Earth is one system with five spheres
A system is a set of parts that work together. Earth is a system. We split it into five parts called spheres.
- Geosphere: the solid Earth. Rocks, soil, sand, mountains and the hot inside of the planet.
- Hydrosphere: all liquid water. Oceans, rivers, lakes, rain and water under the ground. About 97% of it is salty sea water.
- Cryosphere: all frozen water. Ice sheets at the poles, glaciers on the Himalayas, snow and frozen ground. It stores most of the fresh water on Earth.
- Atmosphere: the blanket of air around Earth. It is mostly nitrogen (about 78%) and oxygen (about 21%), with a small amount of carbon dioxide and water vapour.
- Biosphere: every living thing, and the places where they live. It reaches from the deep sea to high in the air.
The spheres touch each other
Nothing works alone. A tree (biosphere) grows in soil (geosphere), drinks water (hydrosphere) and breathes air (atmosphere). A melting glacier (cryosphere) fills a river (hydrosphere). Change one sphere and the others change too.
Solar radiation and the electromagnetic spectrum
Almost all energy on Earth's surface comes from the Sun. The energy travels through empty space as radiation: waves that do not need air or water to move. We call these waves electromagnetic (EM) waves. They all move at the speed of light.
EM waves come in different wavelengths (the gap from one wave top to the next). Put in order from long to short, they make the EM spectrum:
radio → microwave → infrared → visible light → ultraviolet (UV) → X-rays → gamma rays
- Visible light is the only part our eyes see. It holds the rainbow colours.
- Infrared is what we feel as heat.
- UV causes sunburn. The ozone layer in the atmosphere stops most of it.
- Shorter wavelength = more energy per wave.
Where does sunlight go?
About 30% of sunlight is bounced straight back to space by clouds, ice and bright ground. The rest (about 70%) is soaked up by air, land and sea and warms them. Warm Earth then gives off infrared. Gases like carbon dioxide and water vapour catch some of this infrared and keep Earth warm. This is the greenhouse effect. Without it, Earth would be frozen.
Uneven heating: winds, breezes and ocean currents
The Sun does not heat Earth equally. Two big reasons:
- Angle of rays: Near the equator, the Sun is high and rays hit straight. Near the poles, rays hit at a slant and spread over a bigger area. So the equator is hot and the poles are cold.
- Land vs water: Land heats up fast and cools fast. Water heats up slowly and cools slowly (it has a high specific heat).
Wind
Hot air is lighter, so it rises. That leaves low pressure below. Cooler, heavier air from a high-pressure place rushes in to fill the gap. This moving air is wind. Wind always blows from high pressure to low pressure.
Sea breeze and land breeze
By day, land is hotter than the sea. Air over land rises, and cool air flows from sea to land: a sea breeze. At night, land cools faster and the sea is warmer. Air over the sea rises, and air flows from land to sea: a land breeze. The monsoon is a giant, season-long version of this.
Ocean currents
Ocean water also moves in huge rivers called ocean currents. Winds push surface water. Warm water from the equator flows towards the poles; cold water flows back towards the equator. Cold, salty water is heavier and sinks. So currents carry heat around the world and make coasts warmer or cooler than they would be.
Biogeochemical cycles
Bio = life, geo = Earth, chemical = substances. A biogeochemical cycle is the path a substance takes as it moves between living things and non-living parts of Earth, again and again. Energy flows in from the Sun and leaves as heat, but matter is recycled.
Water cycle
The Sun heats seas and lakes. Water turns into vapour (evaporation). Plants also release vapour from their leaves (transpiration). Vapour cools high up and forms clouds (condensation). Water falls as rain or snow (precipitation). It flows back to the sea in rivers or soaks into the ground.
Carbon cycle
Plants take CO₂ from the air and make food by photosynthesis. Animals eat plants, so carbon moves into animals. All living things release CO₂ when they respire. Dead bodies are broken down by decomposers, which also release CO₂. Some carbon gets buried for millions of years as coal, oil and gas; burning them puts it back into the air fast. The oceans also dissolve a lot of CO₂.
Nitrogen cycle
Air is 78% nitrogen gas (N₂), but plants cannot use it directly. It must first be fixed, that is, changed into nitrates or ammonia. Lightning and special bacteria (some live in the root nodules of pea and bean plants) do this. Plants take up nitrates from soil to make proteins. Animals get nitrogen by eating plants. When they die, decomposers turn their nitrogen into ammonia, other bacteria change it to nitrates (nitrification), and some bacteria change nitrates back to N₂ gas (denitrification).
Oxygen cycle
Plants and algae release oxygen during photosynthesis. Animals and plants use oxygen to respire and give out CO₂. Burning and rusting also use oxygen. Oxygen high up forms the ozone layer (O₃). The oxygen and carbon cycles are linked like two sides of one coin.
