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of the slow carbon cycle. For example, the uplift of the Himalayan Mountains, which began 50 million years ago, reset earth’s thermostat by providing a large source of fresh rock to pull more carbon into the slow carbon cycle through chemical weathering. The resulting drop in temperatures led to the consequent formation of ice sheets on Antarctica.

Over very long time scales (millions to tens of millions of years), the movement of tectonic plates and changes in the rate at which carbon seeps from the earth’s interior may change the temperature on the thermostat. Earth has undergone such a change over the last 50 million years, from the extremely warm climates of the Cretaceous (roughly 145 to 65 million years ago) to the glacial climates of the Pleistocene (roughly 1.8 million to 11,500 years ago).

Factors driving change in the magnitude of stores

Weathering

Through a series of chemical reactions and tectonic activity, carbon takes between 100 and 200 million years to move between rocks, soil, ocean and atmosphere in the slow carbon cycle.

Chapter 3The carbon cycleIntroductionCarbon is the backbone of life on earth. We are made of carbon, we eat carbon, and our civilisations – our economies, homes and transport – are dependent on carbon. We need carbon, but that need is also entwined with one of the most serious problems facing us today – global climate change. Carbon is the fourth most abundant element in the universe. Most of earth’s carbon, about 65,500 billion metric tonnes, is stored in rocks and the soil above them (the lithosphere). The remainder is in the hydrosphere (the oceans), atmosphere and the biosphere (plants and animals).

Carbon flows between each of these stores in a complex set of exchanges called the carbon cycle (Figure 3.1), which has slow and fast components. Any change in the cycle that shifts carbon out of one store, places more carbon in the other stores. It is now widely accepted that changes that put more carbon gases into the atmosphere result in warmer temperatures on earth and hence the carbon cycle has a close connection to climate change.

The slow carbon cycleOver the long term, the carbon cycle maintains a balance that prevents all of earth’s carbon from entering the atmos-phere or from being stored entirely in rocks. This balance helps keep earth’s temperature relatively stable, like a thermostat. This thermostat works over a time period from a few hundred thousand to millions of years, as part

Key termsBiosphere The zone occupied by living organisms such as plants, animals and fungi.Lithosphere The earth’s crust and the soil above it.Atmosphere The zone above the surface of the earth to the edge of space.Hydrosphere The area covered by water, such as rivers, lakes and oceans.

Topic 1 Water and carbon cycles

28 AQA AS | A-Level Core Geography

On average, 1013 to 1014 grams (10–100 million metric tonnes) of carbon move through the slow carbon cycle every year. The movement of carbon from the atmosphere to the lithosphere begins with rain. Atmospheric carbon combines with water to form a weak carbonic acid that falls to the surface in rain. The acid dis-solves rocks, a form of chemical weath-ering, and releases calcium, magnesium, potassium and sodium ions. Rivers then carry these ions to the ocean.

Carbon sequestration in oceansIn the oceans, calcium ions react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes lime-stone. Most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (for example, coccolithophores and foraminifera). After the organisms die,

they sink to the seafloor. Over time, layers of shells, skeletons and sediment are cemented together and turn to rock, storing the carbon in limestone and its derivatives. Carbon locked up in lime-stone can be stored for millions or even hundreds of millions of years.

Up to 80% of carbon-containing rock is currently made this way. The remaining 20% contains carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In some cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal or natural gas instead of sedimentary rock like shale. Coal and other fossil fuels are a conven-ient source of energy, but when they are burned the stored carbon is released into the atmosphere (as part of the fast carbon cycle).

CO2 releasedby combustion

CO2 released byanimal respiration

CO2 released by combustion

Air–sea CO2 exchange

CO2 released by marine organism respiration

Carbon in remains of organisms

Carbon in sediment turns into limestone and organic-rich shale

Oil and gasextraction

Carbon moves from sediment to oil and gas Oil and gas

Carbon released by marine organism decomposition

Carbon buried in animal remains

Carbon buried in plant remains

Carbon released bydecomposition of plants

Carbon released by decomposition of animals

Animals eat plants or other animals (or both), storing carbon in their tissues

Coal mine

Coal includes carbon derived from organic remains

CO2 absorbed by photosynthesis

CO2 in rain weathers rocks

Sediment

CO2 absorbed by photosynthesis

Carbon movementHuman carbon transformation

Photosynthesis

Weathering and erosion

CO2 released by plant respiration Carbon stored in

plant tissues

CO2 released by volcanic eruption

CO2 released by wildfires

CO2 dissolved in water

Rivers carry erodedcarbon to the oceanCO2 released

by automobiles

Figure 3.1 The carbon cycle – processes

29Section 1 Physical Geography

3The carbon cycle

Volcanic activity

The slow cycle also returns carbon to the atmosphere through volcanoes. Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide. When volca-noes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. At present, volcanoes emit between 130 and 380 million metric tonnes of carbon dioxide per year. For comparison, humans emit about 30 billion tonnes of carbon dioxide per year – 100 to 300 times more than volcanoes – by burning fossil fuels.

