Term Paper on the Earth Spheres and Cycles | Geography

Here is a term paper on the ‘Earth Spheres and Cycles’ for class 9, 10, 11 and 12. Find paragraphs, long and short term papers on the ‘Earth Spheres and Cycles’ especially written for school and college students.

1. Term Paper on the Earth’s Spheres- Natural and Man-Made:

Earth science generally recognizes four natural spheres, the geosphere (or lithosphere), the hydrosphere, the atmosphere and the biosphere as correspondent to rocks, water, air, and life. The natural cycles for transfer of material and energy amongst these spheres include, rock, carbon, nitrogen and water (hydrological).

However increasing human activities in recent decades have affected activities of the natural spheres. The new sphere of human activities has been termed as anthrosphere or man-made sphere. All these spheres are inter-linked with each other in their activities. The impact of anthrospheric activities on other spheres and global warming.

The nature’s spheres of activities are confined to atmosphere, geosphere, hydrosphere and atmosphere. All the spheres are linked to each other in their activities (tab.7.1). The activities include natural cycles of materials and/or energy transfer amongst the atmosphere, hydrosphere, geosphere, and the biosphere.

These various “spheres” act as “reservoirs” that keep materials for different amounts of time, called residence times. The global warming is linked to generation and absorption of carbon, nitrogen and water in their respective cycles by atmosphere, geosphere, hydrosphere and biosphere (tab. 7.1).

The three material cycles lead to transfer of chemicals from biological to geological systems and are therefore called biogeochemical cycles. Processes that affect these transfers are biological processes such as respiration, transpiration, photosynthesis, and decomposition, as well as geological processes such as weathering, soil formation, and sedimentation.

However the anthrospheric (or human) activities can affect the natural flows or dynamic cycles of materials and energy. When the rates of anthropogenic disruptions are larger than the balancing capacities of the spheres, the system begins to shift, affecting all levels of the ecosystems through local and global changes.

1. Geosphere:

The geosphere is considered to be the portion of the Earth system that includes the solid Earth, its interior, rocks and minerals, landforms and the processes that shape the Earth’s surface. The term “lithosphere” is also used instead of geosphere. However the lithosphere only refers to the uppermost layers of the solid Earth (oceanic and continental crust rocks and uppermost mantle).

The 94% material of the Earth is comprised of oxygen, iron, silica, and magnesium. The interior of the earth is layered both chemically and mechanically. The geosphere is not static. Its surface (crust) is in a constant state of motion that gives rise to movement of the continents. The unifying theory that explains the continental drift is called plate tectonics.

Geosphere is linked to other spheres as follows:

a. Biosphere:

The weathering of the geosphere to form soils provides terrestrial plants with a firm substrate, vital nutrients (phosphorous, nitrogen), and minerals needed for plant growth. In addition, the chemical weathering of the geosphere by water, transports essential nutrients (phosphorous, nitrogen, silica, etc.) to the oceans which are used by algae (marine plants) during photosynthesis.

b. Hydrosphere:

The chemical and mechanical erosion by water can cause the rocks to break mechanically into finer particles and can chemically dissolve elements contained in rock-forming minerals, which are carried away downstream to the ocean. The great depth of the Grand Canyon was formed through erosive cutting by the Colorado River. Over long periods of geologic time, erosion by water can wear down mountain ranges (process called peneplanation) and transport this material to be deposited in sedimentary basins.

c. Atmosphere:

Volcanic eruptions inject significant amounts of gases, such as water vapor, carbon dioxide, sulfur dioxide, hydrogen, hydrogen sulfide, etc. into the atmosphere, thus changing the composition and other characteristics of the atmosphere. For example, explosive volcanism that injects large amounts of sulfur dioxide to the upper atmosphere is known to result in cooling of climate in the years immediately following the eruption.

d. Anthrosphere:

Man has been exploiting the mineral resources of the geosphere by mining of raw materials. For example, the mining and subsequent combustion of fossil fuels, and use of limestone for cement making result in the transfer of large quantities of carbon from the geosphere to the atmosphere. By accelerating the natural rate of transfer of carbon dioxide to the atmosphere, anthrospheric activities can enhance greenhouse effect to a higher temperature level in a shorter period.

