13. Short and Long Answer Questions: Earth as a System: Energy, Matter, and Life

Short Answer Type Questions

Ques1: What is insolation? What is the value of the solar constant and why is it significant? 

Ans: The amount of solar radiation that reaches the Earth’s surface is called insolation. It is responsible for warming the Earth’s surface and its atmosphere.

The average solar energy received per unit time per unit area, perpendicular to the Sun’s rays at the top of the Earth’s atmosphere, is called the solar constant. Its value is approximately $1.4{\text{ kWm}}^{-2}$ (or $1400{\text{ J s}}^{-1}{\text{m}}^{-2}$).

The solar constant is significant because it helps scientists understand the Earth’s energy balance, climate and weather patterns. The maximum insolation reaching the Earth’s surface is lower than the solar constant (about $1{\text{ kWm}}^{-2}$) because some energy is absorbed and scattered by the atmosphere before reaching the ground.

Ques 2: What is albedo? How does albedo of a surface affect the temperature of that region? Give one example. 

Ans: The fraction of solar radiation reflected by a surface is called its albedo (the word comes from the Latin word meaning ‘whiteness’).

High albedo surfaces stay cool because they reflect more light back into space, while low albedo surfaces heat up quickly because they absorb more solar radiation. This directly affects the temperature of that region – areas with high albedo remain cooler, while those with low albedo become relatively warmer.

For example, snow and ice have a very high albedo (0.80-0.90), meaning they reflect most of the incoming solar radiation, which is why polar regions remain very cold.

Ques 3: Explain what valley breeze and mountain breeze are. At what times of day do they occur? 

Ans: Valley breeze and mountain breeze are local winds formed due to uneven heating of the Earth’s surface in mountainous regions.

Valley breeze occurs during the day. The mountain slopes facing the Sun heat up faster than the valley floor. The warm air over the slopes rises, creating a low-pressure region. Cooler air from the valley moves up the slopes to replace the rising warm air. This upward flow of air is called valley breeze.

Mountain breeze occurs after sunset. The mountain slopes cool down faster than the valley floor. The cool, denser air over the slopes flows downward into the valley. This downward flow of cool air at night is called mountain breeze.

Ques 4: What are ocean currents? Name any two factors, apart from winds, that influence the movement of ocean water. 

Ans: Ocean currents are the continuous movement of large masses of ocean water. Similar to winds, planetary pressure differences also drive these currents. Strong planetary winds drag the surface water of the oceans because of friction, setting surface currents in motion.

Apart from winds, the two important factors that influence ocean water movement are:

1. Difference in temperature and salinity: Water with lower salinity is less dense and tends to remain near the surface, while higher salinity water sinks and moves at deeper levels. Similarly, warmer equatorial water travels on the surface towards the poles.
2. Rotation of the Earth: The Earth’s rotation deflects the moving water masses, forming large circular patterns called gyres, which rotate clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere.

Ques 5: What is a biogeochemical cycle? Name the four major biogeochemical cycles.

Ans: Living organisms constantly exchange matter and energy with the air, water, soil and rocks around them. The cyclic movement of matter and energy between the abiotic (non-living) and biotic (living) components of the Earth is called a biogeochemical cycle.

These cycles ensure that essential nutrients such as carbon, nitrogen and oxygen are recycled and remain available to support life on Earth. They also help ecosystems recover from disturbances and maintain environmental balance.

The four major biogeochemical cycles discussed in this chapter are: (i) the Water cycle, (ii) the Carbon cycle, (iii) the Nitrogen cycle, and (iv) the Oxygen cycle.

Ques 6: State the two roles played by the atmosphere in protecting life on Earth. 

Ans: The atmosphere plays two crucial roles in protecting life on Earth:

1. Absorbing incoming solar radiation: The ozone layer in the stratosphere blocks the harmful ultraviolet (UV) rays from the Sun. Clouds and other gases also absorb some sunlight before it reaches the surface of the Earth, preventing overheating and UV damage to living organisms.
2. Trapping outgoing heat (Greenhouse effect): The Earth’s surface absorbs sunlight and re-radiates it as infrared radiation. Greenhouse gases like ${\text{CO}}_2$, ${\text{CH}}_4$ and water vapour absorb this re-radiated heat, preventing it from escaping into space. Without this, the Earth would be too cold for life to survive.

Ques7: How does the Earth’s spherical shape and latitude lead to uneven heating of its surface? 

Ans: Because the Earth is spherical, the Sun’s rays strike different latitudes at different angles. At the equatorial region, the Sun’s radiation falls nearly perpendicular to the surface and is concentrated over a smaller area, so it heats the region strongly.