Chemistry regulates this movement of carbon between ocean, land and atmos-phere. If amounts of carbon dioxide rise in the atmosphere because of an increase in volcanic activity, for example (Figure 3.1), temperatures rise, leading to more rain, which dissolves more rock, creating more ions that will eventually deposit more carbon on the ocean floor. It takes a few hundred thousand years to rebalance the slow carbon cycle through chemical weathering.

Diffusion The slow carbon cycle also contains a slightly faster component – the ocean. At the ocean surface, where air meets

water, carbon dioxide gas dissolves in and ventilates out of the ocean in a steady exchange with the atmosphere (Figure 3.1). Once in the ocean, carbon dioxide gas reacts with water molecules to release hydrogen, making the ocean more acidic. The hydrogen reacts with carbonate from rock weathering to produce bicarbonate ions.

Before the industrial age, the ocean vented carbon dioxide to the atmos-phere in balance with the carbon the ocean received during rock weathering. However, since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Over millennia, the ocean will absorb up to 85% of the extra carbon people have put into the atmosphere by burning fossil fuels, but the process is slow because it is tied to the movement of water from the ocean’s surface to its depths.

In the meantime, winds, currents and temperature control the rate at which the ocean takes carbon dioxide from the atmosphere. It is likely that changes in ocean temperatures and currents helped remove carbon from and then restore carbon to the atmosphere over the few thousand years in which the ice ages began and ended. This means that for shorter time periods the temperature of earth can vary. The earth has had swings between ice ages and warmer interglacial periods within these time-scales.

Extension box 3.1

Biogeochemical cycles

The carbon cycle is an example of a biogeochemical cycle. Biogeochemical cycles are pathways for the transport and transformation of matter within the four broad zones that make up the planet: biosphere, hydrosphere, lithosphere and atmosphere. Where the biosphere overlaps

with the lithosphere, atmosphere or hydro-sphere, there is a zone occupied by living organisms.

Biogeochemical cycles are components of the broader cycles that govern the func-tion ing of the planet. The earth is a system open to electromagnetic radiation from the sun and outer space, but is a virtually closed system with regard to matter. This



means that the planet has minimal flux of matter, other than meteorite collisions from space and minor amounts of intergalactic particle losses by the upper atmosphere. Therefore matter that earth contained from the time of its birth is transformed and circulated geographically. This is in line with the law of conservation of matter, which states that matter can neither be created nor destroyed but can be trans-formed, including the transformation between matter and energy.

The transfer of matter involves biolog-ical, geological and chemical processes – hence the name ‘biogeochemical cycles’. Biogeochemical cycles may also be referred to as cycles of nature because they link together all organisms and abiotic features on earth. Matter is continu-ally recycled among living and abiotic elements. Biogeochemical cycles facilitate the transfer of matter from one form to another and from one location to another on the planet. Additionally, biogeochem-ical cycles are sometimes called ‘nutrient cycles’ because they involve the transfer of compounds that provide nutritional support to living organisms.

Types of biogeochemical cycles Biogeochemical cycles differ in their pathways and, on this basis, they are usually classified into two groups:

● Sedimentary cycles: These involve the transportation of matter from the ground to water (i.e. from the lithosphere to the hydrosphere). Common examples of cycles under the sedimentary category are:– Phosphorus cycle: Phosphorus is commonly found in water, soil and sedi-ments. Phosphorus cannot be found in air in the gaseous state. This is because phosphorus is usually a liquid at normal temperatures and pressures. However, very small particles in the atmosphere may contain phosphorus or its compounds. Phosphorus moves slowly from deposits

on land and in sediments, to living organ-isms, and much more slowly back into the soil and water sediments. The phosphorus cycle is the slowest of the sedimentary cycles.– Sulphur cycle: Sulphur in its natural form is a solid and, in this form, restricted to the sedimentary cycle. It is transported by physical processes such as wind, erosion by water, and geological events like volcanic eruptions. However, in its compounds such as sulphur dioxide, sulphuric acid, salts of sulphate or organic sulphur, sulphur can be moved from the ocean to the atmosphere, to land and then to the ocean through rainfall and rivers.

● Gaseous cycles: These involve the transportation of matter through the atmosphere. Common examples of gaseous cycles are:– Carbon cycle: Carbon dioxide and methane gases (compounds of carbon) in the atmosphere have a significant effect on the earth’s overall heat balance. They absorb infrared radiation and hence may contribute to a warming effect and to climate change.– Nitrogen cycle: Nitrogen is the most abundant element in the atmosphere and all the nitrogen found in terrestrial ecosys-tems originates from the atmosphere. The nitrogen cycle is by far the most important nutrient cycle for plant life.– Oxygen cycle: Oxygen moves within and between its three main reservoirs – the atmosphere, the biosphere and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis and, because of this, the oxygen and carbon cycles are usually interlinked.

Focus questions 1 Why are biogeochemical cycles crucial

to life on this planet?