2. Biosphere:

The biosphere is the life zone of the Earth. It includes all living organisms, including man, and all organic matter that has not yet decomposed. Life on earth and the corresponding biosphere readily distinguishes our planet from all others in the solar system. The chemical reactions of life (e.g., photosynthesis-respiration) have transformed the atmosphere from reducing conditions, to an oxidizing environment with free oxygen.

The biosphere is structured into a hierarchy known as the food chain whereby all life is dependent upon the first tier (i.e. mainly the primary producers that are capable of photosynthesis). Energy’ and mass is transferred from one level of the food chain to the next with an efficiency of about 10%. All organisms are intrinsically linked to their physical environment and the relationship between an organism and its environment is known as the ecosystem.

Links of the biosphere to other components are as follows:

i. Atmosphere:

Life processes involve chemical reactions, such as, photosynthesis and respiration requiring carbon dioxide and oxygen respectively from atmosphere. Other examples of biogenic gases in the atmosphere include methane, dimethyl-sulfide (DMS), nitrogen, nitrous oxide, ammonia, etc.

ii. Hydrosphere:

Water is essential for all living organisms on Earth. Hence the very existence of biosphere depends on water. Water transports the soluble nutrients (phosphate and nitrate) that are needed for plant growth, and the waste products of life’s chemical reactions.

iii. Geosphere:

The geosphere and biosphere are intimately connected through soils, which consist of mainly mineral matter (alumino-silicate), and also air, organic matter, and water. In fact, one could consider soil as composed of all four spheres (atmosphere, geosphere, biosphere, and hydrosphere). Plant activity, such as root growth, and generation of organic acids, are also important for the mechanical and chemical breakdown (weathering) of the geosphere.

iv. Anthrosphere:

Human population poses a threat to the biosphere by habitat destruction, especially by the destruction of tropical rainforests (deforestation). This process is driving thousands of species each year to extinction, and these results in fewer sinks to absorb carbon dioxide.

3. Hydrosphere:

The hydrosphere includes all water on Earth. Seventy one percent of the Earth is covered by water and only twenty nine percent is land. This unique feature distinguishes our “Blue Planet” from others planets in the solar system, which are devoid of water. It’s the heat from the sun which allows water to exist mainly as a liquid, and a major part of this water is contained in the oceans.

The high heat capacity of this large volume of water (1.35 million cubic kilometers) buffers the Earth surface from large temperature changes. However, depending on surface temperatures and pressures water exists also as solid (ice), and gas (water vapor). Water is the universal solvent, and the basis of all life on our Planet.

Water is the main agent of chemical and mechanical erosion of the earth surface, which includes breaking rocks, dissolving chemical, forming silts and soils. Hydrologic cycle connects the other spheres with the hydrosphere.

i. Atmosphere:

Water is transferred between the hydrosphere and biosphere by evaporation and precipitation. Energy is also exchanged in this process.

ii. Biosphere:

Terrestrial plants withdraw water from the ground using their root systems, and transport water and nutrients through the vascular system to stems and leaves. Evaporation of water from the leaf surface (called transpiration) is effective at transferring water to the atmosphere.

iii. Geosphere:

Water is the primary agent for the chemical and mechanical breakdown of rocks, weathering, to form loose rock fragments (regolith) and soil. By the process of erosion, water sculpts the surface of the Earth. Precipitation that falls on the land makes it way by to the sea. The geomorphology of the Earth is unique because of running water.

iv. Anthrosphere:

Human activity has significantly impacted the supply and quality of water on Earth through agricultural and industrial practices. Chemical contamination of groundwater, lakes, rivers, and the oceans is threatening the quality of the water supply and affecting biosphere in many parts of the world.

4. Atmosphere:

The atmosphere is the gaseous envelope that surrounds the Earth. It begins at the surface of the earth and extends to some 500km above the surface. The lower level, troposphere constitutes the climate system that maintains the conditions suitable for sustaining life and biomes on the earth. The next atmospheric level, stratosphere, contains the ozone layer that protects life on the planet by filtering harmful ultraviolet radiation from the Sun.

Since the Industrial Revolution, the atmospheric heat-trapping “greenhouse” gas contents have consistently increased every year leading to an increase in atmospheric temperatures. This has led to a change in the global climate. Chlorofluorocarbons are effective at depleting the Earth’s ozone shield, which protects the earth surface from the harmful effects of ultraviolet radiation.

