NCERT Solutions Class 9 Science Ch 13 Earth System 2026-27

Monsoon winds are driven by the difference in temperature (uneven heating) between the land and the sea. When the Arabian Sea becomes warmer:

• evaporation increases, so more moisture goes into the air;
• moisture-laden winds blow towards the Indian subcontinent;
• this can bring heavier and more intense rainfall, for example on the west coast of India.

However, too much warming can also disturb the normal wind patterns, which makes the monsoon irregular and hard to predict.

• Less infiltration: without trees and roots less rainwater seeps into the ground and more flows over the surface.
• More runoff: water reaches the river quickly and causes sudden floods in heavy rain.
• Soil erosion: soil is washed into the river, which gets silted up and becomes shallow.
• Irregular flow: too much water in the rains and too little in the dry months.
• Less groundwater recharge: this reduces the flow of the river in the dry season.

The sea level would rise, and coastal cities such as Mumbai, Chennai and Kolkata would be badly affected:

• Flooding of coastal areas, especially during high tides and storms.
• Permanent submergence of some low-lying areas.
• Damage to roads, buildings and ports.
• Saltwater intrusion into freshwater sources, which makes drinking water scarce and harms farming.
• Displacement of many people who live near the coast.

• More CO₂ dissolves in sea water and forms carbonic acid, which lowers the pH of the ocean (ocean acidification).
• Plankton that make calcium carbonate shells find it harder to build and keep their shells.
• Many plankton species may decrease in number or become weak.
• Plankton are the base of the marine food chain, so a decline affects fish and other marine animals.

• Reduces heat stress on crops, so plants recover from the daytime heat and grow well.
• Keeps the soil moist: lower temperatures reduce evaporation, so the soil holds water longer.
• Prevents wilting and drying of crops.
• Improves air circulation, which helps gas exchange for photosynthesis and respiration.
• Cooler, well-ventilated conditions can limit some pests and fungal growth.

• Less CO₂ absorbed: warmer water holds less CO₂, so more stays in the atmosphere and climate change gets worse.
• Changes in acidity: changes in the dissolved CO₂ can make the water acidic, which harms marine organisms.
• Damage to shell-forming organisms like plankton, corals and shellfish.
• Disturbed food chains, because the decline of plankton affects fish and larger marine animals.

• Nutrients: without the nitrogen and carbon cycles, nutrients would not be recycled, so plants would stop growing and die.
• Oxygen and carbon dioxide: oxygen would not be replaced and CO₂ would build up, so animals could not breathe.
• Water: if the water cycle stopped, there would be no rain, leading to drought and the death of plants and animals.

• Food chains would collapse: plants (producers) would not make food, herbivores would starve first and then carnivores.
• Oxygen would fall: photosynthesis is the main source of oxygen, so most organisms could not survive.
• CO₂ would rise: plants would no longer use it, so it would build up in the air and cause extreme warming.

How human activities add greenhouse gases:

• Burning fossil fuels (coal, petrol, diesel) in vehicles, industries and power plants releases CO₂.
• Deforestation: fewer trees absorb CO₂, and burning trees releases the stored carbon.
• Agriculture and livestock: cattle and rice fields produce methane (CH₄).
• Industries release CO₂ and other gases.
• Waste and landfills: decaying garbage releases methane.

What I can do as an individual: use public transport, cycling or walking; save electricity; plant and protect trees; reduce, reuse and recycle waste; and avoid burning garbage and wood.

1. FormulaEnergy = intensity × area × time
2. Convert the units1 kW m⁻² = 1000 J s⁻¹ m⁻² and 1 hour = 3600 s
3. CalculationE = 1000 × 1 × 3600 = 3,600,000 J = 3.6 × 10⁶ J

The energy received is 3.6 × 10⁶ J.

This is equal to 1 kilowatt-hour (kWh), that is, one “unit” of electricity on a household electricity bill.

What happens: warm surface water moves towards the poles as ocean currents. At higher latitudes it loses heat and becomes cooler. The cooled water becomes denser and sinks into the deeper layers of the ocean.

Impact:

• It warms nearby coastal regions, so they have a milder climate than expected.
• It spreads heat from the equator to colder regions and helps regulate the global temperature.
• It affects weather patterns, such as winds, rainfall and storms.
• It circulates nutrients, which supports marine life.

Correct option: (a)

Biogeochemical cycles (carbon, nitrogen, water and others) circulate nutrients between living organisms (biotic) and the environment (abiotic: air, water and soil).

