Cambridge Lower Secondary CheckpointStage 9

Earth and Space: Cycles on Earth

Science Stage 9 Chapter Notes

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Earth and Space: Cycles on Earth
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In this chapter

  1. 1The Global Carbon Cycle
  2. 2The Biological Carbon Pump
  3. 3The Slow Carbon Cycle: Rocks and Tectonics
  4. 4The Water Cycle's Key Processes
  5. 5Disrupting the Cycles: Climate Change

1. The Global Carbon Cycle

The carbon cycle is the continuous process by which carbon atoms travel from the atmosphere to the Earth and then back into the atmosphere. Carbon is essential for all life. It is stored in various 'reservoirs' or 'sinks', including the atmosphere (as carbon dioxide, CO2), the oceans, rocks, soils, and all living organisms. Processes called 'fluxes' move carbon between these reservoirs. Key fluxes include photosynthesis, respiration, decomposition, and the burning of fossil fuels. In a balanced cycle, the amount of carbon moving into a reservoir is equal to the amount moving out, keeping atmospheric concentrations stable over long periods.

Key term

Carbon Sink: A natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period, removing it from the atmosphere.

Examiner insight

Examiners reward answers that clearly link carbon reservoirs to the specific processes that move carbon between them, showing a full understanding of the cycle's dynamics.

Common pitfall

Confusing carbon sinks (the reservoirs where carbon is stored) with carbon fluxes (the processes that move carbon between reservoirs). For example, the ocean is a sink, while gas exchange is a flux.

Worked example 13 marks

The diagram below shows a simplified carbon cycle. Identify the processes labelled A, B, and C. [Atmosphere CO2 -> (A) -> Plants -> (B) -> Soil -> (C) -> Atmosphere CO2]

  1. 1

    Step 1: Identify process A. Carbon moves from the atmosphere to plants. This process is photosynthesis.

  2. 2

    Step 2: Identify process B. Carbon moves from plants to the soil. This occurs when plants die and decompose.

  3. 3

    Step 3: Identify process C. Carbon moves from the soil back to the atmosphere. This is caused by the respiration of decomposers (like bacteria and fungi) in the soil.

  4. 4

    Final Answer: A = Photosynthesis, B = Decomposition, C = Respiration (by decomposers).

Recap

  • The carbon cycle describes the movement of carbon between the atmosphere, oceans, land, and living things.
  • Major carbon reservoirs include the atmosphere, oceans, rocks, and the biosphere.
  • Key processes (fluxes) that move carbon are photosynthesis, respiration, decomposition, and combustion.
  • A balanced carbon cycle is crucial for maintaining a stable climate on Earth.

Quick check

  1. Name two major natural carbon sinks.2 marks

2. The Biological Carbon Pump

This refers to the 'fast' part of the carbon cycle, driven by life. Plants on land and phytoplankton in the ocean take in carbon dioxide (CO2) from the atmosphere or water through photosynthesis. They use sunlight to convert this CO2 and water into glucose (for energy) and oxygen. This 'fixes' the carbon into organic matter. When organisms respire, they break down organic matter to release energy, returning CO2 to the atmosphere. This rapid exchange between photosynthesis and respiration dominates the short-term carbon cycle.

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Aerobic Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Key term

Photosynthesis: The process where green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water, releasing oxygen as a byproduct.

Fun fact

The amount of carbon taken up by photosynthesis and released by respiration each year is about 1000 times greater than the amount of carbon humans release by burning fossil fuels. The problem is that the human contribution is a net addition, disrupting the natural balance.

Worked example 14 marks

A scientist measures CO2 concentration in the air around a large forest over 24 hours. They observe that CO2 levels decrease during the day and increase at night. Explain this observation.

  1. 1

    Step 1: During the day, plants in the forest are exposed to sunlight. They perform photosynthesis, which consumes CO2 from the atmosphere. At the same time, plants and animals are respiring, which releases CO2.

  2. 2

    Step 2: In daylight, the rate of photosynthesis is much greater than the rate of respiration. Therefore, there is a net removal of CO2 from the air, causing its concentration to decrease.

  3. 3

    Step 3: During the night, there is no sunlight, so photosynthesis stops. However, plants and other organisms continue to respire, releasing CO2.

  4. 4

    Step 4: With no photosynthesis to remove it, the respired CO2 accumulates in the air, causing its concentration to increase overnight.

Recap

  • Photosynthesis removes CO2 from the atmosphere and converts it into organic compounds.
  • Respiration releases the energy stored in organic compounds, returning CO2 to the atmosphere.
  • The balance between photosynthesis and respiration drives the 'fast' carbon cycle.
  • Producers (like plants) are the foundation of the biological carbon pump.