Atmosphere is linked to other components as follows:

i. Hydrosphere:

The gases of the atmosphere equilibrate with dissolved gases in water through a process known as gas exchange.

ii. Biosphere:

The process of photosynthesis, which is a respiration cycle, results in exchanges of carbon dioxide and oxygen between the biosphere and atmosphere.

iii. Geosphere:

Volcanic eruptions emit gases to the atmosphere, and atmospheric carbon dioxide dissolves in rainwater to produce a weak acid which is important for the breakdown (weathering) of rock exposed on the surface.

iv. Anthrosphere:

Humans breathe air extracting oxygen and emitting carbon dioxide. In addition, our industrial and agricultural activities have changed the chemical composition of the atmosphere.

5. Anthrosphere:

In order to cater to the needs of growing population, we need factories to produce goods and services mostly at the expense of natural resources. The effect of industrialization and the growing population have caused enormous damage to the biosphere, atmosphere, hydrosphere and geosphere. It is essential to control the rate of growth of the anthrosphere, in order to save our planet from destruction.

Its link to other components are as follows:

i. Atmosphere:

Industrial and agricultural activities have changed the composition of the atmosphere. For example, the concentration of carbon dioxide in the atmosphere has increased by 26%, and doubled the concentration of methane. The production of chlorofluorocarbons is depleting the earth’s ozone layer, our natural defense against ultraviolet radiation.

ii. Hydrosphere:

Humans have impacted the hydrosphere by withdrawing large amounts of groundwater for agriculture and by contaminating rivers, lakes, groundwater, and oceans by organic and industrial wastes.

iii. Biosphere:

Man has clearly altered the natural biosphere through agricultural activities. A prime example is the slash and burn agricultural practice in the tropics, where rainforest is cut and burned, and the land is converted to pasture. Anthropogenic activities like deforestation and pollution of water resources have led to enormous loss of flora and fauna, both in quantity and diversity.

iv. Geosphere:

Mineral and energy resources from the geosphere have fueled the industrial revolution, and that has permitted the human species to increase so prodigiously in number. For example, the exploitation of fossil fuels has increased our standard of living, but an unintended consequence of this action may be climate change and global warming.

2. Term Paper on Cycles amongst the Earth’s Spheres:

The dynamic natural cycles of activities amongst the spheres include, the food chain, carbon cycle, nitrogen cycle, water or hydrologic cycle and rock cycle.

1. Food Chain:

The food chain was an idea developed by a scientist named Charles Elton in 1927. He described the way plants get energy from sunlight, plant-eating animals get their energy from eating plants, and meat-eating animals get their energy from eating other animals. The idea of a “chain” means that ail these animals are linked together, so anything that affects one “link” in the chain affects everything in the chain. The first link in the chain, the plant, is called the producer, while all the links above it are called consumers. The deforestation leads to disruption of food chain with the disappearance of plants, the only producer.

2. The Nitrogen Cycle:

The movement of nitrogen between the atmosphere, biosphere, and geosphere in different forms is described by the nitrogen cycle, one of the major biogeochemical cycles. Similar to the carbon cycle, the nitrogen cycle consists of various storage pools of nitrogen and processes by which the pools exchange nitrogen (Fig.7.2).

The nitrogen cycle is a cycle that mainly exists between microbes and men. All organisms require nitrogen to live and grow. Seventy nine percent of the air is nitrogen. However, due to its inert nature nitrogen present as N₂ molecule cannot be used by plants unless converted or “fixed” in any one of the following usable forms, such as, nitrate ions (NO₃⁺), ammonium(NH₄⁺), and organic nitrogen, e.g., urea (NH₂)₂CO.

Animals secure their nitrogen and all other nitrogen compounds from plants or animals that have fed on plants.

Four processes participate in the cycling of nitrogen through the biosphere:

i. Nitrogen Fixation.

ii. Decay.

iii. Nitrification.

iv. Denitrification.

Microorganisms play major roles in all four of these processes:

i. Nitrogen Fixation:

The inert nitrogen molecules (N₂) need to be broken into active atoms before they can combine (or get fixed) with other atoms. The breaking process requires the input of substantial amounts of energy. Three processes are responsible for most of the nitrogen fixation include, atmospheric fixation by lightning, biological fixation by certain microbe, and industrial fixation.

