Movement of Carbon (AQA A-Level Geography): Revision Notes

Movement of carbon

Carbon constantly moves between different storage locations on Earth through various natural and human-influenced processes. Understanding these movements is essential for comprehending how the carbon cycle functions and how human activities are affecting it.

The carbon cycle overview

The processes that move carbon between these stores are called transfers or fluxes. Think of these as pathways along which carbon travels.

When we analyse carbon stores, we can classify them as either:

• Carbon sinks - stores that absorb more carbon than they release
• Net carbon sources - stores that release more carbon than they absorb

This classification is dynamic and can change over time. For example, a forest might act as a carbon sink whilst it grows, but become a carbon source if it experiences a large fire.

The geological component

The geological component represents the slowest part of the carbon cycle, operating over timescales of millions of years. It involves the interaction between carbon and the rock cycle through several key processes.

Chemical weathering

Carbon dioxide in the atmosphere dissolves in water to form a weak acid called carbonic acid:

$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3$

When this acidic water (rain) reaches the Earth's surface, it slowly reacts with minerals in rocks through chemical weathering. This process gradually breaks down the rocks and dissolves them into their component ions. These dissolved ions are then carried by streams and rivers into the ocean.

In the ocean, the ions settle out and form minerals. A particularly important mineral is calcite - a form of calcium carbonate (CaCO₃). This mineral can be precipitated directly from seawater or formed by marine organisms.

Carbon burial in sediments

Marine organisms such as foraminifera, coccoliths and molluscs use calcium and bicarbonate ions from seawater to build their shells and skeletons from calcium carbonate. When these creatures die, their remains sink to the ocean floor where they accumulate as sediment.

Over time, layer upon layer of this sediment builds up. Eventually, the weight and pressure transform these sediments into sedimentary limestone. This process effectively locks carbon away beneath the ocean floor for millions of years.

Tectonic processes and volcanism

Tectonic forces drive plate movement, which can push the sea floor beneath continental margins through subduction. The carbonaceous sea-floor deposits are pushed deep into the Earth where intense heat causes them to melt.

This molten material can then rise back to the surface through volcanic eruptions, or through seeps, vents and CO₂-rich hot springs. When this happens, carbon dioxide returns to the atmosphere.

These geological processes of weathering, burial, subduction and volcanism control atmospheric carbon dioxide concentrations over timescales of hundreds of millions of years, creating a natural thermostat for Earth's climate.

Photosynthesis

Photosynthesis is the fundamental process by which plants and certain microorganisms capture carbon from the atmosphere and convert it into living tissue.

Tiny marine plants called phytoplankton in the sunlit surface waters of the oceans (the euphotic zone), along with terrestrial plants and photosynthetic algae and bacteria on land, carry out this vital process.

The chemical equation for photosynthesis is:

$\text{carbon dioxide} + \text{water} + \text{sunlight} \rightarrow \text{carbohydrate} + \text{oxygen}$

Or in chemical symbols:

$\text{CO}_2 + \text{H}_2\text{O} + \text{sunlight} \rightarrow \text{CH}_2\text{O} + \text{O}_2$

Through photosynthesis, carbon is transformed from an atmospheric gas into solid organic matter. The carbohydrates produced store chemical energy. This process removes carbon dioxide from the atmosphere whilst adding oxygen.

Respiration

Whilst photosynthesis removes carbon dioxide from the atmosphere, respiration does the opposite by releasing it back.

Plants themselves use some of their stored carbohydrates for respiration to power their life processes. The carbohydrates that remain form the plant's biomass (its bulk and structure). Animals and bacteria obtain their energy by consuming this biomass and using respiration to extract the stored energy.

