Earth as a System: Energy, Matter & Life
Chapter 13 — full, step-by-step solutions for the worked example, every In-Text activity and Pause & Ponder question, and the end-of-chapter “Revise, Reflect, Refine” exercise, with clean diagrams and the original textbook figures.
Worked Examples
The quantitative example and estimation worked through in the chapter.
Energy = power received per unit area (intensity) × area × time.
- Write the formula$E = \text{Intensity} \times \text{area} \times \text{time}$
- Substitute the values$E = (1000\ \text{J s}^{-1}\text{m}^{-2}) \times (1\ \text{m}^2) \times (3600\ \text{s})$
(1 kW = 1000 J s⁻¹ and 1 hour = 3600 s) - Compute$E = 3\,600\,000\ \text{J} = 3.6 \times 10^{6}\ \text{J}$
This is an order-of-magnitude estimate, so we choose reasonable values and state our assumptions.
- Average power India uses$P \approx 200\ \text{GW} = 2\times10^{11}\ \text{W}$ (rough average demand)
- Useful power per m² of panelinsolation $\approx 1000\ \text{W m}^{-2}$; with ~20% efficiency and the Sun shining only part of the day (capacity factor ~0.2):
$1000 \times 0.2 \times 0.2 \approx 40\ \text{W m}^{-2}$ (24-hour average) - Area needed$A = \dfrac{P}{40} = \dfrac{2\times10^{11}}{40} = 5\times10^{9}\ \text{m}^2 = 5000\ \text{km}^2$
- Compare with the Thar desertThar area $\approx 2\times10^{5}\ \text{km}^2$, so we need about $\dfrac{5000}{200000} \approx 2.5\%$ of it.
So only a small fraction of the Thar desert, covered with panels, could in principle meet India’s electricity demand — showing the enormous scale of solar energy reaching the Earth. (Exact numbers vary with the assumptions you choose.)
In-Text — Activities & Pause & Ponder
Think It Over, Activity 13.1–13.2 and every Pause & Ponder question.
Warmer sea water evaporates faster, putting more moisture and energy into the air above the Arabian Sea.
This causes fluctuations in the southwest monsoon and makes rainfall more variable — bringing heavy floods to some regions while leaving others in drought. So warming of one part of the hydrosphere disturbs the whole monsoon system.
Forests release moisture through transpiration and their roots hold the soil and help water infiltrate into the ground. Clearing them changes the river’s flow:
- Less transpiration → less local rainfall.
- Rain now runs off quickly instead of soaking in → flash floods during rains and low flow / drying up in dry months (less groundwater recharge).
- Exposed soil erodes and silts up the river, changing its course and depth.
Melting glaciers and polar ice add huge amounts of water to the oceans, causing sea levels to rise.
Low-lying coastal cities such as Mumbai and Chennai could face flooding and submergence, more damaging storm surges, and saltwater intrusion into freshwater and farmland — threatening homes, ecosystems and livelihoods.
The ocean absorbs extra atmospheric CO₂. This forms carbonic acid, making sea water more acidic (ocean acidification).
Acidic water makes it harder for plankton and other shell-forming organisms to build their calcium-carbonate shells. Combined with warmer water (which holds less dissolved gas), this can harm plankton populations — disturbing the base of the marine food web.
Examples of each sphere in the scene: mountains/rocks/soil = geosphere; lake/streams = hydrosphere; snow on peaks = cryosphere; air/clouds = atmosphere; sheep, grass, shepherd = biosphere.
Snow → lake: in warmer weather the snow melts into water, which flows down the slopes as streams and collects in the lake — moving from the cryosphere to the hydrosphere.
Less snowfall for some years: less meltwater means a lower lake level in summer, so there is less water for the grass to grow — leaving less food for the sheep.
Interconnection: the spheres form one Earth system; a disturbance in one (less snow) flows through the others (less water → less grass → fewer sheep).
Filling the blank rows using typical values from reliable sources:
| Material | Albedo |
|---|---|
| Snow | 0.80 – 0.90 |
| Ice | 0.50 – 0.70 |
| Crushed rock | 0.25 – 0.30 |
| Light coloured soil | 0.30 – 0.40 |
| Black soil | 0.05 – 0.15 |
| Ocean water | 0.05 – 0.10 |
Snow and ice have high albedo (reflect most sunlight → stay cold, keeping polar regions cool). Black soil and ocean water have low albedo (absorb most sunlight → stay warmer).
As you increase the concentration of greenhouse gases in the simulation, more outgoing infrared heat is trapped, so the model’s surface temperature rises; reducing the gases lets more heat escape and the surface cools.
This demonstrates the greenhouse effect: a certain amount keeps the Earth warm enough for life, but excess CO₂/CH₄ leads to global warming.