Atmospheric nitrogen fixation probably contributes some 5-8% of the total nitrogen fixed. In industrial fixation, nitrogen and hydrogen are converted to amonia at high pressure and temperature, which is further processed to fertilisers. Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic environments like rice paddies. Although the first stable product by nitrogen fixing process is ammonia, this is quickly incorporated into protein and other organic nitrogen compounds. This is achieved either by a host plant, the bacteria itself, or another soil organism by the nitrogen uptake process.

ii. Mineralization:

After nitrogen is incorporated into organic matter, it is often converted back into inorganic nitrogen by a process called decay or nitrogen mineralization.

Organic N → NH₄⁺

The decomposition of the dead organisms or their excretions by bacteria and fungi leads to the conversion of a significant amount of nitrogen in the dead organism to ammonium. Once in the form of ammonium, nitrogen is available for use by plants or for further transformation into nitrate (NO₃⁺) through the process called nitrification.

iii. Nitrification:

Ammonia can be taken up directly by plants — usually through their roots. However, most of the ammonia produced by decay is converted into nitrates, by a process called nitrification.

This is accomplished in two steps:

a. Bacteria of the genus Nitrosomonas oxidize NH₃ to nitrites:

NH₄+ → NO₂^–

b. Bacteria of genus Nitrobacter oxidize the nitrites to nitrates:

NO₂ → NO₃^–

Nitrification requires the presence of oxygen, so this process can occur only in oxygen-rich environments like circulating or flowing waters and the surface layers of soils and sediments.

The process of nitrification has some important consequences. The positively charged ammonium ions get attached to negatively charged clay particles and soil organic matter, thus preventing them from being-washed out of the soil (or leached) by rainfall. However, the negatively charged nitrate ions in soil particles can be washed or leached out, leading to decreased soil fertility and nitrate enrichment of downstream surface and groundwater.

iv. Denitrification:

Remove nitrogen from the atmosphere and pass it through ecosystems.

Denitrification reduces nitrates to nitrogen gas and to a lesser extent nitrous gas, thus replenishing nitrogen in the atmosphere

NO₃ → N₂ + N₂O

Denitrification is an anaerobic process that is carried out by denitrifying bacteria, which convert nitrate to nitrogen in the following sequence:

NO₃ → NO₂ →NO → N₂O → N₂.

Nitric oxide and nitrous oxide are both environmentally important gases. Nitric oxide (NO) contributes to smog, and nitrous oxide (N₂O) is an important GHG, thereby contributing to global climate change.

Once converted to nitrogen by denitrification process, it is unlikely to be reconverted to a biologically available form, because it is a gas and rapidly becomes a part of the atmosphere. Denitrification is the only nitrogen transformation that removes nitrogen from ecosystems (essentially irreversibly), and it roughly balances the amount of nitrogen fixed by the nitrogen fixers described above.

Once again, bacteria are the agents for this process. They live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration. Thus they close the nitrogen cycle.

Human alteration of the nitrogen cycle and its environmental consequences:

The consequences of human-caused nitrogen deposition have affected many aspects of the Earth’s ecosystems, such as, precipitation, air and water qualities.

i. Precipitation:

The atmospheric nitrogen oxides form a significant portion of acid rain in the precipitation process. The acid rain has been blamed for forest death and decline in parts of Europe and the Northeast United States. Acid rain can damage and kill aquatic life and vegetation, as well as corrode buildings, bridges, and other structures.

ii. Air Quality:

High concentrations of nitrogen oxides in the lower atmosphere are known to damage living tissues, including human lungs, and decrease plant production. Reactive nitrogen (like NO₃^– and NH₄⁺) present in surface waters and soils can also enter the atmosphere as the smog-component nitric oxide (NO) and the greenhouse gas nitrous oxide (N₂O).

iii. Water Quality:

Adding large amounts of nitrogen to rivers, lakes, and coastal systems, results in eutrophication, a condition that occurs in aquatic ecosystems, when excessive nutrient concentrations stimulate blooms of harmful algae that deplete oxygen, killing fish, and other organisms and ruining water quality. Parts of the Gulf of Mexico, for example, are so inundated with excess fertilizer that the water is clogged with algae, suffocating fish and other marine life.

iv. Effect on Carbon Cycle:

The impacts of nitrogen deposition on the global carbon cycle are uncertain, but it is likely that some ecosystems have been fertilized by additional nitrogen, which may boost the capture and storage of carbon. The use of synthetic nitrogen fertilizers that could be added directly to soil has led to an enormous boom in agricultural productivity. But the decrease in natural nitrogen fixation has serious and potentially harmful effects for humans and other organisms because of the disruption of the natural nitrogen cycle.

