These questions appear inside the chapter itself, not at the end. The book page number is shown against each one so you can find it while reading.
Visit the website given below and study the effect of the concentration of greenhouse gas on surface temperature: https://phet.colorado.edu/en/simulations/greenhouse-effect
This is an online activity, so the readings you get depend on the simulation itself. Below is the method to follow and the result you should expect to see.
Method.
What you will observe and why. With no greenhouse gas, almost all the infrared radiation from the surface escapes to space and the surface settles at a very low temperature. As the greenhouse-gas concentration is raised, more of the outgoing infrared photons are absorbed by the gas and sent back towards the ground, so the surface has to become warmer before the heat it loses again balances the sunlight it receives. So the higher the concentration of greenhouse gases, the higher the steady surface temperature.
This is exactly the chapter’s point: some greenhouse gas (CO2, CH4, water vapour) is needed to keep the Earth warm enough for life, but extra greenhouse gas from human activities traps more heat and causes global warming.
How does the cool mountain breeze benefit agriculture activity, particularly the crops and soil?
After sunset the mountain slopes lose heat faster than the valley floor. The air over the slopes becomes cooler and denser and flows down into the valley — this is the mountain breeze. This cool night-time air helps farming in the valley in several ways:
In short, the mountain breeze regulates temperature and moisture in the valley, which supports both soil and crop health — just as the book says, local winds influence the weather, agriculture and daily life in mountainous regions.
What happens to the warm surface of water from the equator as it travels toward the poles? What impact does this movement have on the area?
What happens to the water. Near the equator the Sun heats the ocean strongly, so the surface water is warm. Planetary winds drag this warm water along the surface towards the poles. On the way it keeps giving its heat to the air above, so it slowly cools down. Cooler water is denser (and evaporation also leaves it saltier and denser), so near the high latitudes this water sinks and flows back slowly towards the equator at deeper levels of the ocean. The Earth’s rotation bends these moving water masses into large circular patterns called gyres.
Impact on the area it reaches.
The CO2 dissolved in the ocean is disturbed when the global temperature increases. What will happen to marine life?
How the dissolved CO2 is disturbed. A gas dissolves less in warm water than in cold water. So as the oceans warm up, they can hold less dissolved CO2, and the ocean becomes a weaker carbon sink — more CO2 stays in the air and warms the Earth further. At the same time, the extra CO2 that the ocean does absorb from the air (because there is so much more of it now) makes sea water more acidic. Warm water also holds less dissolved oxygen.
Effects on marine life.
So a rise in global temperature disturbs the ocean’s carbon balance and threatens marine ecosystems, from tiny plankton to coral reefs and fish.
What would happen to plants and animals on Earth if the biogeochemical cycles were disrupted and stopped? Explain by giving a few examples.
Biogeochemical cycles keep re-using the same limited stock of water, carbon, nitrogen and oxygen by moving them back and forth between living things and air, water, soil and rocks. The Earth gets energy from the Sun all the time, but it does not get a fresh supply of matter. If these cycles stopped, each essential element would get used up or locked away, and life would slowly come to an end. Some examples:
So stopping the biogeochemical cycles would break the link between the living and non-living parts of the Earth, and plants and animals could not survive for long.
Discuss how human activities increase the concentration of greenhouse gases in the atmosphere. What would you do as an individual to reduce the emission of greenhouse gas?
Human activities that add greenhouse gases.
Because of such activities, atmospheric CO2 has risen from about 315 ppm in 1960 to about 420 ppm today.
What I can do as an individual (this is the idea of Mission LiFE):
Choose the most appropriate option to describe the role of biogeochemical cycles in an ecosystem.
Answer: (ii) To recycle essential nutrients between biotic and abiotic components.
A biogeochemical cycle is the cyclic movement of matter (such as carbon, nitrogen, oxygen and water) between the living (biotic) and non-living (abiotic) parts of the Earth, so that these nutrients keep being reused.
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.
Most of the sunlight (mainly visible light) passes through the atmosphere and is absorbed by the land and oceans, warming them. The warm surface then gives out (re-radiates) heat as infrared radiation. Greenhouse gases such as CO2, CH4 and water vapour absorb this outgoing infrared radiation and stop part of it from escaping to space, which keeps the Earth warm.
Explain how climate change affects the water cycle. Illustrate with examples.
The water cycle moves water by evaporation, condensation, precipitation, run-off and infiltration. Climate change (global warming) speeds up and disturbs several of these steps:
These examples show that the water cycle links the cryosphere (glaciers), hydrosphere (rivers and oceans), atmosphere (moisture), geosphere (soil erosion) and biosphere (crops and fisheries) — and global warming affects all of them together.
Describe how albedo affects the Earth’s surface temperature and its climate.
Albedo is the fraction of the incoming solar radiation that a surface reflects. The part that is not reflected is absorbed and heats the surface:
Effect on surface temperature. A high-albedo surface (snow, ice) reflects most of the sunlight and stays cool. A low-albedo surface (black soil, ocean water, dark roads) reflects little, absorbs most of the sunlight and heats up.