Quick check

  1. What are the two main products of photosynthesis?2 marks

3. The Slow Carbon Cycle: Rocks and Tectonics

The geological or 'slow' carbon cycle moves carbon between rocks, soil, ocean, and atmosphere over millions of years. It begins with CO2 from the atmosphere dissolving in rainwater to form a weak carbonic acid. This acid rain weathers rocks on land, releasing calcium and other ions. These ions flow into the ocean, where organisms like corals and plankton use them to form shells of calcium carbonate (CaCO3). When these organisms die, their shells sink and accumulate on the seafloor, eventually forming limestone rock. Plate tectonics then plays a crucial role. Through a process called subduction, the seafloor carrying the limestone is forced deep into the Earth's mantle. Under immense heat and pressure, this rock melts, and the carbon is released. This carbon eventually returns to the atmosphere through volcanic eruptions, completing the long cycle.

Key term

Plate Tectonics: The scientific theory that Earth's outer shell is divided into several large, rigid plates that glide over the semi-molten mantle, causing earthquakes, volcanoes, and the creation of mountains.

Examiner insight

Students who can clearly explain the complete geological loop, from atmospheric CO2 removal by weathering to its return via volcanism, and mention the role of plate tectonics, demonstrate a deep understanding that scores very well.

Common pitfall

Thinking that volcanoes only add CO2 to the atmosphere. While they are a source, the rate of release is naturally very slow and is balanced by rock weathering. It's the human rate of release that is the problem.

Worked example 14 marks

Explain the role of volcanic eruptions in the long-term carbon cycle.

  1. 1

    Step 1: Carbon is stored for millions of years in sedimentary rocks like limestone on the ocean floor.

  2. 2

    Step 2: Through plate tectonics, oceanic plates are subducted (forced under) continental plates, carrying these carbon-rich rocks into the Earth's hot mantle.

  3. 3

    Step 3: The intense heat and pressure in the mantle cause the rock to melt and release its carbon, primarily as CO2.

  4. 4

    Step 4: This CO2 mixes with magma and is eventually released back into the atmosphere during volcanic eruptions. This acts as a natural source of atmospheric CO2, balancing the carbon removed by rock weathering.

Worked example 24 marks

How does the process of rock weathering affect the amount of carbon dioxide in the atmosphere?

  1. 1

    Step 1: Rock weathering is a key process that removes carbon dioxide from the atmosphere over geological timescales.

  2. 2

    Step 2: Atmospheric CO2 dissolves in rainwater to form weak carbonic acid.

  3. 3

    Step 3: This acid rain falls on continental rocks and chemically breaks them down, releasing ions like calcium into rivers.

  4. 4

    Step 4: These ions are carried to the ocean where they are used by marine organisms to form calcium carbonate shells, effectively locking the atmospheric carbon away in ocean sediments.

  5. 5

    Step 5: Therefore, increased rock weathering leads to a decrease in atmospheric CO2.

Recap

  • The slow carbon cycle operates over millions of years.
  • Atmospheric CO2 is removed by the weathering of rocks.
  • Carbon is stored long-term in limestone and other sedimentary rocks.
  • Plate tectonics drives the movement of these rocks into the mantle.
  • Volcanoes return this stored carbon to the atmosphere as CO2.

Quick check

  1. What is the name of the process where one tectonic plate is forced beneath another?1 mark
  2. Name the type of rock that is a major long-term store of carbon.1 mark

4. The Water Cycle's Key Processes

The water cycle, or hydrologic cycle, describes the continuous movement of water on, above, and below Earth's surface. It has no starting or ending point. The sun's energy drives the cycle by causing water from oceans, lakes, and rivers to turn into vapor, a process called evaporation. Plants also release water vapor from their leaves in a process called transpiration. As this moist air rises, it cools, and the water vapor changes back into tiny liquid water droplets, forming clouds. This is condensation. When these droplets become large and heavy enough, they fall back to Earth as precipitation (rain, snow, sleet, or hail). Some precipitation flows over the land as surface runoff, collecting in rivers and lakes, while some soaks into the ground, becoming groundwater. This cycle is vital for life and also plays a role in the carbon cycle by enabling rock weathering.

Key term

Condensation: The process by which water vapor in the air is changed into liquid water, leading to the formation of clouds.

Fun fact

A single large oak tree can transpire over 150,000 litres of water into the atmosphere in one year. This makes forests major players in regional weather patterns.

Worked example 14 marks

A student leaves a glass of water on a sunny windowsill. After a few days, the water level has dropped. Later, they notice small water droplets on the inside of the cold window pane above a boiling kettle. Name and explain the two different processes from the water cycle being observed.

  1. 1

    Step 1: The first observation is the dropping water level in the glass. This is due to evaporation. The sun's energy heats the water, giving the surface molecules enough energy to escape into the air as water vapor.

  2. 2

    Step 2: The second observation is droplets forming on the cold window. This is condensation. The hot, moist air from the kettle (containing water vapor) comes into contact with the cold surface of the window.

  3. 3

    Step 3: The cold surface cools the water vapor, causing it to change its state from a gas back into liquid water droplets.

