The Carbon Cycle (3.1.1.3)

Lithosphere store features

The lithosphere holds carbon in sedimentary rocks, fossil fuels and soil. It  is the largest carbon store holding 99.9% of the world’s carbon with an incredibly long residence time of up to 300 million years making it a slow carbon sink. Carbon enters the store by dead organisms and shells getting buried and overtime pressure and chemical processes can turn their carbon into sedimentary rocks and fossil fuels. Carbon leaves the store through volcanic activity, chemical weathering and fossil fuel combustion.

Hydrosphere store features

The hydrosphere holds carbon mostly in the oceans. It is the world’s second largest store of carbon. Carbon that is held close to the oceans surface exchanges with the atmosphere relatively quickly whilst carbon deep in the ocean water can be held for thousands of years. The oceans absorb 25-30% of anthropogenic (human caused) CO2. Carbon enters by dissolving into the oceans from the atmosphere and rivers carry carbon from land into the oceans. Carbon leaves by going back into the atmosphere or marine organisms use the carbon in the oceans to create shells that sink and are buried forming ocean sediments and becoming part of the lithosphere. 

Cryosphere store features

The cryosphere holds carbon in ice. The most significant is permafrost which holds 1400 gigatons of carbon. Permafrost is partially decomposed matter that is frozen into the soil. Low temps limit microbial activity so carbon can remain frozen for thousands of years. However if permafrost thaws due to warming global temps this will allow microbes to decompose the organic matter releasing carbon and methane into the atmosphere. This creates positive feedback loops because warming melts the permafrost causing a release of CO2 which then causes more warming and more melting. 

Biosphere store features

The biosphere holds carbon in plants, animals and microorganisms. Carbon enters through photosynthesis and then moves through food chains as animals gain carbon by eating plants or other animals. It then returns to the atmosphere through respiration, decomposition or combustion. The average residency time is 18 years. Forests store huge amounts of carbon in vegetation. 

Atmosphere store features

The atmosphere is a small but dynamic store as carbon moves quickly in and out of it. Human activities such as burning fossil fuels have increased the amount of carbon it stores. The atmosphere stores carbon mostly as CO2 with smaller amounts as methane. 

Carbon transfers driving change in the magnitude of these stores

• Weathering

• Photosynthesis

• Respiration

• Decomposition

• Combustion

• Carbon sequestration

How does weathering drive change in the carbon cycle?

Acid rain causes chemical reactions that break down rocks moving carbon into rivers and then oceans.

How does photosynthesis drive change in the carbon cycle?

Plants absorb carbon from the atmosphere and use it to create organic matter transferring carbon to the biosphere. Carbon dioxide + water —light energy—} glucose + oxygen

How does respiration drive change in the carbon cycle?

Plants, animals and microorganisms release carbon dioxide as they break down sugars for energy returning carbon to the atmosphere. Glucose + oxygen —} carbon dioxide + water + energy (oxygen is used to break glucose into energy with CO2 released as a by-product. 

How does combustion drive change in the carbon cycle?

Burning biomass or fossil fuels releases stored carbon into the atmosphere.

How does burial drive change in the carbon cycle?

Dead organisms or organic matter becomes buried in sediments which over long periods some carbon can form into sedimentary rocks or fossil fuels.

How does carbon sequestration drive change in the carbon cycle?

Carbon is captured and stored for a long time e.g. in forests, soils, oceans. In the natural cycle plant growth can sequester atmospheric carbon whilst human methods can also capture and store CO2 underground.

Scales of carbon transfers

1. Plant scale

2. Sere cale

3. Continental scale

Plant Scale

Carbon moves into and out of an individual plant. It enters through photosynthesis and can leave through either respiration or decomposition when the plant dies. These processes can happen in minutes or over seasons.

Sere Scale

Carbon moves as a place changes through ecological succession (gradual development of an ecosystem). As vegetation changes it affects carbon storage and transfers.

Continental Scale

Transfers of carbon operate across whole continents over long periods e.g. weathering moves carbon from rocks into rivers which then travels into the ocean. This carbon can later be stored as marine sediment. Carbon is also released through volcanic eruptions. These processes can take thousands to millions of years.

