere, the hydrosphere, the cryosphere, the geosphere, the atmosphere and the anthroposphere.
James Lovelock's Gaia hypothesis is a model of the Earth as a single integrated system.
The hypothesis, also known as the Gaia theory, was introduced to explain how atmospheric composition and temperatures are interrelated through feedback control mechanisms.
Many variations of the Gaia theory were further developed by James Lovelock and Lynn Margulis.
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The Earth is usually described in six parts. They are not separate systems sitting side by side. Each one exchanges matter and energy with the others constantly, which is why the guide calls the Earth a single integrated system rather than six systems.