Earth as a System: Energy, Matter, and Life — Class 9 Science
Exploration · Chapter 13
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Chapter summary
Key ideas & terms
- Earth system
- The whole planet seen as a set of interacting spheres through which energy and matter flow and influence one another.
- Biosphere
- The part of the Earth system that contains all living organisms and where they interact with air, water and land.
- Biogeochemical cycle
- The cyclic movement of a substance (such as water, carbon or nitrogen) between the living and non-living parts of the Earth.
- Greenhouse effect
- The trapping of heat by certain atmospheric gases that keeps the Earth warm; its strengthening by extra carbon dioxide causes global warming.
- Feedback loop
- A process in which a change produces effects that further increase (positive) or reduce (negative) that change, as with melting ice exposing dark surfaces.
- Spheres of the Earth
- The geosphere, hydrosphere, cryosphere, atmosphere and biosphere, which together make up the Earth system.
Extra questions & answers
What are the main sources of energy that drive the Earth system?
The Sun is the main source, with additional energy coming from the Earth’s hot interior and from chemical reactions taking place in the air, water and rocks.
Name the five spheres of the Earth system.
The geosphere (solid rock and soil), hydrosphere (liquid water), cryosphere (ice and snow), atmosphere (air) and biosphere (all living things).
What is meant by treating the Earth as a system?
It means viewing the planet’s spheres as interconnected parts that exchange energy and matter, so that a change in one sphere produces effects in the others rather than acting in isolation.
What is a biogeochemical cycle? Give examples.
It is the repeated movement of a substance between the living and non-living parts of the Earth; examples are the water cycle, the carbon cycle and the nitrogen cycle.
How does energy from the Sun affect the atmosphere and hydrosphere?
Solar heating drives the winds and powers the water cycle by evaporating water, which later condenses and falls as rain, linking the atmosphere and hydrosphere.
What is the greenhouse effect and why is it useful?
It is the trapping of heat by atmospheric gases such as carbon dioxide and water vapour; in moderation it keeps the Earth warm enough to support life.
How does increasing carbon dioxide affect the planet?
Extra carbon dioxide from human activity strengthens the greenhouse effect, trapping more heat and warming the planet (global warming), which disturbs climate and ecosystems.
Give an example of a positive feedback loop in the climate system.
Melting ice exposes darker land or ocean that absorbs more sunlight, causing more warming and still more melting — a change that reinforces itself.
How can clearing a large forest affect a nearby river?
Without trees to hold soil and water and to release moisture, rainfall runs off faster and the soil erodes, so the river may flood after rain and shrink in dry periods, altering its flow.
Why is studying the Earth as a connected system important for us?
Because human actions in one sphere — such as adding carbon dioxide or melting glaciers — affect monsoons, rivers, coasts and ocean life, so understanding the links helps us protect the environment and live sustainably.
Think It Over
How does the warming of Arabian Sea water affect the southwest monsoon in India?
Warmer Arabian Sea water increases evaporation, so more moisture enters the atmosphere. This makes the southwest monsoon more variable - it can cause heavier rainfall and floods in some regions and delayed or weak rainfall and drought in others, because the temperature and pressure differences that drive the monsoon winds are disturbed.
If a large forest is cleared, how can that affect the flow of a river in that area?
Forests hold soil with roots, increase infiltration and release water slowly through transpiration and groundwater. If forest is cleared, rainfall runs off quickly instead of seeping in, so the river may flood in the rainy season and become very low in the dry season. Soil erosion also increases, making water muddy and reducing groundwater recharge.
What might happen to coastal cities in India if glaciers and polar ice keep melting faster?
Melting glaciers and polar ice add water to the oceans and raise sea level. Low-lying coastal cities like Mumbai and Chennai would face more frequent flooding, erosion of coastline, entry of salty water into freshwater and farmland, and loss of habitat for people and coastal ecosystems.
How would increasing carbon dioxide levels in the atmosphere affect the ocean plankton?
More
Activity 13.1: Let us explore
1. Observe the features of the Earth as shown in Fig. 13.1. Identify and circle one example representing each of the geosphere, hydrosphere, cryosphere, atmosphere, and biosphere.
From Fig. 13.1: Geosphere - mountains/rocks or soil underground; Hydrosphere - lake or river; Cryosphere - snow-covered mountain tops and glaciers; Atmosphere - clouds and clear sky; Biosphere - sheep/grass/trees/humans. Circling one clear example of each shows all five spheres present together in one landscape.
2. How does snow (cryosphere) eventually become part of the lake (hydrosphere)?
Snow melts when temperature rises, becomes meltwater, flows as streams and runoff over the geosphere, and collects in the lake. Some snow sublimates and then condenses as precipitation that also feeds the lake. Thus solid water of the cryosphere enters the liquid water of the hydrosphere.