2 Choose one sedimentary cycle and one gaseous cycle and give an example of their impact on the planet.


3The carbon cycle

The fast carbon cycleFigure 3.2 shows the fast carbon cycle of the movement of carbon between land, atmosphere and oceans. All numbers of stores and fluxes (flows) are in peta-grams. The units used in the carbon cycle can be confusing:

1 petagram of carbon per year (PgC/yr) = 1 gigatonne per year (1 GtC/yr) = 1 billion (109) tonnes = 1015 grams of carbon per year

Key to Figure 3.2

The numbers in Figure 3.2 represent both the carbon ‘stores’ and annual carbon fluxes (flows). Black numbers and arrows indicate stores and fluxes (flows) estimated for the time prior to the industrial era (about 1750) and hence represent the ‘slow’ carbon cycle. These

form the basepoint for the fast carbon cycle and therefore indicate natural stores and flows.

Red arrows and numbers indicate annual ‘anthropogenic’ (i.e. human orig-inated or influenced) transfers (or fluxes) averaged over the 2000–09 time period. These fluxes are a perturbation (distur-bance away from the equilibrium) of the carbon cycle during the industrial era after 1750, due to human activity. These fluxes (red arrows) indicate:

● fossil fuel and cement emissions of CO2;

● net land-use change; ● the other processes resulting in a net

increase of CO2 in the atmosphere. The uptake of anthropogenic CO2

by the ocean and by terrestrial ecosys-tems, often called ‘carbon sinks’ and also shown by red arrows, is part of the net land flux and net ocean flux. Red

Net ocean flux

Net land flux

Atmosphere 589 + 240(average atmospheric increase : 4 (PgCyr–1)

Surface ocean900

50

372

11

2

Rivers0.9 Burial0.2

Marine biota

3

Dissolvedorganiccarbon

700

Vegetation450–650

–30

Soils1500–2400Fossil fuel reserves

Gas: 383–1135Oil: 173–264

Coal: 446–541–365

Permafrost1700

Rockweathering 0.1

UnitsFluxes (PgCyr–1)Stores (PgC)

Export fromsoils to rivers

1.7

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Figure 3.2 The fast carbon cycle – stores and fluxes

Source: IPCC AR5 2013

You should spend some time looking at the amounts of carbon in each of the stores, the size of the fluxes (flows) and the overall direction of movement between stores and within the fluxes.



numbers in the reservoirs denote cumu-lative changes of anthropogenic carbon over the industrial period 1750–2011. A positive cumulative change means that a store has gained carbon since 1750. Similarly, a negative cumulative change means that a store has lost carbon since 1750. The cumulative change of anthro-pogenic carbon in the terrestrial store is the sum of carbon cumulatively lost through land-use change and carbon accumulated since 1750 in other ecosys-tems. Note that the mass balance of the two ocean carbon stocks – surface ocean and intermediate and deep ocean – includes a yearly accumulation of anthropogenic carbon. The flows within the oceans are shown by blue arrows.

Fluxes from volcanic eruptions, rock weathering of silicates and carbonates, export of carbon from soils to rivers, burial of carbon in freshwater lakes and reservoirs, and transport of carbon by rivers to the ocean, are all assumed to be as per the pre-industrial fluxes (i.e. unchanged during 1750–2011).

The time it takes carbon to move through the fast carbon cycle is measured in the lifespan of a life form – a plant or an animal. The fast carbon cycle is the movement of carbon through life forms on earth or the biosphere. Between 1015 and 1017 grams (1000 to 100,000 million metric tonnes) of carbon move through the fast carbon cycle every year through all life forms.

Factors driving change in the magnitude of stores and over time

Photosynthesis, respiration, decomposition and combustionPlants (on land) and phytoplankton (microscopic organisms in the ocean) are key components of the fast carbon cycle. During photosynthesis, plants absorb carbon dioxide and sunlight to create glucose and other sugars for building plant structures. Phytoplankton and plants take carbon dioxide from the atmosphere by absorbing it into their cells. Using energy from the sun, both plants and plankton combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen. The chemical reaction looks like this:

CO2 + H2O + energy = CH2O + O2

Four processes take place to move carbon from a plant and return it to the atmos-phere, but all involve the same chemical reaction. Plants break down the sugar to get the energy they need to grow. Animals (including people) eat the plants or plankton, and break down the plant sugar to get energy (respiration). Plants and plankton die and decay (decom-position), and are eaten by bacteria, at the end of the growing season, or fire consumes plants (combustion). In each

Key termsPhotosynthesis The green pigment chlorophyll, present in green plants, is able to capture light energy and convert it into food energy by manu-facturing carbohydrates (energy storing chemicals) using elements such as oxygen, carbon and hydrogen. Photo-synthesis, therefore, allows a plant to grow and increase its biomass.

Respiration The processes whereby organisms obtain energy from organic mole-cules. Processes take place in the cells and tissues during which energy is released and carbon dioxide is produced.

Decomposition The breakdown of plant and animal remains that releases energy and nutrients into the soil. Organisms such as earthworms, mites and slugs help in the decay and incor-poration of leaf litter in the soil, and fungi and bacteria secrete enzymes that break down organic compounds in this detritus.


