NCERT Class 9 Science Chapter 13 Solutions – Our Environment & Biogeochemical Cycles
Looking for accurate and complete ncert class 9 science chapter 13 solutions for the new CBSE curriculum (2026-27)? You are in the right place! In this guide, we provide step-by-step, direct, and easy-to-understand solutions for all exercise questions from NCERT Exploration Class 9 Science Chapter 13: Natural Resources and Earth's Systems.
These answers are crafted specifically to help students write clean, exam-ready answers in their notebooks without extra confusion or lengthy fluff.
Question 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: (ii) To recycle essential nutrients between biotic and abiotic components.
Explanation: Biogeochemical cycles (like carbon, nitrogen, and water cycles) ensure the continuous flow and reuse of vital chemical elements between living organisms (biotic) and their physical environment (abiotic).
Question 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: (iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.
Explanation: Shortwave solar radiation passes through the atmosphere to heat the Earth's surface. The warm surface emits longwave infrared radiation, which is absorbed and trapped by greenhouse gases (like $\text{CO}_2$, water vapor, methane), creating the natural greenhouse effect.
Question 3
Explain how climate change affects the water cycle. Illustrate with examples.
Climate change intensifies the global water cycle through accelerated warming:
- Increased Evaporation: Higher temperatures increase evaporation rates from oceans and soil, drying out land areas faster.
- Extreme Precipitation: Warmer air holds more moisture, leading to intense downpours and severe flooding in some regions while causing prolonged droughts in others.
- Example: Melting glaciers and polar ice caps disrupt seasonal river flows, altering freshwater availability worldwide.
Question 4
Describe how albedo affects the Earth's surface temperature and its climate.
Albedo is the measure of reflectivity of a surface:
- High Albedo Surfaces (e.g., snow, ice, glaciers) reflect most incoming solar radiation back into space, keeping the surface cool.
- Low Albedo Surfaces (e.g., dark soil, forests, oceans) absorb most solar radiation, converting it into heat and raising surface temperatures.
- Climate Impact: Melting ice reduces Earth's albedo, causing more solar energy to be absorbed, which accelerates global warming in a feedback loop.
Question 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?
- Mountain/Valley Breeze Formation: During the day, mountain slopes heat up faster than valleys, creating an upward wind called a valley breeze. At night, slopes cool rapidly, causing cool air to flow down into the valley as a mountain breeze.
- Grass vs. Barren Rock Mountains: Yes, the breezes would differ. Barren rocks have a lower albedo and high heat absorption, making daytime rock breezes much warmer. Grassy mountains transpire moisture and reflect sunlight better, resulting in cooler, milder breezes.
Question 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?
- Atmospheric Layer: Troposphere (the lowest layer of the atmosphere).
- Primary Reason: Uneven heating of the Earth's surface by the Sun, rotation of the Earth, moisture evaporation from water bodies, and pressure differences create atmospheric convection currents and weather disturbances.
Question 7
Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
- Nitrogen Cycle Steps:
- Nitrogen Fixation: Conversion of atmospheric $\text{N}_2$ into nitrates/ammonia by lightning or nitrogen-fixing bacteria (like Rhizobium).
- Assimilation: Plants absorb nitrogen compounds from soil to build proteins; animals consume plants.
- Ammonification & Nitrification: Conversion of dead organic waste into ammonia and then nitrates by soil bacteria.
- Denitrification: Conversion of soil nitrates back into atmospheric $\text{N}_2$ gas by denitrifying bacteria.
- Impact of No Cycling: Without nitrogen cycling, plants could not synthesize amino acids and proteins, collapsing agricultural food webs, halting nucleic acid formation (DNA/RNA), and ending life on Earth.
Question 8
What are the impacts of deforestation on the Earth's oxygen and carbon cycles? What are the other consequences of deforestation?
- Impact on Carbon & Oxygen Cycles: Cutting down trees reduces photosynthetic absorption of $\text{CO}_2$ and oxygen production. Burning or decaying wood releases massive amounts of stored carbon dioxide into the atmosphere, unbalancing both cycles.
- Other Consequences: Soil erosion, loss of biodiversity and natural habitats, disruption of local water cycles (reduced transpiration and rainfall), and climate destabilization.
Question 9
Explain with suitable diagram/description the path that carbon takes to go back to the atmosphere. You may start from plants using $\text{CO}_2$ from the atmosphere.
Carbon flows through the ecosystem in a continuous loop:
- Photosynthesis: Plants absorb atmospheric $\text{CO}_2$ to synthesize glucose and organic compounds.
- Consumption: Herbivores and carnivores eat plants, transferring carbon through the food chain.
- Respiration: Plants and animals release $\text{CO}_2$ back into the atmosphere during cellular respiration.
- Decomposition: Decomposers break down dead organic matter, releasing $\text{CO}_2$.
- Combustion: Burning fossil fuels and biomass releases stored carbon back into the atmosphere.
Question 10
Why is an excess of $\text{CO}_2$ in the atmosphere considered undesirable even though it is required by plants?
While plants need $\text{CO}_2$ for photosynthesis, excess carbon dioxide acts as a powerful greenhouse gas. Excessive amounts trap too much infrared heat radiation, leading to global warming, climate change, melting polar ice, and severe ecological disruptions.
Question 11
How is heat lost from the surface of the Earth? What is its significance?
- Mechanism of Heat Loss: Earth loses heat primarily through the emission of longwave infrared radiation into space, as well as through conduction, convection, and water evaporation/transpiration.
- Significance: This radiative cooling balances incoming solar radiation, maintaining Earth's stable global temperature equilibrium suitable for life.
Question 12
If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?
If Earth were a flat disc facing the Sun directly:
- Solar radiation would hit the entire surface uniformly at the same angle all year round.
- There would be no equator-to-pole temperature gradients, no distinct climate zones, no seasonal variations, and no differential heating to drive global winds and ocean currents.
Question 13
Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?
- Cryosphere: Accelerated melting of glaciers, ice sheets, and permafrost, reducing global ice cover.
- Hydrosphere: Thermal expansion of seawater and glacial meltwater lead to rising sea levels, coastal flooding, and altered ocean salinity.
- Biosphere: Habitat loss, coral bleaching, species extinction, migration shifts, and disruptions in agricultural crop yields.
Question 14
Explain how the Earth's atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.
The atmosphere acts as an insulating blanket:
- It prevents sudden temperature extremes between day and night by absorbing and trapping outgoing infrared radiation (Greenhouse effect).
- The ozone layer shields the surface from harmful ultraviolet radiation, while atmospheric circulation redistributes heat globally between the equator and poles.
Question 15
Describe the interrelationship between different spheres of the Earth. Illustrate with example how these spheres function in a delicate balance.
- Earth's Four Spheres: Lithosphere (land), Hydrosphere (water), Atmosphere (air), and Biosphere (living organisms).
- Interrelationship & Balance Example: Plants (Biosphere) draw water from the soil (Lithosphere/Hydrosphere), absorb carbon dioxide from the air (Atmosphere), and release oxygen. Deforestation (altering the biosphere) disrupts atmospheric $\text{CO}_2$, which alters climate, melting ice (Cryosphere) and causing sea-level rise (Hydrosphere), proving that all spheres are interconnected in a delicate balance.
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Based on latest NCERT class 9 book.