The chemical equation for respiration is:

$\text{oxygen} + \text{carbohydrate} \rightarrow \text{energy} + \text{water} + \text{carbon dioxide}$

Or in chemical symbols:

$\text{O}_2 + \text{CH}_2\text{O} \rightarrow \text{energy} + \text{H}_2\text{O} + \text{CO}_2$

Notice that photosynthesis and respiration are essentially opposite processes. Photosynthesis removes CO₂ from the atmosphere and adds O₂, whilst respiration removes O₂ and adds CO₂.

However, these processes are not perfectly balanced across the Earth. Not all organic matter is consumed through respiration. Some gets buried in sedimentary rocks before it can be oxidised. Over geological time, this has resulted in more oxygen being added to the atmosphere through photosynthesis than has been removed through respiration, which explains why our atmosphere is oxygen-rich.

Decomposition

When organisms die, their organic matter doesn't simply disappear. It is broken down through decomposition.

Decomposition mechanisms include:

• Physical mechanisms - fragmentation caused by animals, wind, water movement, and even other plants. Leaching and transport in water represents another important physical mechanism.
• Chemical mechanisms - processes such as oxidation and condensation that break down organic compounds.
• Biological mechanisms - feeding and digestion by decomposer organisms, aided by enzymes that speed up the breakdown process.

Through decomposition, the essential elements of life (carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur and magnesium) are continually recycled back into the soil, making them available for new life.

Oceanic carbon pumps

The oceans play a crucial role in the carbon cycle by storing vast amounts of carbon dioxide. Water can dissolve CO₂, and there is an important relationship between water temperature and the amount of CO₂ that can be dissolved.

Vertical deep mixing

As the water cools in high-latitude regions, it becomes denser and sinks below the surface layer. The cold water can hold more dissolved CO₂, so it carries carbon dioxide down with it to the ocean depths.

Meanwhile, cold water from the depths returns to the surface in low-latitude regions through upwelling. As this cold water warms up, it loses its capacity to hold as much dissolved carbon dioxide, so CO₂ is released back into the atmosphere.

This vertical circulation continuously exchanges carbon dioxide between the ocean and the atmosphere. The circulation also acts as an enormous carbon pump, storing much more carbon in the ocean than would be possible if surface water was not being constantly replenished with cold, carbon-rich water from the depths.

The biological carbon pump

Living organisms in the ocean create another vital mechanism for moving carbon from the atmosphere into the deep ocean and eventually into rocks.

The process works like this:

1. Living organisms in the ocean incorporate carbon from seawater into their bodies, either as organic matter or as structural calcium carbonate (for shells and skeletons)
2. When these organisms die, their cells, shells and other remains sink into deep water
3. Decay of this organic material releases carbon dioxide into the deep water
4. Some material sinks all the way to the bottom, where it forms layers of carbon-rich sediments
5. Over millions of years, chemical and physical processes may transform these sediments into rocks, effectively locking up the carbon for geological timescales

Combustion

Combustion is the process that occurs when organic material burns in the presence of oxygen, releasing carbon dioxide, water and energy.

All organic materials contain at least carbon and hydrogen atoms, and may also contain oxygen. When other elements like sulphur or nitrogen are present, combustion produces a variety of pollutant molecules including sulphur oxides and nitrogen oxides.

Biomass combustion

Biomass combustion refers to the burning of living and dead vegetation. This occurs both naturally and through human activities.

Natural and human-induced fires occur globally. Major fire regions include:

• Boreal (northern) forests in Alaska, Canada, Russia, China and Scandinavia
• Savanna grasslands and forests in Africa, Brazil and northern Australia
• Tropical forests in Brazil, Indonesia, Colombia, Ivory Coast, Thailand, Laos, Nigeria, Philippines, Myanmar and Peru
• Temperate forests in the US, western Europe and southern Australia
• Agricultural waste burning after harvests in the US and western Europe

The relationship between forests and carbon dioxide emissions is complex and depends on the forest life cycle. When a forest experiences fire, trees die and decomposition begins. If a forest fully regenerates itself after fire, there will be no net change in carbon over the complete life cycle.