3The carbon cycle

case, oxygen combines with sugar to release water, carbon dioxide and energy. The basic chemical reaction looks like this (a reverse of the formula above):

CH2O + O2 = CO2 + H2O + energy

In all four processes (Figures 3.1 and 3.2), the carbon dioxide released in the reaction usually ends up in the atmosphere. The fast carbon cycle is so tightly tied to plant life that a growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the northern hemisphere during winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the earth is breathing.

The ebb and flow of the fast carbon cycle is visible in the changing seasons. As the large land masses of the northern hemisphere green in the spring and

summer, they draw carbon out of the atmosphere. Figure 3.3 shows the average monthly changes in carbon dioxide levels for the northern hemi-sphere averaged over a time period of 60 years (a lifespan). The cycle peaks in August, with about 2 parts per million of carbon dioxide drawn out of the atmos-phere. In the autumn and winter, as vege-tation dies back in the northern hemi-sphere, decomposition and respiration return carbon dioxide to the atmosphere.

Annual variations in the fast carbon cycle can also be shown spatially. Figure 3.4 shows net primary produc-tivity (NPP) – the amount of carbon consumed by plants – on land (green) and in the oceans (blue) during August and December 2010. In August the green areas of North America, Europe and Asia represent plants using carbon from the atmosphere to grow. In December, net primary productivity at high lati-tudes is negative, which outweighs the seasonal increase in vegetation in the

CO

2 flu

x (p

pm

)

–2

–1

0

+1

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3.3 Average monthly changes in the fast carbon cycle for the northern hemisphere, 1949–2010

Sprig of the aquatic plant Elodea, a pondweed, producing oxygen bubbles from photosynthesis

Net primary productivity (kg carbon/m2/yearLand

Ocean–0.5 0 0.5 1 1.5 2 2.5

August

December

Figure 3.4 Global net primary productivity, August and December 2010

Source: NASA

Source: NASA



southern hemisphere. As a result, the amount of carbon dioxide in the atmos-phere increased. Also note the constant green areas of the low latitudes of South America, Africa and Southeast Asia. These are the areas associated with tropical forests that photosynthesise all year round.

Hydrocarbon fuel extraction and burning

Humans have interfered with the carbon cycle where fossil fuels have been mined from the earth’s crust and subsequently burnt. Fossil fuels are fuels formed by natural processes, such as the decompo-sition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years and sometimes exceeds 650 million years. Fossil fuels contain high percentages of carbon and include coal, oil and natural gas. They range from volatile materials with low carbon/hydrogen ratios, such as methane, to liquid petroleum and to non-volatile materials composed of almost pure carbon, such as anthracite coal. Methane can be found in hydrocar-bon fields, alone or associated with oil.

The use of fossil fuels raises serious environmental concerns. The burning of fossil fuels produces around 21 peta-grams of carbon dioxide (CO2) per year, but it is estimated that natural processes can only absorb up to 60% of that amount, so there is a net increase of about 8 petagrams of atmospheric carbon dioxide per year. Carbon dioxide is one of the greenhouse gases that enhances atmospheric heating and contributes to climate change, causing the average surface temperatures of the earth to rise in response.

The IPCC (The Intergovernmental Panel on Climate Change) Fifth Assess-ment Report (AR5), published in 2014, states the following:

● The atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years. Carbon dioxide concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions.

● The atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) have all increased since 1750 due to human activity. In 2011 the concentrations of these greenhouse gases.... exceeded the pre- industrial levels by about 40%, 150% and 20% respectively.

● Annual CO2 emissions from fossil fuel combustion and cement production in 2011 were 54% above the 1990 level. From 1750 to 2011, CO2 emissions from fossil fuel combustion and cement produc-tion have released 375 Gt of carbon to the atmosphere.

Wildfires

Scientists are analysing the role forests play in the carbon cycle and in particular whether forest fires produce more carbon dioxide than the trees capture through photosynthesis. The relationship between forests and carbon dioxide emissions is complex, but it appears that forest fires can release more carbon into the atmos-phere than the forests can capture and that this may be a growing problem.

Every year wildfires burn 3 million to 4 million km2 of the earth’s land area and release tonnes of carbon into the atmosphere in the form of carbon dioxide. However, massive northern latitude forests are also considered a carbon ‘sink’ because these trees are repositories of decades or centuries of carbon and because their heavy canopy blocks sunlight from reaching the forest floor and decomposing the forest litter.

In the natural system, after a fire, new


3The carbon cycle

vegetation moves onto the burned land and over time reabsorbs much of the carbon dioxide that the fire had released. Such new vegetation stores the carbon as organic matter. For this reason, it has been suggested that planting trees is a good way to remove carbon dioxide from the atmosphere. Eventually, however, those trees decompose or burn, thus returning carbon to the atmosphere.

