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

Answer: (ii) To recycle essential nutrients between biotic and abiotic components.

Cycles like the carbon, nitrogen and water cycles move matter between living things, air, water and soil again and again.

Answer: (iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.

This is the greenhouse effect. Gases like CO₂, methane and water vapour trap the heat re-radiated by the surface.

Climate change raises the temperature of the Earth. This speeds up and disturbs the water cycle.

• More evaporation: warm air holds more water vapour. This leads to very heavy rainfall and floods in some places, like cloudbursts in hilly areas.
• Droughts: some regions get less rain and soil dries faster, causing water shortage.
• Melting of glaciers: Himalayan glaciers melt faster. Rivers first get more water, but later the flow may decrease.
• Irregular monsoon: warming of the Arabian Sea can change the timing and amount of monsoon rain.

Albedo is the fraction of solar radiation reflected by a surface.

• High albedo surfaces (snow, ice) reflect most sunlight. They stay cool. This is why polar regions are very cold.
• Low albedo surfaces (black soil, ocean water, forests, roads) absorb more sunlight. They become warmer.

When ice melts due to warming, dark water or land is exposed. It absorbs more heat, which causes more melting. So albedo strongly affects local and global climate.

Valley breeze (day): the sunny mountain slopes heat up faster than the valley floor. Warm air over the slopes rises. Cooler air from the valley moves up the slopes.

Mountain breeze (night): after sunset, the slopes cool faster. The cool, dense air flows down into the valley.

Yes, the temperatures would differ. Barren rocks heat up a lot by day but also lose heat quickly at night. So the mountain breeze from the rocky mountain would be colder. The grass-covered mountain changes temperature more slowly because of moisture and transpiration. So its mountain breeze would be milder and more moist.

The troposphere, the lowest layer of the atmosphere.

It contains almost all the water vapour and most of the air. It is heated unevenly by the Earth’s surface. This causes air to rise, cool and move, which forms winds, clouds, rain and storms.

If nitrogen were not cycled: plants could not get usable nitrogen. They could not make proteins and DNA. Plant growth would stop, and animals would not get food. Life on Earth would not be able to continue.

Oxygen and carbon cycles:

• Fewer trees means less photosynthesis. Less CO₂ is absorbed and less oxygen is released.
• Burning or decay of cut trees releases stored carbon as CO₂. This adds to global warming.

Other consequences: soil erosion, floods and landslides, less rainfall, drying of rivers and springs, loss of habitat and biodiversity, and man-animal conflict.

1. Plants take in CO₂ from the air and make food by photosynthesis.
2. Plants release some CO₂ back by respiration.
3. Animals eat plants. Carbon moves into their bodies. They release CO₂ by respiration.
4. When plants and animals die, decomposers break them down and release CO₂.
5. Some dead remains get buried for millions of years and form fossil fuels. Burning these fuels releases CO₂.

CO₂ is a greenhouse gas. Too much of it traps more heat and increases the greenhouse effect. This causes global warming. Melting glaciers, rising sea levels, extreme weather and changes in rainfall follow. Excess CO₂ also dissolves in oceans and harms marine life. Plants cannot absorb all the extra CO₂.

The Earth’s surface loses heat mainly by re-radiating it as infrared radiation. Heat is also carried away by warm air rising (convection) and by evaporation of water.

Significance: it balances the heat received from the Sun. Without heat loss, the Earth would keep getting hotter. A part of this heat is trapped by greenhouse gases, which keeps the Earth warm enough for life.

On a flat disc facing the Sun, the rays would fall at nearly the same angle everywhere. All places would get almost equal heat. There would be no hot equator and cold poles.

So temperature differences across the Earth would be very small. Without uneven heating, global winds and ocean currents would be very weak, and climate zones would not form as they do now.

Cryosphere: glaciers, snow and polar ice would melt faster.

Hydrosphere: sea level would rise. Evaporation would increase, causing heavy rain in some places and droughts in others. Oceans would become warmer.

Biosphere: many plants and animals would lose their habitats. Coral reefs may die. Crop yields may fall, and diseases may spread to new areas.

• Greenhouse gases trap part of the heat re-radiated by the Earth. This keeps the Earth warm, especially at night.
• The ozone layer absorbs harmful UV rays.
• Clouds reflect some sunlight during the day and prevent overheating.
• Winds carry heat from hot regions to cold regions.

So the atmosphere acts like a blanket. It prevents very hot days and very cold nights.

The Earth has five interacting spheres: geosphere (rocks, soil), hydrosphere (liquid water), cryosphere (ice and snow), atmosphere (air) and biosphere (living things). A change in one sphere affects the others.

Example — a forest near a river:

• Trees (biosphere) take water from soil (geosphere) and release water vapour into the air (atmosphere).
• This vapour forms clouds and falls as rain into rivers (hydrosphere).
• Snow on mountains (cryosphere) melts and feeds the river in summer.
• Roots hold the soil and prevent erosion.

If the forest is cleared, rainfall drops, soil is washed away and the river flow changes. This shows the delicate balance between the spheres.