In contrast, at polar regions, the same amount of radiation is spread over a much larger area due to the angle at which it strikes the surface, resulting in much less heating. This is why equatorial regions remain relatively warm throughout the year, while polar regions experience much colder conditions.

This uneven heating creates large temperature differences between the equator and the poles, which is the primary driving force behind global winds and ocean currents.

Ques8: What is nitrification? Name the bacteria responsible for carrying out this process. 

Ans: Nitrification is the process by which ammonia (${\text{NH}}_3$) present in the soil is converted first into nitrite (${\text{NO}}_2^{-}$) and then into nitrate (${\text{NO}}_3^{-}$) by specific bacteria in the soil.

The bacteria responsible for this process are:

1. Nitrosomonas – converts ammonia into nitrite $({\text{NO}}_2^{-})$.
2. Nitrobacter – converts nitrite into nitrate $({\text{NO}}_3^{-})$.

Plants can then absorb these nitrate compounds from the soil and use them for synthesising proteins and nucleic acids. Nitrification is thus an important step in the nitrogen cycle that makes nitrogen available to plants in a usable form.

Ques 9: What is eutrophication? How does overuse of fertilisers in agriculture lead to it? 

Ans: Eutrophication is the process in which excessive nutrients, especially nitrates from fertilisers, enter water bodies such as rivers and lakes, causing widespread growth of algae (algal blooms). These algal blooms deplete oxygen levels in the water and kill fish, threatening water bodies and coastal fisheries.

When farmers overuse nitrogenous fertilisers in agriculture, the excess nitrogen is not absorbed by the crops and gets washed off into nearby rivers and lakes through rain and runoff. This large input of nutrients triggers rapid and uncontrolled algal growth.

As the algae die and decompose, bacteria use up the dissolved oxygen in the water. The resulting depletion of oxygen makes the water body unable to support aquatic life, severely disrupting the ecosystem.

Ques 10: How does climate change affect the water cycle? Give two examples.
​Ans: Climate change is significantly altering the water cycle by changing the way water evaporates, precipitates and flows across the Earth’s spheres.

1. Intensified monsoons and droughts: A warmer atmosphere holds more moisture. This causes heavier rains in some areas, such as intensified monsoons in India, while causing droughts in others where evaporation exceeds precipitation.
2. Melting glaciers and rising sea levels: Rising atmospheric temperatures accelerate the melting of glaciers and polar ice in the cryosphere. This adds more water to rivers and oceans, raising sea levels in the long run and threatening low-lying coastal cities such as Mumbai and Chennai.

Ques 11: Name the five interacting spheres of the Earth. Give one example of each. 

Ans: The Earth system is made up of five interacting spheres:

1. Geosphere: Solid rocks, soil and landforms – e.g., the Deccan plateau.
2. Hydrosphere: Liquid water in the form of surface water and groundwater – e.g., the Ganga-Brahmaputra river system.
3. Cryosphere: Solid form of water such as ice and snow – e.g., the Himalayan glaciers and polar ice caps.
4. Atmosphere: The air surrounding the Earth that we breathe – e.g., cleaner air found in mountains and forests.
5. Biosphere: All living organisms and their habitats – e.g., mangroves, forests and coral reefs.

Ques12: What is the urban heat island effect? What causes it? 

Ans: Cities are often warmer than their surrounding rural areas, especially during summer and at night. This phenomenon is known as the urban heat island effect.

It is caused primarily because cities have large built-up areas consisting of buildings made of steel, concrete and brick, and roads of concrete and asphalt. All these materials absorb solar radiation and retain heat. The heat re-radiated by these materials warms cities more than surrounding rural areas.

In contrast, rural areas and forests have more vegetation, which keeps them cool through shade and plant transpiration. The urban heat island effect shows how human land use can alter the local climate and increases the energy demand for air conditioning, further stressing urban ecosystems.

Long Answer Type Questions

Ques1: What is the electromagnetic spectrum? Explain how the different regions of the solar spectrum – ultraviolet, visible and infrared – interact with the Earth’s atmosphere and surface, and support life. 

Ans: The entire range of electromagnetic radiation, from very high frequency gamma rays and X-rays to low frequency infrared and radio waves, is called the electromagnetic spectrum. Solar radiation reaches the Earth as electromagnetic (EM) waves that travel through vacuum at the speed of light $(3\times 10^8{\text{ ms}}^{-1})$.