Researchers have found that wildfires have a significant impact on the carbon balance in forested areas. When fire destroys an area of forest, it forces the forest to start all over again. It sweeps away everything, including the old-gen-eration trees. The trees and plants that move in after a fire are often different species from the ones they are replacing, and the new, immature forest is not an immediate carbon sink. Furthermore, thinning the canopy can allow more sunlight to peek through, which speeds up decomposition and thus releases more carbon dioxide. They have also found that the incidence and severity of the fires in the northern forests have increased in recent years, possibly due to warming global temperatures and changing precip-itation levels. The increasing frequency of fires is shrinking the old-growth forest areas and replacing them with younger forests. This means that parts of the forests have changed from carbon sinks to carbon sources. Younger forests do absorb carbon dioxide from the atmos-phere, but they are not the same strong-holds of carbon storage as older forests.

An interesting dimension to this is that researchers in the USA have found that the main reason fires are expanding – and thus releasing more carbon dioxide into the atmosphere – is fire suppres-sion. In order to suppress a forest fire, firefighters create long gaps in the trees (called firebreaks) to stop the fire from progressing. In the USA, fire suppres-sion has been commonplace for the past

100 years for reasons of self-preserva-tion. More and more people have moved into forested areas, putting yet more homes and lives at risk. Keeping a fire suppressed means keeping it away from those homes and lives. Recently, some experts have also begun advocating fire suppression as a way to reduce the carbon dioxide in the atmosphere. Fire suppression preserves trees, and trees absorb carbon dioxide. However, protecting and growing forests has an in-built disadvantage. Fire suppression protects forests from burning, but at the same time it increases the fuel supply for the next fire that will inevitably happen and thereby makes that fire even more damaging.

A helicopter drops water on a wildfire threatening a home near San Marcos, California, May 2014



Land-use change

Land-use change has a large impact on CO2 emissions (Figure 3.5). For example, the IPCC estimates that land-use change, such as the conversion of forest into agri-cultural land, contributes a net 1.6 Gt carbon per year to the atmosphere.

Various types of land-use change can result in changes to carbon stores:

● conversion of natural ecosystems to permanent croplands;

● conversion of natural ecosystems for shifting cultivation;

● conversion of natural ecosystems to pasture;

● abandonment of both croplands and pastures;

● harvesting of timber; ● establishment of tree plantations

(afforestation).When forests are cleared for conver-

sion to agriculture or pasture, a very large proportion of the above-ground biomass may be burned, releasing most of its carbon rapidly into the atmos-phere. Forest clearing also accelerates the decay of dead wood and litter, as well as below-ground organic carbon. Local climate and soil conditions will deter-mine the rates of decay; in tropical moist regions, most of the remaining biomass decomposes in less than ten years. Some carbon or charcoal also accretes to the soil carbon pool. When wetlands are drained for conversion to agriculture or pasture, soils become exposed to oxygen.

Carbon stocks, which are resistant to decay under the anaerobic conditions prevalent in wetland soils, can then be lost by aerobic respiration.

Forest clearing for shifting cultiva-tion releases less carbon than perma-nent forest clearing because the fallow period allows some forest regrowth. On average, the carbon stocks depend on forest type and the length of fallow, which vary across regions. Under some conditions, shifting cultivation can increase carbon stocks in forests and soils, from one cut–regrowth cycle to another. However, because shifting cultivation usually has lower average agricultural productivity than perma-nent cultivation, more land would be required to provide the same outputs. In addition, shorter rotation periods deplete soil carbon stores more rapidly.

Abandonment of cultivated land and pastures may result in recovery of forest at a rate determined by local condi-tions. Selective logging or harvesting often releases carbon to the atmos-phere through the indirect effect of damaging or destroying up to one-third of the original forest biomass, which then decays as litter and waste in the forest. The harvested wood decays at rates dependent on their end use; for example, fuelwood decays in one year, paper in a few years, and construction material in decades. The logged forest may then act as a sink for carbon as it grows at a rate determined by the local

Buildings

Energy

Industry

Transport

Agriculture,forestryand otherland use

8%

39 Gt CO2-equivalent 45 Gt CO2-equivalent 49 Gt CO2-equivalent1990s 2000s 2010

32%

19%

14%

27%22% 21%

15% 14%

20%

7% 7%

36% 37%

21%

Figure 3.5 Changes in greenhouse gas emissions by sector, 1990–2010


The carbon cycle 3

soil and climate, and it will gradually compensate for the decay of the waste created during harvest. Clearcutting of forest can also lead to the release of soil carbon, depending on what happens after harvesting. For example, harvesting followed by cultivation or intensive site preparation for planting trees may result in large decreases in soil carbon – up to 30–50% in the tropics over a period of up to several decades. Harvesting followed by reforestation, however, in most cases has a limited effect (±10%). This effect is particularly prevalent in the tropics, where recovery to original soil carbon contents after reforestation is quite rapid.

Where tree plantations are raised on land that has been specifically cleared, initially there would be net carbon losses from the natural biomass and the soil. The plantations would then begin to fix carbon at rates dependent on site condi-tions and species grown. An estimation of the timescale of carbon uptake in forest plantations depends on the nature and location of the plantations. These values have been estimated: 1.5–4.5 tonnes per hectare per year in coniferous temperate plantations of Europe and the USA; 0.9–1.2 tonnes per hectare per year in Canada and Russia; and 6.4–10.0 tonnes per hectare per year in tropical Asia, Africa and Latin America.