3. The Hydrologic Cycle:

The amount of water on the earth is constant, and is divided up between reservoirs in the oceans, in the air, and on the land. The hydrologic cycle can be thought of as a series of reservoirs, or storage areas, and a set of processes that cause water to move between those reservoirs.

Table 7.2 indicates the storage capacities of the reservoirs. The largest reservoir by far is the oceans, which hold about 95% of the earth’s water. The remaining 3.5% is the freshwater which is so important to our survival, but about 78% of that is stored in the ice in Antarctica and Greenland.

About 21% of freshwater on the earth is groundwater, stored in sediments and rocks below the surface of the earth. The freshwater that we see in rivers, streams, lakes, and rain is less than 1% (0.735%) of the freshwater on the earth, and soil moisture constitutes about 1.5% of water.

The sun is the primary source of energy for all hydro meteorological processes. Although water in the hydrologic cycle is constantly in motion, it never leaves the Earth. The Earth is nearly a “closed system” like a terrarium. This means that the Earth neither gains nor loses much matter, including water. In addition, earth’s water is constantly cycling through these reservoirs in a process called the hydrologic cycle. Therefore more water stored in ice sheets means less water in the oceans, and more water in ocean means less ice in artic.

A simplified hydrologic cycle starts with heating caused by solar energy and progresses through stages of evaporation (or sublimation), condensation, precipitation (snow, rain, hail, glaze), groundwater, and runoff (Fig.7.3). Understanding the processes and reservoirs of the hydrologic cycle are fundamental to dealing with many issues, including pollution and global climate change.

Unit processes in the cycle:

The unit processes involve in the hydrological cycle are as follows:

i. Interception:

Intercepted moisture, stored in the canopy, is the first component of the hydrological cycle to be lost directly back to the atmosphere. The total quantity of intercepted water lost by evaporation can be a significant proportion of the total rainfall. Interception of raindrops by canopies is also a major factor in reducing soil erosion. This has an indirect effect on the hydrological cycle, in that, by conserving surface soil, infiltration is maintained.

Precipitation that is not intercepted can be influenced by the following processes:

a. Stem Flow:

Stem flow is the process that directs precipitation down the branches and stems of plants. The redirection of water by this process causes the ground area around the plant’s stem to receive additional moisture. In general, deciduous trees have more stem flow than coniferous vegetation.

b. Canopy Drip: 

Some plants has an architecture that directs rainfall or snowfall along the edge of the plant canopy. This is especially true of coniferous vegetation. On the ground, canopy drip creates areas with higher moisture content that are located in a narrow band at the edge of the plant canopy.

c. Through Fall:

Through fall describes the process of precipitation passing through the plant canopy. This process is controlled by factors like, plant leaf and stem density, type of the precipitation, intensity of the precipitation, and duration of the precipitation event. The amount of precipitation passing through varies greatly with vegetation type.

ii. Evaporation and Transpiration:

Solar energy evaporates large quantities of water from the oceans and a smaller portion from the land. Plants add some water vapor to the atmosphere through transpiration. Water vapor condenses into clouds and finally precipitates, and most of it falls right back into the oceans, which make up most of this planet.

iii. Precipitation:

The circulating water vapour in the atmosphere condenses back to liquid water around particulates like dust, called condensation nuclei, with the fall in temperature. The water droplets create clouds, and on collision with other moving clouds form larger droplets, which eventually become heavy enough to precipitate as rain, snow, or hail.

Though the amount of precipitation varies widely over the surface of the earth, evaporation and precipitation are globally balanced. In other words, if evaporation increases, precipitation also increases. Rising global temperature is one factor that can cause a worldwide increase in evaporation from the world’s oceans, leading to higher overall precipitation.