A worked comparison. Take 1 m2 of ground for 1 hour in clear-sky sunlight of 1 kW m−2 (as in Example 13.1):
Use the book’s Table 13.1. Even comparing the least reflective snow (albedo 0.80) with the most reflective crushed rock (albedo 0.30):
Keeping the products unevaluated, the common factor 3.6 × 106 J cancels:
So the rock absorbs 3.5 times as much solar energy as the snow and gets much hotter.
Effect on climate.
How are mountain and valley breezes formed? Suppose there are two mountains, one covered with grass and another covered with barren rocks; would the temperature of the two mountain breezes be different? If so, how?
Valley breeze (daytime). During the day the mountain slopes facing the Sun heat up faster than the valley floor. The air over the slopes becomes warm and rises, creating a low-pressure region. Cooler air from the valley moves up the slopes to take its place. This up-slope wind is the valley breeze.
Mountain breeze (night). After sunset the slopes lose heat faster and become cooler than the valley floor. The air over the slopes is cooled, becomes denser and flows down into the valley. This down-slope wind is the mountain breeze.
Grassy mountain versus barren rocky mountain. Yes, the two mountain breezes would most likely have different temperatures:
This is a reasoned prediction. The actual difference also depends on the type of rock, how wet the ground is, the cloud cover and the season.
You have witnessed weather phenomena, such as winds, storms, rainfall, etc. Which atmospheric layer is mainly responsible for such phenomena and what is the primary reason for its occurrence?
Layer: the troposphere, the lowest layer of the atmosphere, from the ground up to about 12 km on average (highest above the equator and lowest above the poles). Nearly all weather phenomena — winds, clouds, storms, rain — take place here.
Primary reason. The troposphere is heated from below, by the Earth’s surface, which absorbs sunlight and re-radiates heat. So the air is warmest near the ground and its temperature decreases with height, by about 6.5 °C for every kilometre. For example, across the whole troposphere:
so if the ground is at 30 °C, the air at the top of the troposphere is at about
Because warm, light air lies below cold, heavy air, the warm air keeps rising. This vertical mixing (convection) carries water vapour upwards, where it cools and condenses into clouds and rain, and the rising and sinking air sets up winds and storms. Almost all of the atmosphere’s water vapour is also in this layer.
In the stratosphere above, by contrast, the ozone layer absorbs UV rays and the temperature increases with height, so there is little vertical mixing and the weather stays confined to the troposphere.
Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
Nitrogen is needed to make proteins and nucleic acids. The atmosphere is its largest reservoir (as N2), but N2 gas is non-reactive and cannot be used directly by plants and animals. The nitrogen cycle moves nitrogen between air, soil, water and organisms through these steps:
| Process | What happens |
|---|---|
| Nitrogen fixation | Nitrogen-fixing bacteria such as Rhizobium (in the root nodules of legumes) and Azotobacter (in the soil) convert atmospheric N2 into ammonia (NH3). Lightning also fixes a little nitrogen as nitrogen oxides, and today most nitrogen is fixed in factories by the Haber–Bosch process to make fertilisers. |
| Nitrification | Nitrifying bacteria convert ammonia into nitrite (NO2−) — Nitrosomonas — and then nitrite into nitrate (NO3−) — Nitrobacter. |
| Assimilation | Plants absorb nitrates (and ammonium) from the soil and use them to make proteins and nucleic acids. Animals get their nitrogen by eating plants or other animals. |
| Ammonification | When plants and animals die or give out wastes, decomposers (bacteria and fungi) break down the organic matter and return nitrogen to the soil as ammonia. |
| Denitrification | Denitrifying bacteria such as Pseudomonas convert some nitrates back into N2 gas, which returns to the atmosphere and completes the cycle. |
If nitrogen were not cycled:
What are the impacts of deforestation on the Earth’s oxygen and carbon cycles? What are the other consequences of deforestation?
Impact on the oxygen and carbon cycles.
Other consequences of deforestation.
Explain with suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using CO2 from the atmosphere.
Step 1 — Carbon enters plants. Green plants take in CO2 from the atmosphere and, using sunlight, convert it into glucose by photosynthesis. The carbon now forms part of the plant’s body (carbohydrates, proteins, fats).
Step 2 — Carbon moves along the food chain. Animals eat the plants (or other animals), so the carbon becomes part of their bodies.
From here carbon returns to the atmosphere by several paths:
The ocean is also part of the cycle: it absorbs CO2 from the air, phytoplankton use it for photosynthesis, and marine organisms build shells from it. When they die their carbon may be stored on the ocean floor for a long time, and the ocean also gives CO2 back to the air. In this way carbon keeps circulating between the atmosphere, biosphere, geosphere and hydrosphere.
Why is an excess of CO2 in the atmosphere considered undesirable even though it is required by plants?