Farming practices

In January 2015 a study published in Global Change Biology stated that green-house gas emissions of carbon dioxide, methane and nitrous oxide from growing crops and raising livestock are now higher (11%) than those from deforest-ation and land-use change (10%). This small change may mean that greater emphasis may be placed on examining the impact of agricultural practices than has previously been the case.

Cropland soils can lose carbon as a consequence of soil disturbance such as tillage. Tillage increases aeration and soil temperatures, making soil aggre-gates more susceptible to breakdown and protected organic material more available for decomposition. In addition, erosion can significantly affect soil carbon stocks through the removal or deposition of soil particles and associ-ated organic matter. Soil carbon content can be protected and even increased through alteration of tillage practices, crop rotations, residue management, reduction of soil erosion, improvement of irrigation and nutrient management, and other changes in forestland and cropland management.

Livestock grazing on grasslands, con-verted cropland and permanent pastures have the largest areal extent of farming land use. Heavy grazing alters the ground cover and can lead to soil compaction and erosion, as well as alteration of nutrient cycles and runoff. Soil carbon, in turn, is affected by these changes. Avoiding overgrazing can reduce these effects.

Croplands as well as pastures are the dominant anthropogenic source of CH4 and N2O, although estimates of the CH4 and N2O budgets remain uncertain. Rice cultivation and livestock have been esti-mated to be the two primary sources of CH4. Alteration of rice cultivation prac-tices, livestock feed and fertiliser use are therefore potential management prac-tices that could reduce CH4 sources.

Carbon sequestrationThis is the process of capture and long-term storage of atmospheric carbon dioxide and includes:

● removing carbon from the atmosphere and depositing it in a reservoir;

● removing carbon dioxide from fuel exhaust gases, such as from power



stations, before storage in underground reservoirs, aquifers and even ageing oilfields;

● storage of carbon either by natural processes or by human activities in the oceans and their sediments.

Carbon sequestration has been sug-gested either to mitigate or to defer warming of the atmosphere and thereby reduce climate change. It has been pro-posed as a way to slow the atmospheric and marine accumulation of greenhouse gases, which are released by the burning of fossil fuels. As we have seen, carbon dioxide is naturally captured from the atmosphere through biological, chemical or physical processes – some anthropo-genic sequestration techniques exploit these natural processes, while some use entirely artificial processes. Carbon dioxide may be captured as a pure byproduct in processes related to oil refining or from flue gases from power generation.

Some natural sequestration carbon stores

Peat bogs These are a very important carbon store. By creating new bogs, or enhancing existing ones, carbon can be sequestered (see p. 44).

Reforestation Reforestation is the replanting of trees on marginal crop and pasture lands to incorporate carbon from atmospheric CO2 into biomass. For this process to succeed, the carbon must not return to the atmosphere from burning or rotting when the trees die. The trees must grow for long periods of time or the wood from them must itself be seques-tered, e.g. via biochar, bioenergy with carbon capture storage (BECCS) or landfill. Reforestation with long-lived trees (>100 years) will sequester carbon effectively.

An important carbon store – moorland peat bogs, Valley of Kergord, Shetland Isles


The carbon cycle 3

Wetland restoration Wetland soil is an important carbon sink: 14.5% of the world’s soil carbon is found in wetlands, while only 6% of the world’s land is composed of wetlands.

The role of agriculture Globally, soils are estimated to contain approximately 1500 gigatonnes of organic carbon to a 1 m depth – more than the amount in vegetation and the atmosphere. Modification of agricul-tural practices is a recognised method of carbon sequestration as soil can act as an effective carbon sink.

Carbon emission reduction methods in agriculture can be grouped into two categories:

● reducing and/or displacing emissions; ● enhancing carbon removal. Some of these reductions involve

increasing the efficiency of farm oper-ations (for example, more fuel-efficient equipment), while some involve interrup-tions in the natural carbon cycle. Some effective techniques (such as the elimina-tion of stubble burning) may have a neg-ative impact, resulting in increased herb-icide use to control weeds not destroyed by burning. Increasing yields and effi-ciency also generally reduces emissions, since more food results from the same or less effort. Such techniques could include more targeted use of fertilisers, less soil disturbance (tillage), better irrigation techniques, and crop strains bred for local requirements and increased yields.

Replacing more energy-intensive farming operations can also reduce emissions. Reduced or no-till farming requires less machine use and burns cor-respondingly less fuel per acre. However, no-till farming usually increases the use of weed-control chemicals and the residue now left on the soil surface is more likely to release its CO2 to the atmosphere as it decays, reducing the net carbon reduction.