Human impact on the Earth system
Humans are part of the biosphere, but we now change all five spheres.
- Burning fossil fuels adds extra CO₂ to the atmosphere. CO₂ is now about 420 parts per million (ppm), up from about 280 ppm before factories. More CO₂ traps more heat: global warming.
- Melting ice: warming shrinks glaciers and polar ice (cryosphere) and raises sea level (hydrosphere).
- Cutting forests: fewer trees take in CO₂, soil washes away, and rain patterns change.
- Too much fertiliser: extra nitrates wash into lakes, algae grow wildly, and fish die from lack of oxygen.
- Pollution: smoke, plastics and chemicals harm air, water, soil and living things. CFC gases thinned the ozone layer.
What we can do
Use less energy and more solar and wind power, plant and protect trees, use fertiliser carefully, cut plastic, save water, and walk, cycle or use buses. Small actions by many people add up.
Try it yourself: make a sea breeze at home
You need: two same bowls, one with dry soil or sand, one with water, a sunny window, and a thermometer (or just your hand).
- Put both bowls in the sun for 20 minutes. Predict first: which will be hotter?
- Touch or measure both. The soil is hotter. Land heats faster.
- Move both into shade for 20 minutes. Check again. Now the water is warmer. Land cools faster.
- With an adult, light an incense stick between the bowls right after the sun step. Watch the smoke drift toward the warm soil side. That is a mini sea breeze.
In the 3D above, use the Sun height slider in step 3 and the Day/Night buttons in step 4 to check your idea.
Key formulas and definitions
- Five spheres: geosphere (rock), hydrosphere (liquid water), cryosphere (ice), atmosphere (air), biosphere (life)
- EM spectrum (long → short wavelength): radio, microwave, infrared, visible, UV, X-ray, gamma
- Shorter wavelength → more energy; all EM waves travel at the speed of light (3 × 10⁸ m/s)
- Wind blows from high pressure (cool air) to low pressure (warm, rising air)
- Sea breeze: day, sea → land. Land breeze: night, land → sea
- Photosynthesis: CO₂ + water + sunlight → food (glucose) + O₂. Respiration: food + O₂ → CO₂ + water + energy
- Nitrogen cycle: fixation → nitrification → uptake by plants → eaten by animals → decay (ammonification) → denitrification
- Albedo ≈ 30%: share of sunlight Earth reflects back to space
Worked examples
1. A glacier in the Himalayas melts and fills the Ganga. Which two spheres are interacting?
The glacier is part of the cryosphere (frozen water). The river is part of the hydrosphere (liquid water). So cryosphere → hydrosphere. If farmers use the river water for crops, the biosphere joins in too.
2. Sunlight of 100 units reaches the top of the atmosphere. About 30% is reflected. How many units warm the Earth and air?
Reflected = 30% of 100 = 30 units. Absorbed = 100 − 30 = 70 units. These 70 units warm the land, sea and air.
3. At 3 pm on a beach in Kerala, which way does the wind blow, and why?
In the afternoon the land is hotter than the sea. Warm air over the land rises, making low pressure. Cooler air from over the sea (high pressure) moves in. So the wind blows from sea to land: a sea breeze.
4. A torch beam falls straight down and lights a 1 m² patch. When tilted, the same beam lights a 2 m² patch. How does the energy per square metre change?
Same total energy spread over twice the area. Energy per m² becomes half. This is why slanted sunlight near the poles heats the ground less than straight sunlight at the equator.
5. Trace one carbon atom from a coal power plant to your body.
Burning coal releases the carbon as CO₂ into the air. A wheat plant takes in that CO₂ during photosynthesis and stores the carbon in grain (starch). You eat a roti made from the wheat, and the carbon becomes part of your body. When you breathe out, some of it goes back to the air as CO₂.
6. Pea farmers often need less nitrogen fertiliser. Explain using the nitrogen cycle.
Pea plants have root nodules with Rhizobium bacteria. These bacteria fix nitrogen gas from the air into ammonia/nitrates that the plant can use. So the plant makes its own nitrogen supply and the soil also gets richer for the next crop.
Common mistakes
- Mixing up hydrosphere and cryosphere: liquid water is hydrosphere; ice and snow are cryosphere.
- Saying a sea breeze blows from land to sea. A breeze is named after where it comes FROM: a sea breeze comes from the sea, by day.
- Thinking energy is recycled like matter. Matter (carbon, water, nitrogen) goes round in cycles; energy comes in from the Sun and leaves as heat.
- Believing plants take nitrogen gas straight from the air. Most plants need it fixed into nitrates first, by bacteria or lightning.