The solar radiation reaching Earth is concentrated mainly in three regions of the spectrum – ultraviolet (UV), visible and infrared (IR) – which together account for about 99% of the Sun’s energy. Each interacts with the Earth differently:

1. Ultraviolet (UV) radiation: Short wavelength UV radiation is mostly absorbed by the ozone layer in the upper stratosphere. This absorption protects living organisms from UV damage, which can cause cancer and harm ecosystems. It also contributes to some atmospheric heating. Without the ozone layer, life as we know it could not survive on land.
2. Visible light: Visible light from the Sun passes through the atmosphere and reaches the Earth’s surface. It provides energy for photosynthesis, which is the primary source of food for most organisms. It also partly warms the land and water surface.
3. Infrared (IR) radiation: Infrared radiation warms the Earth’s surface. The surface then re-radiates this heat back into the atmosphere. A portion of this outgoing heat is trapped by greenhouse gases such as carbon dioxide $({\text{CO}}_2)$, methane $({\text{CH}}_4)$ and water vapour, keeping the Earth warm enough to support life. This is known as the greenhouse effect.

High frequency EM waves such as gamma rays and X-rays are mostly filtered by the Earth’s upper atmosphere, while microwaves and radio waves carry very little energy to significantly warm the Earth. Thus, the three principal regions of the solar spectrum shape the Earth’s climate and make it habitable.

Ques2: Explain the formation of planetary winds. How does the Earth’s rotation affect the path of these winds? How do planetary winds drive ocean currents, and what is the significance of ocean currents for climate? ​

Ans: Planetary winds are large-scale winds that result from the uneven heating of the Earth between the equator and the poles, creating belts of high and low pressure across the globe.

Formation of planetary winds:

1. Near the equator, intense solar heating causes warm air to rise, forming an equatorial low pressure belt. As this warm air rises, it moves poleward at higher altitudes.
2. On cooling, this air becomes denser and sinks around $30\mathrm{°}$ North and South latitudes, forming sub-tropical high pressure belts. Some of this air flows back to the equator along the surface, completing one circulation cycle.
3. The remaining sinking air at the sub-tropical belt moves poleward along the surface and rises again around $60\mathrm{°}$ North and South latitudes, where it meets cold air from the polar regions. This creates sub-polar low pressure belts.
4. In polar regions, very low temperatures cause dense cold air to sink, forming polar high pressure belts. Air from these regions flows towards the sub-polar belts, completing another circulation cycle.

Effect of Earth’s rotation (Coriolis Effect): The Earth’s rotation causes winds to be deflected from their straight paths. In the Northern Hemisphere, winds are deflected towards the right, while in the Southern Hemisphere they are deflected towards the left. As a result, planetary winds follow curved paths rather than moving directly from high to low pressure areas.

Planetary winds and ocean currents: Strong planetary winds drag the surface water of the oceans through friction, setting surface ocean currents in motion. The Earth’s rotation deflects these water masses, forming large circular patterns called gyres.

Significance of ocean currents: Ocean currents play a major role in regulating the Earth’s climate by transporting heat from the equator towards the poles, reducing temperature differences across the planet. For example, the North Atlantic Drift keeps the ports of northwestern Europe ice-free during winter even at high latitudes. Ocean currents also support massive ecosystems by transporting nutrients.

Ques3: Describe the nitrogen cycle in detail. Name the key bacteria involved at each step and explain what would happen if nitrogen were not cycled back into the atmosphere. 
Ans: Nitrogen is an essential element for the synthesis of proteins and nucleic acids in all living organisms. Though the largest reservoir of nitrogen is in the atmosphere $({\text{N}}_2$ gas = about 78%), it cannot be directly used by plants and animals. It must first be converted to soluble compounds. The overall movement of nitrogen between air, soil, water and organisms is called the nitrogen cycle.

The nitrogen cycle involves the following steps:

1. Nitrogen Fixation: Atmospheric ${\text{N}}_2$ is converted into ammonia $({\text{NH}}_3)$ by nitrogen-fixing bacteria. Rhizobium is found in the root nodules of legumes and Azotobacter is found free in the soil. Lightning also contributes by fixing nitrogen into nitrogen oxides.
2. Nitrification: Ammonia in the soil is converted first into nitrite $({\text{NO}}_2^{-})$ by Nitrosomonas bacteria, and then into nitrate $({\text{NO}}_3^{-})$ by Nitrobacter bacteria. Plants absorb nitrates from the soil and use them to build proteins.
3. Assimilation: Plants assimilate nitrate compounds from the soil and convert them into organic molecules like proteins. Animals obtain nitrogen by consuming plants or other animals.
4. Ammonification: When plants and animals die or produce waste, decomposers like bacteria and fungi break down the organic matter, returning nitrogen compounds as ammonia to the soil. This process is known as ammonification.
5. Denitrification: Denitrifying bacteria like Pseudomonas convert some nitrates back into nitrogen gas $({\text{N}}_2)$, which is released back into the atmosphere, completing the cycle.

