NCERT Class 9 Science Exploration Chapter 13 Earth as a System Energy Matter and Life PDF Download

Chapter 13: Earth as a System: Energy, Matter, and Life

Think It Over

• How does the warming of Arabian Sea water affect the southwest monsoon in India?
• If a large forest is cleared, how can that affect the flow of a river in that area?
• What might happen to coastal cities in India if glaciers and polar ice keep melting faster?
• How would increasing carbon dioxide levels in the atmosphere affect the ocean plankton?

Life on Earth is powered by a constant flow of energy and matter. The Sun is the main source of energy. In addition, the Earth's hot interior and chemical reactions in the air, water, and rocks also drive the flow of energy and matter. During the middle stage in Curiosity (Grades 6 - 8) and in earlier chapters of this textbook, you explored these ideas as separate content pieces. For example, you learnt how sunlight drives winds and the water cycle; how plants and microbes cycle nutrients; how the Earth's tilt causes seasons; and how human activities are modifying air, water, soil and climate. In this chapter, we will consider these processes together as belonging to one Earth system made up of interacting 'spheres' -

• Geosphere: Solid rocks, soil, landforms (like the Deccan plateau and the Thar desert), and the Earth's interior.
• Hydrosphere: Liquid water in the form of surface water, such as oceans, rivers (like the Ganga - Brahmaputra river system), lakes and groundwater.
• Cryosphere: Solid form of water, such as ice and snow (like the Himalayan glaciers, snow in Ladakh and polar ice caps).
• Atmosphere: The air surrounding the Earth that we breathe (cleaner air in the mountains and forests).
• Biosphere: All living organisms and their habitats (including mangroves, forests, farms, ocean plankton and coral reefs).

We will explore how energy and matter move, interact across the Earth's spheres, and how a change in one affects others. Understanding these processes has been an important part of the continuing journey of the exploration of science. Natural processes, such as heating by solar radiation, movement of air and water, and nutrient cycling connect these spheres in a delicate balance. Let us understand this by conducting Activity 13.1 and explore how different spheres on the Earth continuously interact with each other.

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.
2. How does snow (cryosphere) eventually become part of the lake (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?
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.

[Figure 13.1: Some features of the Earth's surface, See in your textbook]

From Activity 13.1, we can infer that a disturbance in one sphere can lead to changes in others. For example, less snowfall in winters may lead to less water in the lake in summers, resulting in less water to support the growth of grass. Similarly, on a large scale, warmer Arabian Sea water lead to more evaporation from the sea. This in turn, causes fluctuations in the southwest monsoon, which results in variability in rainfall, bringing floods to some regions of India while leaving others in drought. This disrupts the hydrosphere. At the same time, the rise in atmospheric temperature could eventually accelerate the melting of glaciers and polar ice in the cryosphere, which may lead to the flooding of low-lying regions, and in the long run, it can raise sea levels that may threaten coastal cities. This could disturb the ecosystems within the biosphere by causing a habitat loss.

Teacher's Note

Remember that the five spheres do not work in isolation. A change in one sphere (like less snow in the cryosphere) triggers a chain reaction through the others (less water in hydrosphere, less grass in biosphere). In exam questions about climate or weather, always trace how the spheres connect rather than treating them separately.

Does the solar radiation heat the Earth's surface evenly? Let us explore how solar radiation varies from the equator to the poles, the oceans to the mountains, and drives many of Earth's natural processes, such as wind, ocean current, water cycle, etc.

13.1 Uneven Heating of the Earth

Solar radiation is the main source of energy on the Earth. It reaches the Earth as electromagnetic (EM) waves that travel through a vacuum at the speed of light (unlike EM waves, sound waves, which you studied in Chapter 10, Sound Waves: Characteristics and Applications, are mechanical waves and require a medium to travel). The speed of light in vacuum is \( 3 \times 10^8 \) m s\(^{-1}\). EM waves cover a wide range of frequencies, from high frequency or short wavelength radiation (gamma rays and X-rays) to low frequency or long wavelength radiation (infrared and radio waves). The high frequency EM waves, such as gamma rays and X-rays have very high energy and can be harmful for life on Earth.

The entire range of electromagnetic radiation called electromagnetic spectrum is shown in Fig. 13.2. However, the solar radiation reaching Earth is concentrated mainly in the ultraviolet (UV), visible and infrared (IR) range - about 99 per cent of the Sun's energy falls within these wavelengths. These three regions of the spectrum shape the Earth's climate and support life. 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.

[Figure 13.2: Electromagnetic spectrum, See in your textbook]

Short wavelength UV radiation is mostly absorbed by the ozone layer in the upper atmosphere, protecting life and contributing to some atmospheric heating. Visible light from the Sun reaches the Earth's surface and provides energy for photosynthesis, which is the primary source of food for most organisms. It also partly warms the land and water. Infrared radiation warms the Earth's surface, which then re-radiates this heat back into the atmosphere. A portion of this outgoing heat is trapped by the greenhouse gases, such as carbon dioxide (\( \ce{CO2} \)), methane (\( \ce{CH4} \)) and water vapour, keeping the Earth warm enough to support life.

Ready to Go Beyond

The UV rays lie in the wavelength range of 100 nm to 400 nm [1 nanometre (nm) = \( 10^{-9} \) m] and have a much higher energy than the visible light. Prolonged exposure to these rays can damage the eyes and skin, and increase the risk of cancer. Therefore, people should use UV protective glasses and sunscreen if they are likely to be exposed to UV rays. UV rays are also useful in killing germs in water purifiers and help power fluorescent lights.

The amount of the Sun's radiation that reaches the Earth's surface is called insolation. It is responsible for warming the Earth's surface and its atmosphere. The average amount of solar energy received per unit time per unit area that is 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 kilowatts per square metre (1.4 kW m\(^{-2}\)), or about 1400 joules per second per square metre (1400 J s\(^{-1}\) m\(^{-2}\)). It represents the Sun's energy available on the Earth before any absorption, scattering or reflection occurs in the atmosphere.

Teacher's Note

Do not confuse the electromagnetic spectrum with just visible light. The spectrum includes many types of radiation - UV, infrared, X-rays, and others - all traveling at the speed of light but with different wavelengths and energy. Remember that longer wavelength means lower frequency and lower energy, so radio waves are the weakest and gamma rays are the strongest.

Key Points

• The Earth system is made up of five interacting spheres: geosphere, hydrosphere, cryosphere, atmosphere, and biosphere. A change in one sphere can trigger changes in all the others.
• Solar radiation is the main source of energy for Earth. About 99 per cent of the Sun's energy reaches Earth in the ultraviolet, visible, and infrared ranges.
• Electromagnetic waves vary in wavelength and frequency: shorter wavelengths have higher frequency and higher energy, while longer wavelengths have lower frequency and lower energy.
• Ultraviolet radiation is absorbed by the ozone layer, visible light drives photosynthesis, and infrared radiation warms the Earth's surface. Greenhouse gases trap some of the outgoing heat.

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