3. If there is less snowfall during winters for a few years, how would this affect the lake’s level and the grass available for the sheep?
Less snowfall means less meltwater in spring and summer, so the lake level falls. Less water means less growth of grass and other plants around the lake, so sheep get less food and may need to move or produce less milk and wool.
4. Discuss with your classmates and write down how all the spheres are interconnected, and how a disturbance in one can lead to changes in others.
All spheres exchange energy and matter. Sun heats atmosphere and geosphere, evaporates hydrosphere, melts cryosphere, and supports biosphere. Example: warmer atmosphere melts cryosphere, more water enters hydrosphere raising sea level, flooding geosphere soils, and destroying biosphere habitats. A change in one sphere spreads to all others, showing Earth as one interconnected system.
Activity 13.2: Let us find out
Complete Table 13.1 using information from authentic sources like websites and books. Table 13.1: Reflection of solar radiation by surfaces of materials S. No. 1. Snow 0.80–0.90 2. Ice 0.50–0.70 3. Crushed rock 0.25–0.30 4. Light coloured soil 5. Black soil 6. Ocean water
Typical albedo values are: 4. Light coloured soil
Think as a Scientist
An interesting estimation problem that helps us to appreciate the enormous amount of energy that we get from the Sun is to estimate how much of the Earth’s surface would be needed to be covered with solar panels to supply all the electric power that our country uses today (Fig. 13.4). To make this estimate, you can find these numbers on the internet, assume some insolation on the Earth’s surface and consider that some fraction of this energy is converted into electricity. You will probably find that even a fraction of the area of the Thar desert, if covered with solar panels, could supply India’s electricity needs.
Take average insolation at surface as about
Pause and Ponder
1. Visit the website given below and study the effect of the concentration of greenhouse gas on surface temperature, https://phet.colorado.edu/en/simulations/greenhouse-effect
In the PhET simulation, increasing greenhouse gas concentration (
Pause and Ponder
2. How does the cool mountain breeze benefit agriculture activity, particularly the crops and soil?
Cool mountain breeze at night lowers temperature, reduces water loss by evaporation, brings moist valley air that can form dew, and prevents heat stress on crops. Gentle drainage of cold air reduces frost damage in valleys and keeps soil moisture and nutrient cycling balanced, helping plants grow steadily.
3. What happens to the warm surface of water from the equator as it travels toward the poles? What impact does this movement have on the area?
Warm equatorial surface water is carried poleward by ocean currents like the Gulf Stream and North Atlantic Drift. It releases heat to the cooler atmosphere, warming coastal lands, keeping ports ice-free in winter in northwest Europe, moderating climate, bringing moisture for rainfall, and carrying nutrients that support fisheries.
Pause and Ponder
4. The CO2 dissolved in the ocean is disturbed when the global temperature increases. What will happen to marine life?
Warmer water holds less dissolved
Pause and Ponder
5. What would happen to plants and animals on Earth if the biogeochemical cycles were disrupted and stopped? Explain by giving a few examples.
If cycles stopped, nutrients would not be recycled and life would run out of essentials. Without the water cycle, no rain would reach land and plants would dry. Without carbon cycle, plants would have no
6. Discuss how human activities increase the concentration of greenhouse gases in the atmosphere. What would you do as an individual to reduce the emission of greenhouse gas?
Burning coal, oil and gas, deforestation, vehicle emissions, rice paddies and livestock release
Revise, Reflect, Refine
1. Choose the most appropriate option to describe the role of biogeochemical cycles in an ecosystem. (i) To provide food directly to all organisms. (ii) To recycle essential nutrients between biotic and abiotic components. (iii) To create new elements for use by living things. (iv) To remove pollutants and toxins from the organism.
Correct option is (ii) To recycle essential nutrients between biotic and abiotic components. Cycles continuously move water, carbon, nitrogen and oxygen between air, water, soil, rocks and living things, making them available again without creating new elements.
2. Which of the following is primarily responsible for warming of the Earth? (i) Solar radiation is immediately absorbed by carbon dioxide, which then releases it as heat. (ii) The atmosphere’s tiny particles absorb incoming solar radiation, which directly heats the Earth. (iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases. (iv) The Earth’s environment is heated only by the solar radiation reflected by the clouds.
Correct option is (iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases. Visible light warms the surface, surface emits infrared, and
3. Explain how climate change affects the water cycle. Illustrate with examples.
Warmer air holds about
4. Describe how albedo affects the Earth’s surface temperature and its climate.
Albedo is the fraction of sunlight reflected. High albedo (snow
5. How are mountain and valley breezes formed? Suppose there are two mountains, one covered with grass and another covered with barren rocks; would the temperature of the two mountain breezes be different? If so, how?