The cool mountain breeze (cool, dense air flowing down into the valley at night) helps farming in several ways:
- It lowers night temperatures, reducing heat stress on crops and slowing water loss.
- It helps retain soil moisture by reducing evaporation, keeping the soil cooler and moist.
- Steady air movement improves ventilation around plants, lowering humidity-related fungal disease and supporting healthy growth.
Warm surface water moves polewards as warm ocean currents (e.g. the Gulf Stream → North Atlantic Drift). As it travels it gradually cools and becomes denser, and at high latitudes the cold, dense water sinks and flows back towards the equator at depth.
Impact: this transport of heat warms coastal regions (keeping many European ports ice-free in winter), reduces temperature differences across the planet, supports trade, and carries nutrients that sustain marine ecosystems.
Warmer water can hold less dissolved CO₂ (and oxygen), so the ocean becomes a weaker carbon sink and more CO₂ stays in the air. At the same time, the extra CO₂ already absorbed makes the water more acidic.
Effect on marine life: acidification makes it hard for plankton, corals and shellfish to form calcium-carbonate shells/skeletons; warming and lower oxygen stress many species. This damages coral reefs and the food web, threatening fisheries.
Biogeochemical cycles keep recycling water, carbon, nitrogen and oxygen so they stay available to life. If they stopped:
- Water cycle stops → no rain; plants and animals die of drought.
- Carbon/oxygen cycle stops → no recycling of CO₂ and O₂; if photosynthesis stopped, plants couldn’t make food and would not release O₂, so animals would run out of food and oxygen.
- Nitrogen cycle stops → no usable nitrogen for proteins; plants can’t grow, so food chains collapse.
Nutrients would get locked up, ecosystems would break down, and most life on the Earth could not survive.
Human activities that raise greenhouse gases:
- Burning fossil fuels (electricity, vehicles, industry) → releases CO₂.
- Deforestation → fewer trees to absorb CO₂.
- Agriculture and livestock, landfills → release methane (CH₄); overuse of fertilisers adds nitrogen gases.
What I can do as an individual: save electricity and water, use public transport / cycle / walk, switch to renewable energy where possible, plant trees, and reduce, reuse and recycle to cut waste.
Exercise — Revise, Reflect, Refine
Full solutions to the end-of-chapter questions 1–15.
- To provide food directly to all organisms.
- To recycle essential nutrients between biotic and abiotic components.
- To create new elements for use by living things.
- To remove pollutants and toxins from the organism.
Biogeochemical cycles continuously move nutrients such as carbon, nitrogen and oxygen between living (biotic) and non-living (abiotic) parts of the Earth, keeping them available to life. They do not create new elements, nor directly feed organisms, nor exist to remove toxins.
- Solar radiation is immediately absorbed by CO₂, which releases it as heat.
- The atmosphere’s tiny particles absorb incoming solar radiation, directly heating the Earth.
- The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.
- The Earth is heated only by solar radiation reflected by clouds.
The Earth’s surface absorbs incoming solar radiation and then re-radiates it as infrared heat. Greenhouse gases (CO₂, CH₄, water vapour) trap part of this outgoing heat, keeping the Earth warm. Solar radiation is not directly absorbed mainly by CO₂, nor does it warm the Earth only via reflection by clouds.
A warmer atmosphere holds more moisture and speeds up evaporation, so the water cycle becomes more intense and uneven:
- Heavier rainfall and intensified monsoons in some regions, but droughts in others.
- Melting glaciers add water to rivers and raise sea levels, threatening coastal cities (Mumbai, Chennai).
- Intense downpours cause more run-off and soil erosion, while less infiltration reduces groundwater recharge, making dry-season farming harder.
Thus climate change links the cryosphere, hydrosphere, atmosphere, geosphere and biosphere.
Albedo is the fraction of sunlight a surface reflects.
- High-albedo surfaces (snow, ice) reflect most sunlight, absorb little, and stay cool — keeping polar regions cold.
- Low-albedo surfaces (black soil, ocean, dark roads) absorb most sunlight and become warm.
Climate link: albedo controls how much solar energy is absorbed regionally. It also drives feedback — e.g. melting ice exposes darker water/land, which absorbs more heat and causes further warming. Human changes (dark cities, deforestation) lower albedo and warm local climates.
Valley breeze (day): Sun-facing slopes heat faster than the valley floor; warm air over the slopes rises (low pressure) and cooler valley air flows up the slopes.
Mountain breeze (night): slopes cool faster than the valley; the cool, dense air flows down into the valley.
Grass vs barren rock: Yes, the breezes would differ. Bare rock has low albedo and heats/cools faster, so at night it cools more and produces a colder, stronger mountain breeze. A grass-covered slope holds moisture and loses heat more slowly (transpiration/shade), so its mountain breeze is relatively warmer.
