Earth as a System: Energy, Matter, and Life

Class 09 Science

Life on Earth is powered by a constant flow of energy and matter. The Sun is the main source of energy. In addition, the Earth’s hot interior and chemical reactions in the air, water, and rocks also drive the flow of energy and matter.

Earth system made up of interacting spheres:

  1. Geosphere: Solid rocks, soil, landforms (like the Deccan plateau and the Thar desert), and the Earth’s interior.
  2. Hydrosphere: Liquid water in the form of surface water, such as oceans, rivers (like the Ganga - Brahmaputra river system), lakes and groundwater.
  3. Cryosphere: Solid form of water, such as ice and snow (like the Himalayan glaciers, snow in Ladakh and polar ice caps).
  4. Atmosphere: The air surrounding the Earth that we breathe (cleaner air in the mountains and forests).
  5. Biosphere: All living organisms and their habitats (including mangroves, forests, farms, ocean plankton and coral reefs).

Uneven Heating of the Earth

Solar radiation is the main source of energy on the Earth. It reaches the Earth as electromagnetic (EM) waves that travel through a vacuum at the speed of light. The speed of light in vacuum is 3 × 108 ms-1. EM waves cover a wide range of frequencies, from high frequency or short wavelength radiation (gamma rays and X-rays) to low frequency or long wavelength radiation (infrared and radio waves). The high frequency EM waves, such as gamma rays and X-rays have very high energy and can be harmful for life on Earth.

The entire range of electromagnetic radiation called electromagnetic spectrum. However, the solar radiation reaching Earth is concentrated mainly in the ultraviolet (UV), visible and infrared (IR) range - about 99 per cent of the Sun’s energy falls within these wavelengths. These three regions of the spectrum shape the Earth’s climate and support life. Gamma rays and X-rays are mostly filtered by the Earth’s upper atmosphere, while microwaves and radio waves carry very little energy to significantly warm the Earth.

Short wavelength UV radiation is mostly absorbed by the ozone layer in the upper atmosphere, protecting life and contributing to some atmospheric heating. Visible light from the Sun reaches the Earth’s surface and provides energy for photosynthesis, which is the primary source of food for most organisms. It also partly warms the land and water. Infrared radiation warms the Earth’s surface, which then re-radiates this heat back into the atmosphere. A portion of this outgoing heat is trapped by the greenhouse gases, such as carbon dioxide (CO2), methane (CH4) and water vapour, keeping the Earth warm enough to support life.

Role of the Atmosphere

The air surrounding the Earth is called atmosphere. It is held in place by the force of Earth’s gravity. It consists mainly of nitrogen (78%) and oxygen (21%) along with small amounts of argon, carbon dioxide, water vapour, and other gases.

The Earth’s atmosphere has a layered structure. This structure of the atmosphere is important as it helps explain key weather patterns.

Nearly, all weather phenomena take place in the troposphere (with an average height of about 12 km), which is heated from the Earth’s surface. In this layer, temperature decreases with height (~6.5 °C/km). As the warm air rises, it drives winds and storms. The height of the troposphere is maximum above the equator and lowest above the polar regions.

In the stratosphere (12 – 50 km), where the ozone layer is located, ozone absorbs UV rays and heats up the atmosphere, so temperature rises with height. This increase in temperature with height in the stratosphere calms the layer due to the lack of vertical mixing of air, keeping weather confined to the troposphere.

Above the stratosphere are the mesosphere, thermosphere and exosphere, but they play only a minor role in regulating climate on the surface of the Earth. At about 100 km above the Earth is the start of the region, we call the outer space.

The atmosphere plays two crucial roles in protecting the life on the Earth. Firstly, it partly absorbs the incoming solar radiation. The ozone layer blocks the harmful UV rays. Clouds and other gases also absorb some sunlight before it reaches the surface of the Earth. Secondly, it traps outgoing heat. The Earth’s surface absorbs sunlight and re-radiates it in the infrared region. Greenhouse gases like CO2, CH4 and water vapour absorb this re-radiated heat, preventing it from escaping into space. Without the atmosphere, the Earth would be too cold for life to survive. However, excess CO2 from human activities enhances the greenhouse effect, causing global warming, which if left unchecked could make the Earth uninhabitable.