During the day the sun heats the mountain slopes faster than the valley, so warm slope air rises and valley air flows up as a valley breeze. At night the slopes cool faster, and the dense cold air sinks down as a mountain breeze. Of the two mountains, the barren rocky one has a low heat capacity, so it heats up quickly by day and cools quickly at night, while the grass-covered mountain holds moisture and changes temperature more slowly. Therefore the rocky mountain drives the stronger slope breezes, and its night-time mountain breeze is the colder of the two.
6. You have witnessed weather phenomena, such as winds, storms, rainfall, etc. Which atmospheric layer is mainly responsible for such phenomena and what is the primary reason for its occurrence?
The troposphere (
7. Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
Steps: nitrogen fixation (Rhizobium, Azotobacter and lightning convert
8. What are the impacts of deforestation on the Earth’s oxygen and carbon cycles? What are the other consequences of deforestation?
Trees absorb
9. Explain with suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using CO2 from the atmosphere.
Plants take
10. Why is an excess of CO2 in the atmosphere considered undesirable even though it is required by plants?
A small amount of
11. How is heat lost from the surface of the Earth? What is its significance?
Surface absorbs sunlight and re-radiates heat as infrared radiation. Part escapes to space, part is absorbed by greenhouse gases and clouds and re-radiated back. Significance: this trapped heat keeps average temperature about
12. If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?
On a sphere, sun rays hit equator almost perpendicularly over small area and poles at slant over large area, so equator is hot and poles cold, creating uneven heating, winds and ocean gyres. On a flat disc facing the sun, all places would get nearly same angle and same insolation at same time, so little latitudinal temperature difference, no polar cold zones, no global pressure belts or seasonal variation from shape, and climate would be uniform.
13. Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?
Cryosphere: glaciers, snow and polar ice melt faster, reducing ice cover and raising albedo feedback. Hydrosphere: warmer water expands, sea level rises, evaporation and heavy rainfall increase in some areas while drought in others, rivers get more meltwater initially then less. Biosphere: habitats shift or disappear, coral bleaching, plankton decline, crops suffer from heat and water stress, and species may migrate or go extinct.
14. Explain how the Earth’s atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.
Atmosphere filters harmful UV by ozone and reduces intense sunlight by scattering. It absorbs sunlight and traps outgoing infrared via greenhouse gases
15. Describe the interrelationship between different spheres of the Earth. Illustrate with example how these spheres function in a delicate balance.
Geosphere, hydrosphere, cryosphere, atmosphere and biosphere exchange matter and energy. Example: Sun heats ocean (hydrosphere) causing evaporation (atmosphere), moisture forms clouds and snow on mountains (cryosphere), melt feeds rivers (hydrosphere) that carry nutrients from rocks (geosphere) to plains where plants and animals (biosphere) grow. Cutting forests (biosphere) changes albedo and runoff (geosphere/hydrosphere), altering rainfall (atmosphere) and melting ice (cryosphere), showing delicate balance.
The Journey Beyond
Consider two hypothetical Earth-sized planets that have an atmosphere. Assume that one planet is entirely covered by oceans and the other is entirely by land. Knowing that the Sun heats the equator more than the poles, how would the wind patterns on these planets compare with the wind systems we observe on Earth, with its combination of land and sea?
Both planets would still have planetary winds from equator-to-pole heating: equatorial low, sub-tropical highs and polar highs, with winds deflected by rotation. Ocean-covered planet would have very uniform winds and strong, steady ocean-driven currents with no monsoon-like seasonal reversal, because water heats and cools slowly. Land-covered planet would heat and cool quickly, creating stronger seasonal pressure changes and dusty winds but no sea breezes. Earth with both land and sea has mixed system: large ocean gyres plus seasonal monsoons where land heats faster in summer drawing moist ocean air inland.
Choose any one meal you ate recently (it could be anything, for example, roti and dal, rice and sambar, idli and chutney, and so on). For each main item in the meal, find out and explain: (i) how the carbon in it originally came from carbon dioxide in the air through photosynthesis, and (ii) how the nitrogen in it likely came from the atmosphere into the soil (for example, by bacteria or through the Haber-Bosch process and fertilisers) and then into the plant. Further, list other human activities involved in producing or cooking this meal that add extra carbon dioxide or nitrogen to the environment.
Example meal: roti (wheat) and dal (lentil). (i) Carbon: Wheat and lentil plants took
Using data from the India Meteorological Department (IMD, https://mausam.imd.gov.in/) or newspaper records, find the average monsoon rainfall (June–September, or the local season) for your city or district for 5 years during two decades, such as the 1980s and 2020s. Note any trend you may find (increasing or decreasing, or the total number of days) with heavy rain (>50 mm). How can this be connected to the warmer Arabian Sea temperatures or changes in land use (forests to farms to cities) as discussed in the chapter?
Sample approach: For a city like Pune, IMD data might show 1980s average June-September about