Some of the clouds are transported over land, where their precipitation falls onto the ground. Most of that precipitation sinks into the shallow layers of soil near the surface (or onto plant surfaces), where it is used immediately by plants, animals, and people. Precipitation comes as rain, snow, sleet, ice pellets, dew, hail, and other types, but the main process is still rain, except in polar regions.

When the water reaches the ground, one of two processes may occur:

(i) Some of the water may evaporate back into the atmosphere, or

(ii) The water may penetrate the surface and become groundwater.

Groundwater either seeps its way to into the oceans, rivers, and streams, or is released back into the atmosphere through transpiration. The balance of water that remains on the earth’s surface is runoff, which empties into lakes, rivers and streams and is carried back to the oceans, where the cycle begins again.

iv. Transpiration:

Plants take up water through their root systems; the water is then pulled up through all parts of the plant and evaporates from the surface of the leaves, a process called transpiration. Water that soaks into the soil can also continue to percolate down through the soil profile into groundwater reservoirs, called aquifers.

v. Runoff:

When too much rain falls onto the ground, to conveniently infiltrate the soil, part of it “runs off’. Gravity pulls it directly downhill. This runoff adds greatly to water levels in rivers and lakes. Occasionally, when runoff radically exceeds infiltration, it creates floods and fresh floods.

vi. Infiltration and Percolation:

Water infiltrates the soil by moving through the surface. Percolation is the movement of water through the soil itself. Finally, as the water percolates into the deeper layers of the soil, it reaches ground water, which is water below the surface. The upper surface of this underground water is called the “water table”. The ground water can intersect with surface streams, it can appear at the surface as springs, and it flows generally downhill toward the ocean.

The Ocean and the Atmosphere:

The oceans are the largest reservoir of liquid water, and it’s here where most of the evaporation occurs. The amount of water vapor in the atmosphere varies widely over time and from place to place; these variations are reflected as changes in the humidity. Water vapor belongs to GHG family and thus can trap heat in the atmosphere while gases like nitrogen (N₂) and argon (Ar) allow heat to escape to space.

The presence of water vapor in the atmosphere helps keep surface air temperatures on the earth comfortable for survival of living organisms and plants, in the range from about -40° C to 55° C. Temperatures on planets without water vapor in the atmosphere, like Mars, stay as low as -100° C.

Since oceans cover around 70% of the earth’s surface, most precipitation falls right back into the ocean and the cycle begins again.

Humans and the Hydrologic Cycle:

Atmospheric and oceanic circulations are two of the major factors that determine the distribution of climatic zones over the earth. Changes in the cycle or circulation can result in major climatic shifts. For example, if average global temperatures continue to increase as they have in recent decades due to anthropogenic increase in GHGs, water that is currently trapped as ice in the polar ice sheets will melt, causing a rise in sea level. Water also expands as it gets warmer, causing further sea level rise.

Many heavily populated coastal areas like New Orleans, Miami, and Bangladesh will be inundated by a meager 1 meter increase in sea level. Additionally, the acceleration of the hydrologic cycle with increasing atmospheric temperature may result in severe weather and extreme conditions. Some scientists believe that the increased frequency and severity of El Nino events in recent decades are due to the acceleration of the hydrologic cycle induced by global warming.

4. Carbon Cycle:

An important ecosystem in connection with global warming is the carbon cycle. Carbon is the basis of all organic molecules. The carbon dioxide is the main source for natural process of heat generation in the atmosphere.

A simplified carbon cycle diagram is shown in Fig.7.4. Carbon cycle is the process through which carbon is transferred through the air, ground, plants, animals, and fossil fuels. Large amount of carbon exists in the atmosphere as carbon dioxide (CO₂). Carbon dioxide is cycled by green plants during the process known as photosynthesis to make carbohydrate. The product provides the nourishment of every heterotrophic organism.

The concentration of carbon in living matter (18%) is almost 100 times greater than its concentration in the earth (0.19%). Living matters, therefore extract carbon from the nonliving environment. For life to continue, this carbon must be recycled.

Carbon exists in the nonliving environment as:

i. Carbon dioxide (CO₂) in the atmosphere and dissolved in water (forming HCO₃);

ii. Carbonate rocks (limestone and coral = CaCO₃);

iii. Deposits of coal, petroleum, and natural gas derived from once-living things or dead organic matter, e.g., humus in the soil.