The troposphere (0–12 km), the lowest layer, is where nearly all weather occurs.
Primary reason: it is heated from below by the Earth’s surface, so temperature decreases with height. The warm air near the surface rises and cooler air sinks (convection), driving winds, clouds, storms and rainfall.
Atmospheric N₂ is plentiful but unusable directly, so it is converted through these steps:
- Nitrogen fixation — bacteria (Rhizobium, Azotobacter) and lightning convert N₂ → ammonia (NH₃).
- Nitrification — Nitrosomonas turns ammonia → nitrite (NO₂⁻); Nitrobacter turns nitrite → nitrate (NO₃⁻).
- Assimilation — plants absorb nitrates; animals get nitrogen by eating plants.
- Ammonification — decomposers break down dead matter/waste back into ammonia.
- Denitrification — Pseudomonas converts nitrates back to N₂ gas, completing the cycle.
If nitrogen were not cycled: usable nitrogen would run out, so plants could not make proteins and nucleic acids, growth would stop, and the food chains depending on plants would collapse.
On the oxygen and carbon cycles:
- Fewer trees → less photosynthesis → less O₂ released and less CO₂ absorbed, so atmospheric CO₂ rises (intensifying the greenhouse effect).
- Burning/decay of cleared trees releases stored carbon as CO₂.
Other consequences: reduced transpiration → less local rainfall; soil erosion (no roots to bind soil); altered surface albedo; loss of habitats → decline in biodiversity.
Starting from plants taking in CO₂:
- Plants absorb atmospheric CO₂ and make glucose by photosynthesis.
- Carbon passes to animals when they eat plants.
- Plants and animals release CO₂ back through respiration.
- When they die, decomposition returns CO₂ to the air.
- Buried remains form fossil fuels over millions of years; burning (combustion) of these fuels releases CO₂ back quickly.
- The ocean also exchanges CO₂ with the atmosphere.
So carbon returns to the atmosphere mainly by respiration, decomposition and the combustion of fossil fuels.
Plants do need CO₂ for photosynthesis, and some CO₂ keeps the Earth warm enough for life. But too much CO₂ upsets the balance:
- It intensifies the greenhouse effect, causing global warming.
- This melts glaciers and Arctic ice, raises sea levels, and brings more extreme weather and erratic monsoons.
- Oceans absorb the excess and become acidic, harming corals and plankton.
So the issue is not CO₂ itself but its excess, which disturbs climate and ecosystems.
The Earth’s surface, warmed by sunlight, re-radiates heat as infrared (long-wave) radiation back towards the atmosphere and space.
Significance: greenhouse gases trap part of this outgoing heat, keeping the Earth warm enough for life; the rest escapes to space. This balance between incoming sunlight and outgoing heat regulates the Earth’s temperature. If too much heat is trapped (excess greenhouse gases), the planet overheats.
On the real spherical Earth, the Sun’s parallel rays strike the equator almost head-on (concentrated) but hit the poles at a slant (spread over a larger area) — so the equator is hot and the poles are cold.
On a flat disc facing the Sun, the rays would strike evenly everywhere, so all parts would be heated almost equally. There would be no equator-to-pole temperature gradient, and so the global winds and ocean currents driven by that uneven heating would be very different (or absent).
- Cryosphere: glaciers, snow and polar ice melt faster, shrinking the ice cover.
- Hydrosphere: meltwater and thermal expansion raise sea levels; more evaporation makes rainfall erratic (floods and droughts); oceans warm and become more acidic.
- Biosphere: habitats are lost, species must migrate or face extinction; coral reefs bleach; agriculture and food webs are disturbed.
The atmosphere balances the energy the Earth gains and loses:
- It absorbs and filters incoming radiation — the ozone layer blocks harmful UV, and clouds/gases reflect or absorb some sunlight.
- It traps outgoing heat — the surface re-radiates infrared, and greenhouse gases (CO₂, CH₄, water vapour) absorb part of it, keeping the surface warm.
Without this blanket the Earth would be too cold for life; with the right amount of greenhouse gases it stays comfortably warm. (Too much, however, causes overheating.)
The geosphere, hydrosphere, cryosphere, atmosphere and biosphere are not separate — energy and matter constantly flow between them, so a change in one affects all.
Example of the delicate balance: rising atmospheric temperature (atmosphere) melts Himalayan glaciers (cryosphere); the meltwater swells rivers and raises sea levels (hydrosphere), which floods coastal land (geosphere) and destroys habitats and farmland (biosphere). One disturbance thus cascades through every sphere — showing how finely balanced the Earth system is.
Ready for new horizons…
Energy and matter flow through one connected Earth system — and the next discoveries about our changing planet may come from someone learning science today.