All crops absorb CO2 during growth and release it after harvest. The goal of agricultural carbon removal is to use the crop and its relation to the carbon cycle to sequester carbon permanently within the soil. This is done by selecting farming methods that return biomass to the soil and enhance the conditions in which the carbon within the plants will be reduced to its elemental nature and stored in a stable state. Methods for accomplishing this include:

● using various cover crops such as grasses and weeds as temporary cover between planting seasons;

● concentrating livestock in small pad-docks for days at a time so they graze lightly but evenly. This encourages roots to grow deeper into the soil. Stock also till the soil with their hooves, grinding old grass and manure into the soil;

● covering bare paddocks with hay or dead vegetation. This protects soil from the sun and allows the soil to hold more water and be more attractive to carbon-capturing microbes;

● restoring degraded land, which slows carbon release while returning the land to agriculture or another use.

Such agricultural sequestration prac-tices may have positive effects on soil, air and water quality, may be benefi-cial to wildlife and may expand food production. It has been estimated that on degraded croplands, an increase of 1 tonne of carbon in the soil may increase crop yield by 20 to 40 kg per hectare for wheat and 10 to 20 kg per hectare for maize.

The effects of soil sequestration can be reversed, however. If the soil is disrupted or tillage practices are aban-doned, the soil becomes a net source of greenhouse gases. Typically after 15–30 years of sequestration, soil becomes saturated and ceases to absorb carbon. This suggests that there is a limit to the amount of carbon that soil can hold.



Since reduction of atmospheric CO2 is a long-term concern, farmers can be reluc-tant to adopt more expensive agricultural techniques when there is not a clear crop, soil or economic benefit. In response to this, the governments of Australia and New Zealand are considering allowing farmers to sell carbon credits once they document that they have sufficiently increased soil carbon content.

Ocean-based sequestration

Iron fertilisation

Iron fertilisation is the deposition of iron-rich dust into the waters of the oceans. This attempts to encourage phyto-plankton growth, which removes carbon from the atmosphere for a period of time. This technique is controversial due to limited understanding of its complete effects on the marine ecosystem. Such effects potentially include release of nitrogen oxides and disruption of the ocean’s nutrient balance.

Sperm whales act as agents of natural iron fertilisation when they transport iron from the deep ocean to the surface during prey consumption and defeca-tion. Sperm whales have been shown to increase the levels of primary production and carbon export to the deep ocean by depositing iron-rich faeces into surface waters of the Southern Ocean. The iron-rich faeces cause phytoplankton to grow and take up more carbon from the atmosphere. When the phytoplankton die, some sink to the deep ocean and take the atmospheric carbon with them. By reducing the abundance of sperm whales in the Southern Ocean, whaling has resulted in an extra 2 million tonnes of carbon remaining in the atmosphere each year.

Urea fertilisation It has been proposed that fertilising the oceans with urea, a nitrogen-rich

substance, could also help encourage phytoplankton growth. An Australian com pany called Ocean Nourishment Corporation (ONC) plans to sink hundreds of tonnes of urea into the ocean off western Australia to boost CO2-absorbing phytoplankton growth as a way to combat climate change.

Mixing layers By encouraging various ocean layers to mix, it is possible to move nutrients and dissolved gases around. Mixing may be achieved by placing large vertical pipes in the oceans to pump nutrient-rich water to the surface, triggering algal blooms, which store carbon when they grow and export carbon when they die.

Other non-natural sequestration options

Carbon capture and storage (CCS) This is a process that extracts carbon dioxide chemically from air, either from the free atmosphere or from the waste flues of fossil fuel power stations. The captured carbon dioxide is pumped underground into vacant geological strata.

Bioenergy with carbon capture and storage (BECCS) BECCS refers to the burning of biomass in power stations and boilers that use carbon capture and storage. The carbon sequestered by the biomass is captured

Key termPhytoplankton Very small plants that float near the surface of seas and oceans. Like land-based plants, they produce food by photosynthesis, obtaining their carbon from atmospheric carbon dioxide dissolved in ocean water and fuelled by sunlight, which only penetrates the surface layers of the oceans. In places, they often form dense blooms, which may have a density of 200,000 individual plants per cubic metre of seawater.


3The carbon cycle

and stored, thus removing carbon dioxide from the atmosphere.

Biochar Biochar is the name for charcoal when it is used for the purpose of being added to the soil. It is created by a process called ‘pyrolysis’ (an intense burning of wood in an oxygen-free environment). The biochar is dug into the ground and thereby increases soil fertility and agri-cultural productivity. Most importantly, the process places carbon back into the ground, which can remain there for thousands of years.

Subterranean injection Carbon dioxide can be injected into depleted oil and gas reservoirs and other geological features, or can be injected into the deep ocean. The first large-scale CO2 sequestration project (Sleipner) began in 1996 and is located in the North Sea where Norway’s Statoil Hydro strips carbon dioxide from natural gas and disposes of this carbon dioxide in a deep saline aquifer.