Every year, fires burn 3-4 million km² of the Earth's land surface. These fires release more than a billion tonnes of carbon into the atmosphere as carbon dioxide. Interestingly, old-growth northern latitude forests are sometimes considered carbon 'sinks' even though fires kill trees, because mature forests contain decades or centuries of accumulated carbon. Their dense canopy blocks sunlight from reaching the forest floor, which slows decomposition of dead material.

Volcanic activity

Volcanic eruptions release carbon dioxide that has been stored in the Earth's interior. However, recent research suggests volcanic activity is not a major contributor to current global warming.

According to the United States Geological Survey (USGS), "the carbon dioxide released in recent volcanic eruptions has never caused detectable global warming of the atmosphere."

This is probably because:

• The warming effect of emitted CO₂ is counterbalanced by a large amount of sulphur dioxide released simultaneously. This sulphur dioxide converts to sulphuric acid, which forms fine droplets that increase the reflection of solar radiation back into space, actually cooling the lower atmosphere.
• The amounts of carbon dioxide released by present-day volcanic activity are very small compared to human emissions. All research to date indicates that current sub-aerial and submarine volcanoes release less than 1% of the CO₂ currently emitted by human activities.

Hydrocarbon extraction and burning – cement manufacture

Fossil fuel formation and combustion

Dead plants and animals gradually transform into fossil fuels following burial. The pressure from multiple layers of sediment creates an anoxic (oxygen-free) environment. In these conditions, decomposition occurs without oxygen.

When heat from the Earth is added, the carbon in organic molecules is rearranged to form different compounds. The type of material that is buried determines the final product:

• Animal remains tend to form petroleum (crude oil)
• Plant remains are more likely to form coal and natural gas

When these fossil fuels are extracted from underground and then burnt, carbon dioxide and water are released back into the atmosphere. This represents a major transfer of carbon from long-term geological storage into the active atmosphere.

Cement production

The cement industry is a significant contributor to atmospheric CO₂. Cement manufacture contributes CO₂ through two pathways:

1. Chemical process emissions - When calcium carbonate is heated to produce cement, it releases lime and carbon dioxide. This chemical process itself accounts for approximately 50% of cement industry emissions.
2. Fuel combustion emissions - The fossil fuels burnt to provide heat for the cement manufacturing process contribute approximately 40% of the industry's emissions.

The remaining 10% comes from other sources.

The graph above shows the dramatic acceleration in global emissions from fossil fuel combustion and industrial processes since 1750. Between 2017 and 2018, global energy-related CO₂ emissions rose by 1.7% to reach a historic high of 33.1 GtC (gigatonnes of carbon).

Emissions from fossil fuel burning have increased substantially, with electricity generation accounting for just over 60% of this growth. Coal burning, mainly in China, India and the US, accounted for 10 GtC of CO₂ emissions and 85% of the net increase in emissions. However, there was a decline in Germany, Japan, Mexico and France in the United Kingdom as these countries increased their use of renewable energy.

Farming practices

Agricultural activities significantly impact the carbon cycle through several mechanisms.

When soil is ploughed, the soil layers become inverted and mixed with air. This dramatically increases soil microbial activity. The result is that soil organic matter breaks down much more rapidly, releasing carbon from the soil into the atmosphere.

In addition to the direct effect on soil, emissions from farm machinery (tractors, harvesters) increase carbon dioxide levels in the atmosphere during ploughing and harvesting operations.

Land use change

Deforestation

CO₂ emissions resulting from land use change, mainly through deforestation, account for up to 30% of anthropogenic CO₂ emissions.

Most deforestation is driven by the need for additional agricultural land. Often, subsistence farmers in tropical regions clear a few hectares to feed their families through a process known as 'slash and burn' agriculture.

However, commercial logging operations also remove substantial forest areas. Loggers, some operating illegally, build roads to access increasingly remote forests. This road construction leads to further deforestation as it opens up previously inaccessible areas. Forests are also cleared to accommodate growing urban areas.