Uneven Heating Causes Wind and Ocean Currents

The movement of air from a region of high pressure to a region of low pressure. These pressure differences are mainly caused by the uneven heating of the Earth’s surface by the Sun. Uneven heating of land and water is also responsible for phenomena, such as land and sea breezes.

Uneven heating of the Earth’s surface also produces local winds, such as valley and mountain breezes. In mountainous Warm air regions, the slopes and the valley floor do not heat up and cool rising V all down at the same rate. During the day, the mountain slopes ze ey ree b b facing the Sun are heated more rapidly than the valley floor. y re lle ez e Va As a result, the air over the slopes becomes warm and rises, creating a low pressure region. Cooler air from the valley moves up the slopes to replace the rising warm air. This flow of air is called a valley breeze.

After sunset, the situation reverses. The mountain slopes lose heat faster and become cooler, while the valley floor remains relatively warmer. The air over the slopes becomes cooler and denser, and flows down into the valley. This is known as a mountain breeze.

Planetary Winds

Uneven heating of the Earth between the equator and the poles create belts of low and high pressure. This large scale pressure difference sets the air in motion over long distances, giving rise to planetary winds.

Near the equator, intense heating by the Sun causes warm air to rise, forming an equatorial low pressure belt. As this air rises, it moves poleward at higher altitudes. On cooling, it becomes denser and sinks around 30° North and South latitudes, forming the sub-tropical high pressure belts.

From these high pressure regions, air flows back towards the equator along the Earth’s surface, completing one circulation cycle. However, not all the sinking air at the sub-tropical belt returns to the equator. A part of it moves towards the poles along the surface, and rises again around 60° North and South latitudes, where it meets the cold air from the polar regions. This creates the sub-polar low pressure belts.

In the polar regions (around 90° North and South), very low temperature causes cold and dense air to sink, forming polar high pressure belts. Air from these regions flows towards the sub-polar belts, completing another circulation cycle. The Earth’s rotation causes these winds to be deflected from their straight paths. As a result, planetary winds follow curved paths rather than moving directly from high pressure to low pressure regions. The winds are deflected towards the right in the Nothern Hemisphere and towards the left in the Southern Hemisphere due to the rotation of the Earth on its axis.

Ocean Currents

Ocean currents are the continuous movement of large masses of ocean water. Similar to winds, planetary pressure differences also drive the ocean currents. Strong planetary winds drag the surface water of the oceans because of friction, setting surface currents in motion. In addition to winds, differences in temperature and salinity, the rotation of the Earth, and the distribution of land masses influence the movement of ocean water.

The water of the equatorial and the polar regions have different temperatures. While warm equatorial waters travel over the surface toward the poles, colder and denser waters slowly flow back towards the equator through deeper ocean levels. Ocean water has different salinities depending on the location. Water with lower salinity, being less dense, tends to remain near the surface, while higher salinity, denser water sinks and moves at deeper levels in the ocean. The Earth’s rotation deflects these moving water masses, forming large circular patterns called gyres. These rotate clockwise in the Northern Hemisphere and counter clockwise in the Southern Hemisphere. Continents further modify these paths by blocking and redirecting currents.

Ocean currents play a major role in regulating the Earth’s climate and supporting life. By transporting heat from the equator towards the poles, they reduce temperature differences across the planet. For example, the warm ocean current - the North Atlantic Drift, an extension of the Gulf Stream (a strong ocean current carrying warm water from the southern part of the North American east coast across the Atlantic Ocean) - flows toward the northwestern coast of Europe and keeps many ports ice-free during winter, even at high latitudes. This moderating influence on climate also supports human activities, such as trade and commerce. Ocean currents also support a massive ecosystem by transporting nutrients.