Plants do need CO2 for photosynthesis, and some CO2 is needed in the air to keep the Earth warm enough for life. The problem is the balance: too much CO2 does more harm than good.
How large is the rise? Using the book’s figures (Fig. 13.14), CO2 rose from 315 ppm in 1960 to 420 ppm:
That is roughly one-third more CO2 in about 65 years — an unprecedented rise in the history of human civilisation. (The book rounds this to ‘about 35%’.)
How is heat lost from the surface of the Earth? What is its significance?
How heat is lost. The Earth’s surface absorbs sunlight during the day and warms up. It then loses heat mainly in these ways:
Significance.
If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?
On the real (spherical) Earth. The Sun’s rays are nearly parallel. Near the equator they strike the surface almost straight on, so a beam of sunlight is concentrated on a small area. Towards the poles the surface is tilted away from the rays, so the same beam is spread over a larger area and each square metre gets less energy. At latitude θ (for the Sun overhead at the equator) the beam is spread over an area larger by the factor
For example, at latitude 60°, since cos 60° is one-half:
so where the equator gets 1 kW m−2, a place at 60° gets only half of it:
In one hour each square metre there receives
which is half of the 3.6 × 106 J at the equator. This is why the equator is warm and the poles are cold, and this uneven heating drives the planetary winds and ocean currents.
On a flat disc facing the Sun.
Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?
| Sphere | Effect of a rise in atmospheric temperature |
|---|---|
| Cryosphere (ice and snow) | Himalayan glaciers, polar ice caps and Arctic sea ice melt faster; less snow falls in winter and snow cover lasts for a shorter time. As bright ice is replaced by darker water or land, the surface albedo falls, more sunlight is absorbed and warming speeds up further. |
| Hydrosphere (water) | Melt-water first raises river flow and then, over time, sea level, threatening low-lying coastal cities such as Mumbai and Chennai. A warmer atmosphere holds more moisture, giving heavier rain and floods in some places and droughts in others. Oceans become warmer, hold less dissolved CO2 and oxygen, and become more acidic. Lakes fed by snow may shrink when snowfall drops. |
| Biosphere (living things) | Habitats are lost (for example, coastal and polar habitats); coral reefs bleach and plankton are affected; species move to cooler places or may become extinct. Crops suffer from heat, floods and droughts, so food production falls; with less water, grass for grazing animals such as sheep decreases. Fisheries and human health are also affected. |
The three spheres are linked: less snow (cryosphere) means less water in the lake (hydrosphere), which means less grass for the sheep (biosphere) — so a change in one sphere spreads to the others.
Explain how the Earth’s atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.
The atmosphere acts like a blanket and a shield at the same time:
The balance matters: too little greenhouse gas would make the Earth freezing, while too much traps excess heat. Venus, with its thick CO2 atmosphere, has an uncontrolled greenhouse effect and is hotter than Mercury even though Mercury is closer to the Sun.
Describe the interrelationship between different spheres of the Earth. Illustrate with example how these spheres function in a delicate balance.
The Earth is one system made of five interacting spheres — the geosphere (rocks, soil, landforms), hydrosphere (liquid water), cryosphere (ice and snow), atmosphere (air) and biosphere (living things). Energy from the Sun and the cycling of matter (water, carbon, nitrogen, oxygen) constantly connect them, so a change in one sphere leads to changes in the others.
Example 1 — snow, lake, grass and sheep (Fig. 13.1). In summer, snow on the mountains (cryosphere) melts and flows into the lake (hydrosphere). This water keeps the soil (geosphere) moist so that grass grows, and the sheep (biosphere) graze on it. The air (atmosphere) brings the winter snowfall in the first place. If there is less snowfall for a few years, the lake level falls, less grass grows and the sheep go hungry — one disturbance travels through all the spheres.
Example 2 — the monsoon. Warmer Arabian Sea water (hydrosphere) increases evaporation into the air (atmosphere). This disturbs the south-west monsoon, bringing floods to some regions and droughts to others, which affects soil erosion (geosphere) and crops and fisheries (biosphere). Rising air temperature also melts glaciers (cryosphere), which can flood low-lying areas and raise sea levels.
Example 3 — the carbon cycle. Plants (biosphere) take CO2 from the air (atmosphere); the ocean (hydrosphere) dissolves CO2; dead organisms become fossil fuels in rocks (geosphere). When humans burn fossil fuels and cut forests, extra CO2 builds up in the atmosphere, warming the planet, melting ice and making oceans acidic.
The delicate balance. Under natural conditions these give-and-take processes balance each other — for example, photosynthesis restores the oxygen used in respiration and burning, and decomposers return nutrients to the soil. This balance keeps the temperature, air, water and soil suitable for life. Human activities that push one sphere too hard (burning fossil fuels, deforestation, overuse of fertilisers causing eutrophication) upset this balance, and its effects spread to all the other spheres.
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