Correct option: (d)

• Sunlight is absorbed by the land and oceans.
• The Earth sends this energy out again as heat (infrared radiation).
• Greenhouse gases (like CO₂ and methane) trap this heat and warm the Earth.

• More evaporation: rising temperatures make more water evaporate from oceans, rivers and soil.
• Changed rainfall: some areas get heavy rain and floods, while others face droughts.
• Melting glaciers: glaciers melt faster, which increases river flow at first but reduces the long-term availability of water.

Albedo is the measure of how much sunlight a surface reflects.

• High albedo (ice, snow, clouds) reflects most sunlight back into space, so less heat is absorbed and it is cooler.
• Low albedo (oceans, forests, dark soil) absorbs more sunlight, so more heat is kept and it is warmer.

Effect on the climate:

• Polar regions stay cold because ice and snow reflect the sunlight.
• Warming feedback: when ice melts, it exposes darker water or land that absorbs more heat, which causes more melting and more warming.
• Regions with low albedo (deserts, cities) tend to be hotter.

They are formed by the uneven heating and cooling of the mountain slopes.

• Valley breeze (day): the slopes heat up faster, the warm air rises and cooler air from the valley moves up the slope.
• Mountain breeze (night): the slopes cool quickly, and the cold, dense air flows down into the valley.

Yes, the temperatures differ. Barren rock heats up and cools down faster, so the breezes there are warmer in the day and colder at night. A grass-covered mountain heats and cools more slowly, so its breezes are milder.

The troposphere is mainly responsible for weather.

• It contains most of the air and the water vapour of the atmosphere.
• It is heated unevenly by the Earth’s surface, which causes air movement and the formation of winds, clouds and rain.

Processes in the nitrogen cycle:

1. Nitrogen fixation: bacteria change nitrogen of the air into usable forms.
2. Nitrification and assimilation: nitrates are formed and absorbed by plants, and animals get nitrogen by eating plants.
3. Denitrification: nitrates are changed back into nitrogen gas.

If nitrogen were not cycled: the soil would lose its fertility and plants could not grow; proteins and DNA could not be formed; and life on Earth would not survive.

On the oxygen and carbon cycles:

• Less photosynthesis: fewer trees absorb less CO₂ and release less oxygen.
• More CO₂: cutting and burning trees release stored carbon, which adds to global warming.

Other consequences: soil erosion and loss of fertility, loss of biodiversity as animals and plants lose their habitat, and a disturbed water cycle with floods and droughts.

1. Photosynthesis: plants take in CO₂ from the atmosphere and change it into food (organic carbon).
2. Food chain: animals eat the plants, so the carbon moves into animals.
3. Respiration: plants (including their roots) and animals release CO₂ back into the air.
4. Decomposition: decomposers break down dead organisms and waste products and release CO₂.
5. Burning: burning of fuels and wood (vehicle and factory emissions) releases CO₂.

Plants need CO₂ for photosynthesis, but an excess of it strengthens the greenhouse effect and traps more heat in the atmosphere. This leads to global warming and climate change, with melting glaciers, rising sea levels and extreme weather. It also upsets the balance of gases in the atmosphere and can harm ecosystems.

How heat is lost: mainly by radiation in the form of infrared waves, and also by conduction and convection to the surrounding air.

Significance:

• It keeps a balance in the Earth’s temperature.
• It prevents the surface from getting too hot.
• The uneven heating drives the winds and weather patterns.

• More uniform solar radiation: sunlight would fall more evenly on most places instead of spreading over a curved surface, so there would be less variation in heating.
• No clear climate zones: there would be little difference between equatorial, temperate and polar regions.
• Smaller temperature differences: with little difference in the angle of sunlight, winds and ocean currents would be weaker.

• It lets sunlight reach the Earth and warm its surface.
• It traps some of the outgoing heat with greenhouse gases (like CO₂ and water vapour), which prevents the Earth from getting too cold.
• It spreads heat by winds and air circulation, which reduces extreme temperature differences.

The Earth has several connected spheres: the lithosphere (land), hydrosphere (water), cryosphere (ice), atmosphere (air) and biosphere (living things). They exchange matter and energy all the time (water, gases and nutrients), and a change in one sphere affects the others. This keeps a delicate balance.

Example: plants (biosphere) take CO₂ from the atmosphere and water from the hydrosphere to make food, and release oxygen into the atmosphere. The soil (lithosphere) gives them the nutrients.