Biogeochemical Cycles

Living organisms constantly exchange matter and energy with the air, water, soil and rocks around them. This continuous interaction between the non-living (abiotic) and living (biotic) components of the Earth results in the transfer of matter and energy across various spheres of the Earth.

This process ensures that essential nutrients, such as carbon, nitrogen and oxygen are recycled, and remain available to support life on the Earth. This cyclic movement of matter and energy between the abiotic and biotic components is called the biogeochemical cycle.

Water Cycle

In this cycle, water evaporates from water bodies, such as rivers, lakes, oceans, etc., and condenses to form clouds. It returns as precipitation in the form of rain, hail or snow to the surface and finally flows back to the ocean. Some of the water seeps through soil and rocks beneath the Earth’s surface. Water dissolves minerals from soil and rocks. It also supports all terrestrial organisms and transports these nutrients to oceans, and support marine life.

Carbon Cycle

In the fast cycle, which happens over days to years, plants convert atmospheric CO2 into glucose using sunlight through photosynthesis. CO2 is released back into the atmosphere through respiration. Animals eat plants and other animals. When they die, CO2 returns to air because of decomposition.

In the slow cycle, which happens over millions of years, dead plants and animals get buried, and are converted to fossil fuels, such as coal, oil and gas. These fuels are burnt to provide energy for various needs like heating, cooking, transportation and industrial purposes. When fossil fuels are burnt, carbon is released back as CO2 on a very short time scale. The atmosphere and ocean water continuously exchange CO2. The ocean water absorbs atmospheric CO2 to form carbonate and bicarbonate ions. Phytoplankton use them for photosynthesis. Some marine organisms also use them to form shells. When organisms die, they sink to the ocean floor and their organic matter is stored as carbon for a long period.

Nitrogen Cycle

Nitrogen is an essential element for the synthesis of proteins and nucleic acids in all living organisms. The largest reservoir of nitrogen is in our atmosphere. However, nitrogen gas (N2) is rather non-reactive, and cannot be directly used by plants and animals. It must first be converted to soluble compounds that living beings can absorb. The overall movement of nitrogen between air, soil, water and organisms is called the nitrogen cycle. The nitrogen cycle contains several steps - nitrogen fixation, assimilation, ammonification, nitrification and denitrification.

Nitrogen-fixing bacteria, such as Rhizobium in the root nodules of legumes and Azotobacter in the soil, convert atmospheric N2 into ammonia (NH3). Nitrifying bacteria like Nitrosomonas convert ammonia into nitrite (NO2-), while Nitrobacter convert nitrite into nitrate (NO3-). This process is known as nitrification.

Plants assimilate these nitrogen compounds from the soil, whereas animals obtain nitrogen by consuming plants or other animals. When plants and animals die or produce waste, decomposers like bacteria and fungi break the organic matter, returning nitrogen compounds like ammonia to the soil. This process is known as ammonification. Denitrifying bacteria, such as Pseudomonas convert some nitrates back into nitrogen gas. This process is known as denitrification. This completes the cycle and maintains a balance of nitrogen in ecosystems. Lightning also contributes to the fixation of nitrogen oxides.

Oxygen Cycle

Oxygen is one of the Earth’s most abundant elements. About 21% of the atmosphere consist of free oxygen gas (O2). It is an essential component of most biological molecules like carbohydrates, proteins, nucleic acids and fats. Oxygen also exists in combined forms - in the Earth’s crust as metal oxides and minerals, and in the air as carbon dioxide.

Organisms like plants and animals use oxygen for respiration, and releases CO2. Combustion of fuels uses oxygen and releases CO2. Plants restore oxygen through photosynthesis using sunlight, water and CO2 to form glucose and release O2. This balance between consumption (respiration and combustion) and production (photosynthesis) circulates oxygen between the atmosphere, land, oceans and living organisms, sustaining life across all spheres of the Earth.