Recap

  • The water cycle is the continuous movement of water on Earth.
  • Evaporation is the process of liquid water turning into water vapor.
  • Condensation is water vapor turning back into liquid water, forming clouds.
  • Precipitation is water falling from clouds as rain, snow, or hail.
  • Transpiration is the release of water vapor from plants.
  • The sun's energy is the primary driver of the water cycle.

Quick check

  1. What is the main source of energy that drives the water cycle?1 mark

5. Disrupting the Cycles: Climate Change

The Earth is kept warm by a natural process called the greenhouse effect. Gases in the atmosphere, like CO2 and methane, trap some of the Sun's heat, preventing it from escaping back into space. This is essential for life. However, human activities are disrupting the carbon cycle and enhancing this effect. Since the Industrial Revolution, we have been burning vast quantities of fossil fuels (coal, oil, and gas). These fuels are the remains of ancient organisms, and burning them releases huge amounts of 'fossil' carbon back into the atmosphere as CO2, far faster than the slow carbon cycle can reabsorb it. This extra CO2 acts like a thicker blanket, trapping more heat and causing the planet's average temperature to rise. This is known as global warming, which in turn drives broader changes in weather patterns, sea levels, and ecosystems, collectively called climate change.

Key term

Enhanced Greenhouse Effect: The increase in the natural greenhouse effect caused by human activities, primarily the emission of greenhouse gases like carbon dioxide from burning fossil fuels.

Examiner insight

Examiners look for a clear distinction between the natural greenhouse effect, which is beneficial, and the enhanced greenhouse effect, which is caused by humans and leads to climate change. Using the term 'enhanced' shows a higher level of understanding.

Common pitfall

Confusing the 'greenhouse effect' with the 'hole in the ozone layer'. The greenhouse effect is about heat-trapping gases (like CO2) warming the lower atmosphere. The ozone hole is about CFCs destroying the ozone layer in the upper atmosphere, which protects us from UV radiation. They are two different environmental problems.

Worked example 15 marks

The graph below shows the concentration of atmospheric CO2 measured at Mauna Loa, Hawaii, since 1958 (the 'Keeling Curve'). It shows a steady upward trend and a yearly 'wobble'. Explain both the long-term trend and the annual wobble.

  1. 1

    Step 1: Explain the long-term trend. The steady increase in CO2 concentration from under 320 ppm in 1958 to over 420 ppm today is primarily caused by human activities, especially the burning of fossil fuels and deforestation. This releases more CO2 into the atmosphere than natural cycles can remove.

  2. 2

    Step 2: Explain the annual wobble. The yearly up-and-down pattern is caused by the seasonal activity of plants in the Northern Hemisphere (where most of the Earth's landmass is).

  3. 3

    Step 3: In the Northern Hemisphere's spring and summer, plants grow and take in large amounts of CO2 through photosynthesis, causing the global CO2 level to drop.

  4. 4

    Step 4: In the autumn and winter, plants lose their leaves and decay, and photosynthesis slows dramatically. Respiration continues, leading to a net release of CO2 and causing the global level to rise until the next spring. This creates the annual 'breathing' pattern of the planet.

Recap

  • The natural greenhouse effect keeps Earth warm enough for life.
  • Human activities, like burning fossil fuels, release large amounts of extra CO2.
  • This extra CO2 enhances the greenhouse effect, trapping more heat.
  • The result is global warming and long-term climate change.
  • Fossil fuels are ancient carbon stores being returned to the atmosphere very rapidly.

Quick check

  1. Name the primary human activity responsible for the enhanced greenhouse effect.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the terms 'carbon sink' and 'carbon flux', giving one example of each.4 marks
  2. Write the balanced chemical equations for both photosynthesis and aerobic respiration. Explain why the balance between these two processes is crucial for the fast carbon cycle.5 marks
  3. Describe the journey of a carbon atom from the atmosphere, into the deep ocean as rock, and back to the atmosphere via the slow carbon cycle. Name the key geological processes involved.6 marks
  4. Draw a simple, labelled diagram of the water cycle. Your diagram must include labels for evaporation, condensation, precipitation, and surface runoff.4 marks
  5. Explain why global atmospheric CO2 concentrations are highest in May and lowest in October each year.3 marks
  6. A student claims that 'volcanoes are the main cause of modern global warming because they release CO2'. Evaluate this statement using your knowledge of the carbon cycle.4 marks
  7. How does plate tectonics contribute to both the removal and release of carbon from the atmosphere over geological time?5 marks
  8. Distinguish between the 'natural greenhouse effect' and the 'enhanced greenhouse effect'.4 marks
  9. Deforestation (cutting down large areas of forest) has two major impacts on the carbon cycle. Describe these two impacts.4 marks
  10. If the average global temperature increases, the amount of water vapor in the atmosphere is also expected to increase. Explain why, and describe how this might affect the climate further (a feedback loop).6 marks

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