Earth as a System : Energy, Matter, and Life — NCERT Solutions

Albedo is the fraction of solar radiation reflected by a surface. Surfaces with high albedo like snow and ice reflect more sunlight, so they stay cooler. Surfaces with low albedo like black soil and ocean water absorb more sunlight, so they heat up more. This affects climate because bright icy regions remain cold, while darker surfaces warm more quickly.

During the day, mountain slopes heat faster than the valley floor. Warm air over the slopes rises, and cooler air from the valley moves up to replace it; this is a valley breeze. After sunset, the slopes cool faster than the valley floor. Cooler, denser air flows down into the valley; this is a mountain breeze.

Yes, the breezes could differ in temperature. A mountain with grass would stay relatively cooler because vegetation shades the ground and cools it by transpiration. A barren rocky mountain would heat up and cool down faster, so the breeze there may be warmer in the day and colder at night compared with the grassy mountain.

Such weather phenomena mainly occur in the troposphere. This is because the troposphere is heated from the Earth’s surface, and its temperature generally decreases with height. Warm air rises, creating convection, which drives winds, storms and rainfall.

The nitrogen cycle includes nitrogen fixation, assimilation, ammonification, nitrification and denitrification. Atmospheric nitrogen is fixed by bacteria like Rhizobium and Azotobacter into ammonia. Nitrifying bacteria convert ammonia into nitrite and then nitrate. Plants absorb these compounds, animals get nitrogen by eating plants or other animals, decomposers return ammonia to the soil, and denitrifying bacteria convert some nitrates back to nitrogen gas.

If nitrogen were not cycled, plants would not get usable nitrogen to make proteins and nucleic acids. Then animals would also lack food and essential nutrients, and life on Earth would be severely affected.

Deforestation reduces photosynthesis, so less oxygen is produced. It also reduces the absorption of carbon dioxide, disturbing the carbon cycle and increasing atmospheric CO₂. Other consequences include reduced transpiration, decline in local rainfall, increased soil erosion, altered albedo, habitat loss, and a decline in biodiversity.

Carbon returns to the atmosphere through several paths:

Atmospheric CO₂ → plants (photosynthesis) → animals eat plants → respiration by plants and animals releases CO₂

Also:

• Dead plants and animals → decomposition → CO₂ released to air
• Dead organic matter buried for millions of years → fossil fuels → burning of fuels → CO₂ released to air
• Ocean and atmosphere exchange CO₂ continuously, so some carbon also returns from ocean water to the atmosphere.

Although CO₂ is needed by plants for photosynthesis, too much of it is harmful because it intensifies the greenhouse effect, causing global warming. This can lead to melting of glaciers, rising sea level, extreme weather, and other climate problems. So the amount must stay in balance.

Heat is lost from the Earth’s surface by re-radiation in the infrared region. The surface absorbs sunlight and then sends heat back upward. Greenhouse gases absorb part of this outgoing heat and trap it. This is significant because it keeps the Earth warm enough for life; without the atmosphere, the Earth would be too cold for life to survive.

If the Earth were a flat disc, the Sun’s rays would not fall at different angles as they do on a spherical Earth. In that case, solar radiation would be distributed more nearly evenly across the surface, so the clear pattern of uneven heating between the equator and the poles would not occur in the same way. On the present spherical Earth, the equator receives more concentrated sunlight and the poles receive it spread over a larger area, causing temperature differences. A flat disc would therefore have less variation in temperature by latitude and the present pattern of warm equatorial regions and cold polar regions would be greatly changed.

A rise in atmospheric temperature would affect all three spheres:

• Cryosphere: Glaciers, snow and polar ice would melt faster.
• Hydrosphere: More melting would add water to rivers and oceans, raising sea level and increasing the risk of flooding in low-lying areas.
• Biosphere: Rising sea level and changing water availability would cause habitat loss, disturb ecosystems, and threaten plants, animals and human life.

So, warming of the atmosphere can disturb the balance between ice, water and living organisms.

The atmosphere helps maintain a suitable temperature for life in two main ways:

1. It partly absorbs incoming solar radiation. The ozone layer blocks harmful UV rays, and clouds and gases absorb some sunlight before it reaches the surface.
2. It traps outgoing heat. The Earth’s surface absorbs sunlight and re-radiates it as infrared radiation. Greenhouse gases such as carbon dioxide, methane and water vapour absorb some of this heat and prevent it from escaping too quickly into space.

Because of this, the Earth stays warm enough for life. Without the atmosphere, the Earth would be too cold for living organisms to survive.

The different spheres of the Earth—geosphere, hydrosphere, cryosphere, atmosphere and biosphere—are closely interconnected. A change in one sphere can affect the others.

For example, if snowfall decreases in winter, the cryosphere provides less meltwater. This reduces the water in lakes and rivers in the hydrosphere. Less water then means less moisture for plants, so the biosphere is affected because grass and other vegetation grow less. In a larger example, warmer Arabian Sea water increases evaporation, which can affect the southwest monsoon in the atmosphere, leading to floods in some regions and drought in others, thereby disturbing the hydrosphere and the biosphere.

Thus, Earth functions as a delicate system in which the spheres continuously interact and depend on one another.

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

The correct option is (iii).

If one Earth-sized planet were covered entirely by oceans and the other entirely by land, the wind patterns would still be driven by the same basic cause: uneven heating of the equator and poles.

• On the ocean-covered planet, winds would be influenced strongly by the large heat capacity of water. Heating and cooling would be more gradual, so winds would be somewhat more uniform and less sharply changed by day-to-day land effects.
• On the land-covered planet, land would heat up and cool down much faster, so pressure differences would become stronger and more quickly changing. This would produce stronger local temperature contrasts and more variable winds.

Compared with Earth, which has both land and sea, these planets would lack the land–sea breeze system we observe on Earth. But both would still have large-scale circulation caused by the equator being warmer than the poles, so planetary winds would still form. The exact wind patterns would depend on how quickly the surface stores and releases heat.

New tools that observe the planet in real time can help scientists discover more about how climate, ecosystems and human activity are linked. They may reveal:

• how rapidly glaciers, sea ice and rainfall patterns are changing,
• how oceans, forests and soils act as carbon sinks,
• how urbanisation, deforestation and pollution affect local and global climate,
• how weather extremes such as intense monsoons, droughts and heat waves are becoming more common,
• and how living systems respond to warming, acidification and habitat loss.

These discoveries will improve our understanding of global warming and climate change by giving better evidence, better models and more accurate forecasts. That can help scientists and governments make better decisions to protect ecosystems and reduce human impact.