According to Global Forest Watch, there was a total loss of 361 Mha (million hectares) of tree cover globally between 2001 and 2018 - roughly one and a half times the size of the United Kingdom. This loss released an estimated 98.7 GtC of CO₂ emissions.

At the same time, tree planting initiatives have resulted in forests being established or extended in some regions. Research by NASA and the University of Maryland in 2018, using modern satellite imagery, demonstrates that global tree cover is actually increasing overall.

However, the loss caused by deforestation in the tropics is greater than the gains from reforestation in temperate areas, particularly in agricultural regions of Asia. The long-term outcome could be a complete loss of tropical rainforest but an increase in temperate forest on former natural grasslands.

Impact of deforestation on the carbon cycle

When forests are cleared for conversion to agriculture or pasture, a large proportion of the above-ground biomass may be burnt. This rapidly releases most of the stored carbon into the atmosphere. Some wood may be used for products and thus preserved for a longer period. Forest clearing also accelerates the decay of dead wood, leaf litter and below-ground organic carbon in the soil.

The diagram above illustrates how deforestation dramatically changes carbon storage and fluxes. An undisturbed tropical forest stores 180 tonnes of carbon per hectare in above-ground biomass and 226 tonnes below ground. Ten years after deforestation, above-ground biomass storage has dropped to just 43 tonnes, with below-ground storage reduced to 150 tonnes. Meanwhile, carbon emissions through burning, decay and soil erosion have released significant quantities of carbon to the atmosphere.

Urban growth

For the first time in human history, over half the world's population now lives in urban areas. As a proportion of global population, the urban population is expected to reach 60% by 2030, with urban areas growing at a rate of 1.1 million people every week.

As cities expand, land use changes from either natural vegetation or agriculture to built-up areas. The CO₂ emissions resulting from energy consumption for transport, industry, domestic use, plus the CO₂ emitted during cement manufacture for buildings and infrastructure, have all increased substantially.

This scatter plot reveals the relationship between city population and carbon footprint. Interestingly, there is no simple linear relationship - population size alone doesn't determine emissions. Cities like Seoul and Guangzhou show the highest carbon footprints (around 270-280 Mt CO₂), whilst other large cities emit considerably less. This concentration of emissions in particular urban centres is likely to increase in the future.

Carbon sequestration

Geologic sequestration

Geologic sequestration involves capturing CO₂ at its source (such as power plants or industrial facilities) and then injecting it in liquid form into underground storage locations.

Potential storage sites include:

• Depleted oil and gas reservoirs
• Thin, uneconomic coal seams
• Deep salt formations
• The deep ocean

The diagram shows various geological sequestration methods. CO₂ can be captured from coal-burning power plants and industrial sources, then transported via pipeline and injected into salt formations or depleted oil and gas reserves at depths below 800 metres. Impermeable caprock formations create a seal that prevents the gas from migrating back to the surface.

Ocean-based storage involves dissolving CO₂ into ocean water below 1,000 metres depth, where it can form CO₂ 'lakes' on the ocean floor. However, this approach is still experimental.

Terrestrial sequestration

Terrestrial sequestration involves using plants to capture CO₂ from the atmosphere and then storing it as carbon in plant stems and roots as well as in the soil.

The aim is to develop land management practices that maximise the amount of carbon that remains stored in the soil and in plant material for extended periods. Most environmental authorities also believe that enriching plant ecosystems has positive environmental benefits beyond carbon storage, including enhancement of wildlife populations.

However, there are disadvantages to terrestrial sequestration:

• A forest planted specifically to capture carbon might lose that carbon back to the atmosphere in a catastrophic forest fire, or if the forest suffers disease or infestation
• Land-based sequestration plantations are slow-growing and require active monitoring and management throughout the lifetime of the plantation, typically many decades
• The carbon within terrestrial systems is never permanently removed from the atmospheric system