What Is One Way That Carbon Is Returns From The Atmosphere?

Photosynthesis Returns Carbon to the Biosphere

One of the main ways that carbon is returned from the atmosphere to the biosphere is through photosynthesis by plants and algae. During photosynthesis, plants and algae absorb carbon dioxide (CO2) from the atmosphere. The CO2 molecules are then converted into carbohydrates through chemical reactions powered by sunlight.

These carbohydrates make up the biomass that comprises the plant’s stems, leaves, roots and so on. So the carbon that was originally in the atmospheric CO2 becomes incorporated into the plant tissues as an integral part of the biomass. In this way, photosynthesis draws carbon out of the atmosphere and returns it to the biosphere in the form of living plants and algae.

The carbon remains locked in the plant biomass until the plant dies and decays or is eaten. But overall, photosynthesis is a massive natural carbon sink that can remove billions of tons of carbon from the air each year and store it in the terrestrial biosphere.

Respiration by Organisms

respiration releases co2
All animals, including humans, release carbon dioxide (CO2) into the atmosphere through the process of respiration. This includes breathing, in which oxygen is inhaled and CO2 is exhaled. But more CO2 is actually produced internally through cellular respiration. This is the process in which organisms like animals, plants, and microbes break down carbohydrates, fats, and proteins from food and convert them into energy, releasing CO2 as a byproduct.

The chemical equation for cellular respiration is:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy

This shows that for every molecule of glucose (C6H12O6) broken down, six molecules of CO2 are produced. The CO2 diffuses out of cells and into the bloodstream, from which it travels to the lungs and is exhaled. This process continuously releases carbon from biological sources back into the atmosphere.


Dead organic matter such as dead plants, animals, and microorganisms decay through the process of decomposition. Decomposition is carried out primarily by bacteria and fungi which break down tissues and release carbon dioxide gas (CO2) in the process. As organic matter decomposes, carbon that was previously incorporated into biomass through photosynthesis is returned from the biosphere back into the atmosphere as CO2.

On land, decomposition happens in the soil and leaf litter where dead plant matter such as leaves, bark, and plant roots are broken down. In water, decomposition happens to submerged organic matter such as algae and aquatic plants. The CO2 released from decomposition eventually diffuses into the atmosphere.

Decomposition is a vital component of the carbon cycle, as it allows carbon stored in organic matter to be recycled. Without decomposer organisms breaking down dead biomass, carbon would remain locked up in organic matter and be unavailable for future photosynthesis. The CO2 released from decomposition is utilized by autotrophs such as plants to continue fixing carbon during photosynthesis.


One way carbon dioxide is returned to the atmosphere is through combustion, or burning. The burning of fossil fuels like coal, oil and natural gas releases ancient carbon stored underground back into the air as carbon dioxide. When we burn gasoline in our cars, the carbon in the fuel combines with oxygen to produce CO2 which is then emitted from the tailpipe. Power plants also burn huge amounts of fossil fuels to generate electricity, releasing copious amounts of CO2.

In addition to burning fossil fuels, wildfires in forests also combust biomass and release the carbon stored in trees and plants back into the atmosphere. In a forest fire, the carbon stored in the organic matter of trees and vegetation is converted into carbon dioxide as it burns. Intense wildfires, exacerbated by hot and dry conditions, can release massive plumes of CO2 into the air. Overall, combustion processes including fossil fuel burning and wildfires result in billions of tons of carbon being emitted into the atmosphere each year.

Ocean-Atmosphere Exchange

The world’s oceans play a crucial role in removing carbon dioxide from the atmosphere through the physical process of gas exchange. At the ocean surface, gases like carbon dioxide move freely between the ocean and atmosphere in response to differences in partial pressure. Because surface seawater typically contains much higher dissolved CO2 concentrations compared to air, the oceans act as a net sink for atmospheric CO2.

As CO2 from the air dissolves into the surface ocean, it reacts with seawater to form carbonic acid. This carbonic acid rapidly dissociates into bicarbonate ions and carbonate ions, which makes the CO2 available for biological processes and ocean mixing. The dissolved inorganic carbon is incorporated into marine food webs, with phytoplankton using it for photosynthesis. Plants and animals also convert it into organic carbon through biological activity. Through ocean circulation and the biological pump, this absorbed carbon can become isolated from the atmosphere for centuries.

Experts estimate the oceans have absorbed about 30% of the total anthropogenic CO2 emissions produced over the past 200 years. This ocean carbon sink serves a vital role in moderating the pace of climate change. However, increased CO2 absorption leads to ocean acidification, threatening marine ecosystems.

Carbon Sequestration

Carbon sequestration refers to the long-term capture and storage of carbon dioxide from the atmosphere. It is one of the ways that carbon can be removed from the atmosphere and returned to the land or oceans. There are two main types of carbon sequestration: biological and geological.

