Science: Class 9 — Chapter 13: Earth as a System: Energy, Matter, and Life

The Earth functions as a single, integrated system powered by solar energy and internal heat, composed of five interacting subsystems:

• Geosphere: Solid rocks, soil, landforms (e.g., Deccan plateau, Thar desert), and the Earth's interior.
• Hydrosphere: Liquid water including oceans, rivers (e.g., Ganga-Brahmaputra system), lakes, and groundwater.
• Cryosphere: Solid water including glaciers, ice caps, and snow (e.g., Himalayan glaciers, Ladakh).
• Atmosphere: The gaseous envelope surrounding the Earth, held in place by gravity.
• Biosphere: All living organisms and their habitats (e.g., forests, coral reefs, plankton).

• A disturbance in one sphere propagates changes across others.
• Example: Reduced winter snowfall (cryosphere) leads to lower summer lake levels (hydrosphere), reducing grass growth (biosphere) and impacting livestock.
• Example: Warmer ocean water (hydrosphere) increases evaporation, causing monsoon fluctuations and flooding/droughts (atmosphere & biosphere).

• Solar energy travels through space as electromagnetic (EM) waves at the speed of light ($3 \times 10^8 \text{ ms}^{-1}$).
• About 99% of solar radiation reaching Earth is concentrated in the ultraviolet (UV), visible, and infrared (IR) ranges.
• Insolation: The amount of solar radiation reaching the Earth's surface.
• Solar Constant: The average solar energy received per unit area per unit time perpendicular to the Sun's rays at the top of the atmosphere. Value $\approx 1.4 \text{ kWm}^{-2}$ (or $1400 \text{ J s}^{-1}\text{m}^{-2}$).
• Under clear skies, maximum insolation reaching the surface is lower, around $1 \text{ kWm}^{-2}$.

• Albedo: The fraction of solar radiation reflected by a surface.
• High albedo surfaces (e.g., snow: 0.80–0.90, ice: 0.50–0.70) reflect most light and stay cool.
• Low albedo surfaces (e.g., black soil, ocean water) absorb more radiation and warm up.
• Urban Heat Island Effect: Cities are warmer than rural areas because concrete, asphalt, and steel absorb and retain more heat, coupled with less vegetation.

• Because the Earth is spherical, solar rays strike different latitudes at different angles.
• Equatorial regions receive concentrated, direct rays (warmer), while polar regions receive spread-out, oblique rays (colder).
• This temperature gradient drives global winds and ocean currents.

• Troposphere (0–12 km): Contains nearly all weather phenomena. Temperature decreases with altitude (approx. $6.5^\circ\text{C/km}$).
• Stratosphere (12–50 km): Contains the ozone layer, which absorbs harmful UV radiation, causing temperature to increase with altitude.
• Greenhouse Effect: Greenhouse gases ($CO_2$, $CH_4$, water vapour) trap outgoing infrared radiation, keeping Earth warm enough to support life.

• Valley Breeze: During the day, mountain slopes heat faster than the valley floor. Warm air rises up the slopes, drawing cooler air up from the valley.
• Mountain Breeze: At night, slopes cool faster. Cool, dense air sinks down into the valley.

• Uneven heating creates global pressure belts: equatorial low-pressure and sub-tropical high-pressure (around $30^\circ$ N/S).
• Air flows from high to low pressure, forming planetary winds.
• Winds are deflected due to Earth's rotation (Coriolis effect): to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• Continuous movements of large masses of ocean water driven by winds, temperature/salinity differences, and Earth's rotation.
• Gyres: Large circular current patterns (clockwise in NH, counter-clockwise in SH).
• Climate Regulation: Currents transport heat from the equator to the poles. E.g., the North Atlantic Drift keeps northwestern European ports ice-free in winter.

Involves evaporation, transpiration, condensation, precipitation, infiltration, and groundwater flow, linking all five Earth spheres.

• Fast Cycle: Plants convert atmospheric $CO_2$ into glucose via photosynthesis; respiration and decomposition release it back.
• Slow Cycle: Carbon is stored in fossil fuels, rocks, and deep ocean sediments over millions of years.
• Human activities have raised atmospheric $CO_2$ from 315 ppm (1960) to 420 ppm (2025), intensifying global warming.

• Nitrogen Fixation: Atmospheric $N_2$ is converted to ammonia ($NH_3$) by bacteria (e.g., Rhizobium, Azotobacter) or lightning.
• Nitrification: Ammonia is converted to nitrites ($NO_2^-$) and nitrates ($NO_3^-$) by nitrifying bacteria (e.g., Nitrosomonas, Nitrobacter).
• Assimilation: Plants absorb nitrogen compounds to make proteins.
• Ammonification: Decomposers return nitrogen to the soil as ammonia.
• Denitrification: Anaerobic bacteria (e.g., Pseudomonas) convert nitrates back into $N_2$ gas.

Maintained by a balance between oxygen-consuming processes (respiration, combustion) and oxygen-producing processes (photosynthesis).

• Ocean Acidification: Excess atmospheric $CO_2$ dissolves in oceans, forming carbonic acid, which threatens plankton and coral reefs.
• Eutrophication: Fertilizer runoff adds excess nitrogen to water bodies, causing rapid algal growth (algal blooms) that depletes oxygen and kills aquatic life.
• Deforestation: Reduces photosynthesis and transpiration, leading to local rainfall decline, increased soil erosion, and altered surface albedo.

1. What is the primary role of biogeochemical cycles in an ecosystem?
   Answer: (ii) To recycle essential nutrients between biotic and abiotic components.
2. Which of the following is primarily responsible for warming of the Earth?
   Answer: (iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.