The global carbon cycle, one of the major biogeochemical cycles, can be divided into geological and biological components. The geological carbon cycle operates on a time scale of millions of years, whereas the biological carbon cycle operates on a time scale of days to thousands of years.

The Geological Carbon Cycle:

The geological component of the carbon cycle is where it interacts with the rock cycle in the processes of weathering and dissolution, precipitation of minerals, burial and subduction, and volcanism. In the atmosphere, carbonic acid forms from carbon dioxide (CO₂) and water vapor, reaches the earth as rain, reacts with minerals at the earth’s surface(chemical
weathering), runs along with surface waters like streams and rivers eventually to the ocean, where they precipitate out as minerals like calcite (CaCO₃). The continued deposition and burial, this calcite sediment forms the rock called limestone (rock cycle).

This cycle continues as seafloor spreading pushes the seafloor under continental margins in the process of subduction. As seafloor carbon is pushed deeper into the earth by tectonic forces, it heats up, eventually melts, and can rise back up to the surface, where it is released as CO₂ and returned to the atmosphere.

This return to the atmosphere can occur violently through volcanic eruptions, or more gradually in seeps, vents, and CO₂-rich hot springs. Tectonic uplift can also expose previously buried limestone. One example of this occurs in the Himalayas where some of the world’s highest peaks are formed of material that was once at the bottom of the ocean. Weathering, subduction, and volcanism control atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years.

Carbon Cycle in Biosphere:

Biology plays an important role in the movement of carbon between land, ocean, and atmosphere through the processes of photosynthesis and respiration. Carbon enters the biotic world through the action of autotrophs. Autotrophy is the capability of synthesizing organic molecules from inorganic raw materials. Virtually all multicellular life on Earth depends on the (photosynthesis) and the metabolic breakdown (respiration) of those sugars to produce the energy needed for movement, growth, and reproduction.

Photoautotrophs:

Photoautotrophs plants, algae, and some bacteria – use light as the source of the needed energy, through a process called photosynthesis.

Photosynthesis:

Green plants produces carbohydrate (sugars) from carbon dioxide and water by sunlight in photosynthesis.

Chemoautotrophs:

Chemoautotrophs bacteria and archea that do the same but use the energy derived from an oxidation of molecules in their substrate.

Carbon returns to the atmosphere and water by biological cycle.

Respiration (as CO₂):

Respiration by plants and animals and the metabolism result in a reverse photosynthesis forming carbon dioxide, which is in turn released to the atmosphere:

C₆H₁₂O₆ (Organic matter) + 6O₂ 6CO₂ + 6 H₂O + Energy

Cellular respiration is the process by which organisms release energy from complex organic molecules, typically sugars. All living things, including plants, respire. In the absence of oxygen, anaerobic respiration occurs, producing lactic acid or ethanol.

The majority of the carbon dioxide in the air comes from heterotrophic respiration, and not from the respiration of plants. Other processes of releasing carbon to the atmosphere include the burning & decay of organic bodies, thus producing CO₂ if oxygen is present, otherwise producing methane (CH₄). The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is about 1,000 times greater than the amount of carbon that moves through the geological cycle on an annual basis.

Carbon Cycle in Ocean (Hydro-Biosphere):

In the oceans, phytoplankton (microscopic marine plants that form the base of the marine food chain) use carbon to make shells of calcium carbonate (CaCO₃). The shells settle to the bottom of the ocean when phytoplankton dies and are buried in the sediments. These and other shells are buried and compressed over time to transform eventually into limestone.

Similarly, under certain geological conditions, organic matter can be buried and over time form deposits of carbon-containing fuels, coal and oil. Both limestone and fossil fuel formations are biologically controlled processes and represent long-term sinks for atmospheric CO₂.

Uptake and return of CO₂ are not in balance role of anthropogenic carbon. The carbon dioxide content of the atmosphere is gradually and steadily increasing. The increase in CO₂ began with the start of the industrial revolution, and its concentration has risen over 20% by now. This increase is thus of “anthropogenic” origin, that is, caused by human activities; such as, burning fossil fuels (coal, oil, natural gas), and clearing and burning of forests. In addition to global warming & climate change, the increase in CO₂ retards plant growth.