If nitrogen were not cycled: If nitrogen were not continuously recycled, the soil would quickly become depleted of usable nitrogen compounds. Plants would be unable to synthesise proteins and nucleic acids, leading to stunted growth and eventually death. Since animals depend on plants for nitrogen, animal life would also fail. The entire biosphere would collapse as protein synthesis – fundamental to all life – would cease. The atmosphere would accumulate unusable ${\text{N}}_2$ while living systems would suffer from severe nitrogen deficiency.

Ques 4: What is the carbon cycle? Explain the fast and slow cycles of carbon. How have human activities disrupted the carbon cycle and what are its consequences for the Earth? ​

Ans: Carbon forms the backbone of life. Every protein, carbohydrate, fat and DNA molecule contains carbon. The carbon cycle is the continuous circulation of carbon between the atmosphere $({\text{CO}}_2$ gas), biosphere (plants and animals), geosphere (carbonate rocks and fossil fuels like coal and oil) and hydrosphere (dissolved ${\text{CO}}_2$ and marine shells).

Fast carbon cycle (days to years):

1. Plants absorb atmospheric ${\text{CO}}_2$ through photosynthesis and convert it into glucose.
2. ${\text{CO}}_2$ is released back into the atmosphere through the respiration of plants, animals and decomposers.
3. When organisms die, decomposition also releases ${\text{CO}}_2$. The ocean continuously exchanges ${\text{CO}}_2$ with the atmosphere.

Slow carbon cycle (millions of years):

1. Dead plants and animals get buried over millions of years and are converted into fossil fuels such as coal, oil and gas.
2. These fuels, when burnt, release carbon back as ${\text{CO}}_2$ on a very short time scale.
3. The ocean water absorbs atmospheric ${\text{CO}}_2$ to form carbonate and bicarbonate ions; marine organisms use these to form shells, which eventually settle to the ocean floor as carbonate rock.

Human disruption of the carbon cycle: Human activities like burning fossil fuels and deforestation have raised atmospheric ${\text{CO}}_2$ by about 35% since 1960 (from 315 ppm to 420 ppm) – an unprecedented rise in human history.

Consequences:

1. Excess ${\text{CO}}_2$ intensifies the greenhouse effect, causing global warming, melting of glaciers, rising sea levels and more extreme weather conditions.
2. Increased ${\text{CO}}_2$ dissolved in the ocean makes seawater more acidic, threatening tiny ocean plankton and coral reefs.
3. In India, warmer air holds more moisture, leading to more intense monsoons and threats to agriculture from changing rainfall patterns.

Ques 5: How do human activities impact the five spheres of the Earth? Discuss any three specific impacts and suggest measures that can help restore the balance of the Earth’s natural systems. ​

Ans: Human activities have significantly disrupted the delicate balance among the Earth’s five interacting spheres – the geosphere, hydrosphere, cryosphere, atmosphere and biosphere. Three major impacts are discussed below:

1. Burning of fossil fuels (Atmosphere and Hydrosphere): Burning fossil fuels releases large amounts of ${\text{CO}}_2$ and other greenhouse gases into the atmosphere. This intensifies the greenhouse effect, causing global warming. Excess atmospheric ${\text{CO}}_2$ is also absorbed by the oceans, making seawater more acidic (ocean acidification), which threatens marine ecosystems including coral reefs and plankton. Vehicular emissions also react with sunlight to form ground-level smog and ozone, making city air unhealthy.
2. Deforestation (Biosphere, Geosphere and Atmosphere): Clearing forests reduces photosynthesis and transpiration, which can lead to a decline in local rainfall. It alters the surface albedo. Without tree roots to hold soil together, soil erosion increases. Habitats are destroyed, leading to biodiversity loss as many species lose their natural homes. Deforestation also reduces the Earth’s capacity to absorb ${\text{CO}}_2$, thereby accelerating the carbon cycle disruption.
3. Overuse of fertilisers (Hydrosphere and Biosphere): Excessive nitrogen from fertilisers in agriculture is washed off into rivers and lakes, causing eutrophication – rapid growth of algae (algal blooms) that deplete oxygen and kill fish. This threatens freshwater bodies and coastal fisheries, disrupting the nitrogen cycle and harming aquatic biodiversity.

Measures to restore balance:

1. Switching to renewable energy sources such as solar and wind power to reduce fossil fuel burning and lower ${\text{CO}}_2$ emissions.
2. Planting trees on a large scale to restore forests, enhance carbon absorption and reduce soil erosion – India has already planted billions of trees as part of this effort.
3. Practising sustainable farming by using organic methods and reducing fertiliser overuse to prevent eutrophication and soil degradation.
4. Individuals can contribute by saving water, food and energy, reducing waste, reusing and recycling materials to minimise their environmental footprint.

Together, a combination of local actions and global cooperation – as seen in the Montreal Protocol (which helped the ozone layer recover) – can help maintain the environmental balance on Earth.