Photosynthesis equation

Carbon dioxide + water —light energy—} glucose + oxygen

Respiration equation

Glucose + oxygen —} carbon dioxide + water + energy (oxygen is used to break glucose into energy with CO2 released as a by-product. 

Physical causes of change in the carbon cycle over time

• Glacial and Interglacial periods

• Wildfires

• Volcanic activity

Why do glacial and interglacial periods cause changes in the carbon cycle?

During glacial periods colder oceans are able to absorb more CO2 however less biological activity reduces carbon transfer through ecosystems. During warmer interglacial periods oceans may release more CO2 but vegetation growth can increase photosynthesis and carbon storage.

Why do wildfires cause changes in the carbon cycle?

Fires burn vegetation and release stored carbon into the atmosphere as CO2. They also reduce the amount of vegetation able to absorb CO2 through photosynthesis. However they also burn away dead or decaying organic matter and recycle some of the nutrients and carbon back into the soil which helps to stimulate new plant growth.

Why do volcanic eruptions cause changes in the carbon cycle?

Volcanic eruptions release CO2 from the lithosphere into the atmosphere. Large eruptions can also release ash and gas that can temporarily cool the climate by reducing incoming sunlight which affects the rate of photosynthesis in plants and therefore carbon intake. 

Impacts of cold weather on transfers/stores

• More Co2 dissolves in cold water

• Frozen soils slow transfers

• Decomposition decreases

• Vegetation changes

More Co2 dissolves in cold water

Cold ocean water can hold more dissolved CO₂ than warm water. During colder periods the oceans may therefore take up more carbon from the atmosphere increasing the hydrosphere store.

Frozen soils slow transfers

When soils freeze, biological activity is limited. Carbon transfers between soil, plants and microorganisms slow down as the frozen soils lock the carbon in place and prevent microbial decay. Repeated freeze-thaw cycles disrupt soil structure causing pulses of greenhouse gas release.

Decomposition decreases

Cold temperatures reduce decomposer activity so dead organic matter breaks down more slowly. This means that organic matter accumulates and becomes trapped in soils and permafrost instead of being released quickly into the atmosphere.

Vegetation changes

Cold conditions can disrupt the growth of forests and affect where forests can grow. Slower plant growth means less photosynthesis and therefore carbon intake. 

Advantage of a wildfire

Burns away any dead or decomposing plants adding nutrients into the soil creating more space for new plants to grow and adding nutrients into the soil allowing whole ecosystems to regenerate for future carbon sequestration. 

Human causes of change in the carbon cycle over time

• Combustion of fossil fuels

• Land-use change

• Farming practices

• Deforestation

How is the combustion of fossil fuels a human cause of change to the carbon cycle?

Burning coal, oil and gas transfers carbon stored in the lithosphere to the atmosphere as CO2. This adds carbon faster than natural processes (photosynthesis and ocean absorption) can remove it. It is the cause of 90% of anthropogenic CO2 emissions.

How is land-use change a human cause of change to the carbon cycle?

Areas of previously forested land may be used to build infrastructure on such as roads or houses. The cement needed to build different infrastructure accounts for 2.4% of global emissions. The land may also be used for agricultural purposes. This removes plants that take in and store carbon. Peatlands are often drained to make space for more agricultural land or so the peat can be used as fuel. This releases ancient carbon into the atmosphere.

How are farming practices a human cause of change to the carbon cycle?

Ploughing exposes organic matter that is then decomposed releasing CO2. Livestock and rice farming also releases 40% of anthropogenic methane into the atmosphere. Farm machinery also burns fossil fuels.

How is deforestation a human cause of change in the carbon cycle?

Cutting trees down removes a vital carbon sink. Forests absorb 20% of global emissions. Burning forests immediately releases carbon stored in vegetation as well as reducing future absorption. 

What is the carbon budget?

The data describing the inputs, outputs and stores of carbon in a cycle

What has disrupted the modern carbon budget?

Human activity has massively increased carbon emissions especially through fossil fuel combustion and deforestation. Carbon sinks like forests and the oceans cannot handle the rising levels of atmospheric carbon leading to an enhanced greenhouse effect which causes global warming. 

Positive influence of the carbon cycle on land

Carbon stored in vegetation supports further biomass growth. Carbon held in soils helps the soil develop and maintain fertility allowing further plant growth. 

