Class 9 Science Chapter 13: Earth as a System: Energy, Matter, and Life — Notes, Examples & Practice

Scientists divide the Earth system into spheres: the atmosphere (the layer of air), the hydrosphere (oceans, rivers, lakes, groundwater), the geosphere or lithosphere (rocks, soil, the solid Earth), the cryosphere (ice sheets, glaciers, snow, permafrost) and the biosphere (all living things, in every other sphere).

The spheres interact all the time. A forest (biosphere) takes water from the soil (geosphere) and releases it as vapour into the air (atmosphere), which later falls as rain (hydrosphere). A volcano (geosphere) puts gases and dust into the atmosphere, which can cool climate for a year or two.

Energy flows through the system — from the Sun, used once and eventually radiated to space as heat — while matter cycles within it, because Earth exchanges very little matter with space.

Sunlight passes through the atmosphere and heats the ground and oceans. The warm surface then gives off energy as infrared radiation (heat). The ozone layer in the upper atmosphere absorbs most harmful ultraviolet rays.

Some gases — carbon dioxide, water vapour, methane and nitrous oxide — absorb part of the outgoing infrared and send it back down, keeping the lower atmosphere warm. This natural greenhouse effect keeps Earth's average temperature about 15 °C instead of roughly −18 °C. Adding more greenhouse gases (by burning fossil fuels, cutting forests) strengthens it, causing global warming.

Albedo is how much light a surface reflects. Fresh snow and ice reflect most sunlight (high albedo) and stay cool; dark oceans, forests and asphalt absorb most (low albedo) and warm up. When ice melts, darker water or ground is exposed, which absorbs more heat and melts more ice — a feedback loop. Because Earth is a sphere, sunlight strikes the equator directly and the poles at a slant, spreading the same energy over a larger area — so the tropics are hotter.

When air is heated it expands, becomes less dense and rises, leaving a region of low pressure near the ground. Cooler, denser air sinks and forms high pressure. Wind is air moving from high-pressure to low-pressure regions.

Sea breeze (day): land heats up faster than water, so air over land rises and cooler air from the sea blows in. Land breeze (night): land cools faster, so air over the sea is warmer and rises, and air flows from land to sea. Mountain and valley breezes form the same way — warm air rises up slopes in the day and cool air drains down at night.

On a large scale, the hot tropics and cold poles set up global wind belts. The monsoon is a seasonal giant sea breeze: in summer the Indian landmass heats strongly, drawing moist south-west winds from the Indian Ocean that bring most of India's rain.

Solar energy drives the water cycle. Water evaporates from oceans, lakes and soil, and plants release water vapour through transpiration. The vapour rises, cools and condenses on dust particles to form clouds. When droplets grow big enough they fall as precipitation — rain, snow or hail.

Water then flows over land as runoff into rivers and back to the sea, or infiltrates the soil to recharge groundwater. Some is stored for long periods in glaciers and ice sheets.

The water cycle also moves heat: evaporation takes heat from the surface, and condensation releases it high in the atmosphere, powering storms. A warmer climate means more evaporation and air that holds more moisture — so heavier downpours in some places and droughts in others.

Carbon cycle: plants take in CO₂ from the air in photosynthesis and build it into sugars. Animals eat plants; respiration by living things, decomposition of dead matter, and combustion return CO₂ to the air. Oceans dissolve large amounts of CO₂, and some carbon is locked away for millions of years in fossil fuels, limestone and shells. Burning fossil fuels releases this stored carbon quickly.

Nitrogen cycle: air is about 78% nitrogen, but most living things cannot use N₂ gas directly. Nitrogen-fixing bacteria (in root nodules of legumes like peas and beans, and in soil) and lightning convert it into compounds such as ammonia and nitrates. Plants absorb nitrates to make proteins; animals get nitrogen by eating plants. Decomposers turn dead matter and wastes back into ammonia, and denitrifying bacteria return nitrogen gas to the air.

Oxygen cycle: photosynthesis releases O₂; respiration and combustion use it. The oxygen and carbon cycles are linked through these same processes. About 21% of air is oxygen, and ozone (O₃) high up shields life from UV.

Since the Industrial Revolution, burning coal, oil and gas has raised the CO₂ in the air from about 280 parts per million to over 420 ppm. Together with methane from agriculture and landfills, this has enhanced the greenhouse effect and warmed Earth's average temperature by more than 1 °C.

Effects ripple through all the spheres: glaciers and sea ice melt (cryosphere), sea level rises as water expands and ice melts (hydrosphere), rainfall becomes more extreme, coral reefs bleach, and species move or decline (biosphere). Deforestation removes carbon sinks, reduces transpiration and rainfall, and exposes soil to erosion.

Responses include using renewable energy (solar, wind), improving energy efficiency, protecting and planting forests, saving water, reducing waste, and choosing sustainable farming. Small actions — switching off devices, using public transport, not wasting food — add up across many people.