In 2000, a coal-fuelled synthetic natural gas plant near Beulah, North Dakota, became the world’s first coal-

using plant to capture and store carbon dioxide. CO2 has been used extensively in the USA since the 1970s in recovering crude oil from difficult locations, such as heavy oil reservoirs. There are in excess of 10,000 wells that inject CO2 in the state of Texas alone. The gas comes in part from anthropogenic sources but principally from large naturally occur-ring geological formations of CO2. It is transported to the oil-producing fields through a 5000 km network of pipelines. Similar methods have been suggested for use in the west Canadian oil basin, possibly using carbon dioxide from the Athabasca oil sands to the north. The use of this technique would actually increase heavy oil production.

Deep basalt storage CO2 sequestration in deep ocean basalt has been suggested. If mixed with seawater, it would react with the basalt and produce a number of stable carbonate and hydrate minerals. These are more dense and settle on the seabed, and hence are unlikely to rise and leak. One site suggested for this is the Juan de Fuca Plate off the west coast of the USA. This has a possible storage capacity

Great Plains Synfuels coal gasification plant near Beulah, North Dakota, USA



of 208 gigatonnes – equivalent to the current US carbon emissions for the next 100 years and more.

Acid neutralisation As carbon dioxide forms carbonic acid when dissolved in water, the oceans steadily become more acidic. This means that the amount of CO2 that they can absorb reduces over time. It has been suggested that adding bases such as crushed limestone could neutralise the acid seawater and thus increase CO2 absorption.

The carbon budget

Global scale

Carbon dioxide is the single most impor-tant anthropogenic greenhouse gas in the atmosphere, contributing approx-imately 65% to radiative forcing by long-lived greenhouse gases (LLGHGs). It is responsible for approximately 84% of the increase in radiative forcing over the past decade. The pre-industrial level of 278 ppm represented a balance of relatively large annual two-way fluxes between the atmosphere and oceans and the atmosphere and terrestrial biosphere.

However, the rising levels of CO2 and other LLGHGs in post-industrial times are fuelling fears of climate change through atmospheric warming.

Atmospheric CO2 reached 142% of the pre-industrial level in 2013, primarily because of emissions from combustion of fossil fuels and cement production (Figure 3.6). Relatively small contributions to increased CO2 come from deforestation and other land-use change, although the net effect of terrestrial biosphere fluxes is as a sink. The average increase in atmos-pheric CO2 from 2004 to 2013 corre-sponds to 45% of the CO2 emitted by human activity, with the remaining 55% removed by the oceans and the terrestrial biosphere. The main sinks for CO2 emis-sions from fossil fuel combustion are the oceans and terrestrial biosphere. Uptake of atmospheric CO2 by the oceans results in ocean acidification.

Source: NOAA

Fossil fuel and cement

8.9Land-usechange

0.9

Land sink2.9

Ocean sink2.6

Geological reservoirs

Atmosphericgrowth

4.3

Figure 3.6 The recent global carbon dioxide budget (gigatonnes of carbon per year), 2004–13

Key termsRadiative forcing The difference between incoming solar radiation (insolation) absorbed by the earth and the energy radiated back out into space.Carbon budget The balance of exchanges between the four major stores of carbon.


3The carbon cycle

The globally averaged amount of CO2 in the atmosphere in 2013 was 396 ppm. The average growth rate for the 1990s was 1.5 ppm per year, and the average growth rate for the decade of the 2000s was 2.1 ppm per year (Figure 3.7). Recent increases in emissions of CO2 from fossil fuel combustion have continued to be 2% per year and are expected to be maintained or even increase. Radiative forcing remains a major and increasing influence on global climates.

National scale

Peat soil is the biggest store of carbon in the UK, so it crucial that we under-stand how this is changing over the long term. Is carbon continuing to build up in peat bogs, removing carbon from the atmosphere, or are the peat bogs degrading, losing carbon to rivers and the atmosphere? The answer matters, since it is better if peat remains a carbon ‘sink’ rather than a source of carbon. The main input to peat comes from the atmosphere; there is a balance between carbon uptake via photosynthesis and carbon loss via respiration. In a healthy peat bog the net effect is the storage of carbon and the gradual build up of peat.

There are several processes contrib-uting to the output of carbon from a peat bog:

● Dissolved inorganic carbon losses in river water include dissolved carbon dioxide and carbonate from weathered rock below the peat.

● Losses of dissolved organic carbon (DOC) have been rising steadily over recent decades and the reasons why are not fully understood. It is probably a combination of causes, including higher temperatures and wetter winters. DOC causes problems for the water industry – it must be removed before chlorination or carcinogenic compounds are formed. Better management of peat moorland can reduce DOC losses, saving money on water treatment and helping to conserve carbon storage.

● Carbon is lost as solid, particulate matter when it is eroded by surface runoff. If gullies are blocked and erosion minimised, this keeps the carbon where it should be – in the peat and not in the river.

● Finally, there is loss of methane to the atmosphere and again this may be on the increase where peat bogs are in a poor state.

1

2

3

4

01985 1990 1995 2000 2005 2010

Source: WMO (2014)

CO

2 gr

owth

rat

e (p

pm/y

ear)

Figure 3.7 Changes in the rate of increase of CO2 in the atmosphere, 1984–2013



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