Biological carbon sequestration involves increasing carbon storage in soils, plants, and other biomass through land management practices. Things like planting trees, restoring wetlands, and improving soil quality can increase the ability of ecosystems to absorb and store carbon. Sustainable agriculture and forestry practices aimed at increasing biomass production can result in additional carbon storage as plants grow. The carbon absorbed by plants during photosynthesis is stored in roots, stems, branches and leaves. When plants die and decompose, some of that carbon makes its way into soils. In this way, biological carbon sequestration uses natural processes to pull CO2 out of the atmosphere.

Geological carbon sequestration, also known as carbon capture and storage, involves capturing CO2 from industrial processes or directly from the atmosphere, transporting it by pipeline, then injecting it deep underground where it can be stored for centuries or longer. Potential underground storage sites include depleted oil and gas reservoirs, deep saline aquifers, and basalt formations. The CO2 is trapped underground by impermeable cap rock layers that prevent it from escaping. While promising, geological sequestration is still an emerging technology with some risks and uncertainties. Overall, carbon sequestration through land management and underground injection provides ways for carbon to be returned from the atmosphere to long-term storage reservoirs.

Weathering of Rocks

One of the ways carbon dioxide is removed from the atmosphere is through chemical weathering of rocks. When carbon dioxide dissolves in rainwater, it forms a weak acid called carbonic acid. The carbonic acid can react with minerals in rocks to form new compounds. For example, carbonic acid reacts with minerals like calcium silicate to form calcium bicarbonate, which is dissolved in water and transported to the oceans. The calcium bicarbonate eventually forms limestone on the ocean floor, locking up the carbon from the carbon dioxide in solid sedimentary rock.

Over geologic timescales, the weathering of silicate rocks on land and the subsequent formation of carbonates like limestone on the seafloor is a major process for removing carbon dioxide from the atmosphere. The carbon dioxide removed through chemical weathering and locked up in carbonate sediments helps regulate Earth’s climate over millions of years.

Volcanic Emissions

Volcanic eruptions release carbon dioxide (CO2) from magma (molten rock) below the Earth’s surface. When a volcano erupts, gases dissolved in the magma are released into the atmosphere. One of these gases is CO2.

While volcanic eruptions can release large amounts of CO2 over short periods of time, their overall impact on the carbon cycle is small compared to human CO2 emissions from burning fossil fuels. Scientists estimate that volcanoes release about 200 million tons of CO2 into the atmosphere each year. In contrast, human activities like driving cars and burning coal release over 100 times more – about 26 billion tons of CO2 per year.

So while volcanic eruptions do release CO2, their emissions are dwarfed by those from human industrial activities. Volcanoes are not a major player in returning carbon to the atmosphere compared to the impact of humanity’s carbon footprint.

Limestone Formation

Limestone rock is formed from the skeletal remains of marine organisms like coral, clams, oysters, and coccolithophores (a type of phytoplankton). Over geologic timescales, the calcium carbonate shells of these organisms accumulate on the seafloor and are compressed into sedimentary limestone deposits. This geological process removes carbon dioxide from the atmosphere and oceans and locks it away in solid mineral form.

The marine organisms extract calcium and carbonate ions from seawater to build their shells and skeletal structures in a process called biomineralization. When the organisms die, the calcium carbonate shells sink to the seafloor and accumulate in shallow marine environments, often forming large reef structures or carbonate platforms. Over long periods of time, these skeletal remains are lithified into limestone rock by heat and pressure.

Limestone deposits can stretch for miles and reach thousands of feet thick. The formation of limestone represents a significant carbon sink, sequestering atmospheric CO2 and storing it in Earth’s rocks where it can remain locked up for millions of years. This long-term carbon storage helps regulate Earth’s climate and the concentration of greenhouse gases in the atmosphere.

Human CO2 Emissions

Human activities such as fossil fuel burning and land use changes are a major cause of rising atmospheric carbon dioxide (CO2) levels. The burning of coal, oil, and natural gas for energy, transportation, heating, and industry releases billions of tons of carbon into the atmosphere every year. Deforestation and changing land use patterns also release significant amounts of CO2.

As a result, atmospheric CO2 levels have risen over 40% since pre-industrial times, from about 280 parts per million (ppm) to over 400 ppm today. Much of this anthropogenic CO2 persists in the atmosphere, as natural carbon sinks like oceans and vegetation cannot absorb it all. Human CO2 emissions are therefore the leading cause of rising atmospheric carbon that drives climate change and ocean acidification.

Reducing the burning of fossil fuels, switching to renewable energy sources, curbing deforestation, and protecting natural CO2 sinks can help mitigate this impact. But steep reductions in anthropogenic emissions are needed to stabilize atmospheric CO2 levels and meet global climate goals. Human activities have dramatically increased the flow of carbon from fossil fuel reserves to the atmosphere, disrupting the natural carbon cycle.

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