Negative influence of the carbon cycle on land

Loss of carbon in soils e.g. through deforestation means the soil is less fertile, decreasing agricultural productivity. Higher temperatures also speed up the rate of decomposition of dead organic matter releasing more C02 into the atmosphere. 

Positive influence of the carbon cycle on oceans

Marine organisms use carbon to build shells from calcium carbonate. When these organisms die some of their remains sink and are buried in sediments storing carbon for a very long time as sedimentary rock. Phytoplankton near the ocean surface absorb carbon through photosynthesis. Their carbon is then passed through the marine food web and eventually reaches the deep ocean where it can be stored for a long time or eventually will become buried in seafloor sediments. This process is called the biological pump. 

Negative influence of the carbon cycle on oceans

Due to increased carbon levels in the atmosphere the ocean has to absorb additional amounts. This causes ocean acidification which damages coral reefs and marine organisms that have shells.

Positive influence of the carbon cycle on the atmosphere

Carbon dioxide in the atmosphere contributes to the natural greenhouse effect. It traps some heat, keeping Earth warm enough to support life. 

Negative influence of the carbon cycle on the atmosphere

Extra C02 in the atmosphere enhances the greenhouse effect raising global temperatures. This can shift precipitation patterns because warmer air is able to hold more moisture contributing to heavier rainfall and therefore flooding in some areas. Rising temperatures also increases the chance of extreme weather events occurring such as heat waves and droughts. Changes in ocean temperature and atmospheric circulation can also increase risk of tropical storms in some regions.

Regional factors affecting the carbon cycle

• Vegetation

• Deforestation

• Increased amounts of phytoplankton

• Volcanic eruptions

How can more vegetation be seen as a regional factor affecting the carbon cycle?

In a very densely vegetated region like the Amazon rainforest there may be more carbon taken in through photosynthesis meaning more is stored in their biomass. Leaf litter and dead/decomposing plants also add carbon into the soil. Plants will release some carbon through decomposition and respiration but if photosynthesis is greater overall the region acts as a carbon sink. Densely packed vegetation will also release high amounts of water vapour affecting local humidity, cloud cover and rainfall. These conditions then influence the growth of further plants.

How can deforestation be seen as a regional factor affecting the carbon cycle?

When trees are cut down or burned, carbon stored in vegetation is released. This is especially if the wood is burned or left to decay. Some reasons for deforestation include commercial/subsistence farming, logging and mineral extraction/mining. Commercial farmers clear huge areas or forested land to make way for large scale cattle ranching or soy bean farming. In the Amazon rainforest around 20% of the vegetation has been cleared to make way for commercial farming practices. For subsistence farmers a lot of tribes in the Amazon use a method called slash and burn which allows the ash from the trees to create very nutrient rich soil to grow crops on. However this regularly gets out of control and can cause large areas to burn. Removing trees means less photosynthesis will occur so less carbon is being removed from the atmosphere. It also reduces further carbon sequestration in the future.

How can increased amounts of phytoplankton be seen as a regional factor affecting the carbon cycle?

Increased phytoplankton can affect the carbon cycle across a particular region of ocean by absorbing more dissolved carbon dioxide through photosynthesis. This can encourage more carbon dioxide to diffuse from the atmosphere into the surface water increasing the hydrosphere store. When phytoplankton are eaten or die, some of their carbon sinks to deeper water in their remains and waste, transferring carbon away from the atmosphere and surface ocean through the biological pump. Once it reaches the bottom of the ocean some of this carbon will form sedimentary rock after burial and compaction. The effects of this depend on ocean temperature and upwelling. Colder water is able to absorb more carbon dioxide whilst upwelling brings nutrient-rich deep water to the surface supporting phytoplankton growth though it can also bring carbon rich water back to the surface which would create less of a concentration gradient so less carbon could be absorbed.

How can volcanic eruptions be seen as a regional factor affecting the carbon cycle?

A volcanic eruption affects the carbon cycle in the region around it by releasing carbon dioxide into the atmosphere. It can also release ash and volcanic gas that block the sun’s rays, causing temporary cooling known as a volcanic winter. The cooling can reduce photosynthesis in affected areas, so vegetation may absorb less carbon dioxide. The eruption’s direct carbon dioxide release and its effects on plant uptake can therefore alter the regional carbon budget.