Summary
Summary notes on the carbon cycle
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- Physical geography
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- PEARSON (PEARSON)
Summary notes on the carbon cycle
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Evaluate the extent to which today’s increasing demand for energy is the most important factor modifying the globalcarbon cycle (20)
Modifying vs changing?
Is increasing demand the most important factor in modifying global carbon cycle?
Not all energy is the same. Fossil fuels have much higher carbon emissions per unit of energy than others like coal and tar
sands
Energy demand isn’t the only factor (deforestation, land use change and farming)
Most important = coal and dirty fossil fuels -> gas and other energy sources -> deforestation -> farming -> land use
changes
Global energy demand increased by nearly 50% between 1990 and 2010 and is expected to increase by another 40% to
2030. Fossil fuels supply more than 80% of the world’s energy and unlikely to change despite increases in renewable
energy. According to the US Environmental Protection Agency, 70% of CO2 is due to emissions from fossil fuel burning
which has increased CO2 concentrations in the atmosphere. Burning fossil fuels release CO2 from geological stores into
the atmosphere and oceans which take up some CO2 and become more acidic.
However, not all energy sources are the same. Some sources like nuclear and wind power are low carbon but don’t meet
the increasing demand for energy. Coal and oil are the main sources but are inefficient. The development of Canada’s tar
sands and the US’ oil shales are likely to release even greater amounts of geological carbon. This is because a lot of energy
is used in extracting and processing unconventional fossil fuels even before they are used – so emissions are far greater.
Growing global energy demand makes it likely these sources will need to be tapped in order to increase supply.
Some energy sources are more complex. Biofuels like maize or sugar cane emit CO2 when combusted, but when they
grow in the following season, they sequester carbon from the atmosphere. In theory, this means their impact is less than
fossil fuels. However, in Indonesia, palm oil plantations have resulted in burning primary tropical forest which releases
enormous amounts of CO2.
Land use change, especially deforestation is responsible for up to 25% of global GHG emissions. Up to 8 million hectares of
tropical forest have been destroyed in Indonesia alone just for palm oil. However, deforestation can be resolved through
afforestation schemes. Some countries like India and Costa Rica have net afforestation so their biological carbon sinks are
increasing.
Farming is also a major contributor to atmospheric GHG emissions. 80% of all methane emissions are from farming,
including farm animals and wet rice cultivation. Fertilisers are a major source of NO2 emissions. As the global population
increases toward 9 billion forecasted by the UN by 2050, the challenge of feeding these additional people is likely to
increase emissions further and lead to more deforestation to convert forests to food production. Poor farming practices
can lead to more rapid decay of soil organic matter and greater CO2.
Overall, energy demand does have the greatest impact on modifying the carbon cycle, by releasing fossil carbon into the
atmosphere and oceans. However, not all energy sources have the same impact, with coal and unconventional sources
being the most significant. Biofuels are more complex, and their impact isn’t easy to quantify. Deforestation and farming
are two other factors, which are linked. The importance of increasing demand is likely to grow as there are more people.
However, it may be possible to reduce fossil fuel use with renewable energy technologies in the future.
Evaluate the extent to which geological processes control the carbon cycle (20)
Most of the Earth’s carbon is geological, resulting from the formation of sedimentary carbonate rocks in the oceans.
,Combination of all: biogeochemical processes
1. Carbon stores
2. Formation of sedimentary rock
Formed from the cancerous ooze and shells and skeleton (e.g. foraminifera) that collect at the bottom of oceans
Marine creatures like corals and phytoplankton absorb carbon from seawater and when their remains collect on
the seabed the calcium carbonate is compacted by the weight of new layers and cemented together to form an
organic limestone rock
Calcareous oozes aren't found where the ocean is too deep (below 6000 metres) because the pressure causes
calcium carbonate to dissolve
3. Formation of biologically derived carbon rock
Geochemical process formed from the remains of living organisms deposited in layers within sedimentary rocks
Organic carbon from tropical coastal swamps is buried to produce coal, the hardness of which depends on the
amount of pressure and heat when squashed during rock formation
Anaerobic reactions may convert organic carbon into a liquid that moves within rock layers until trapped
This process forms coal and oil and methane is a natural by product which is trapped within layers of rock
Geological processes
1. Volcanic outgassing
Volcanic activity is estimated to release 300 million tonnes of CO2 every year and some carbon monoxide. CO2 is
the least soluble of the gases and so is degassed earlier in eruption processes. Mt Etna is the most actively
degassing volcano due to limestone from the ocean underneath.
However, it only releases 0.1 PgC a year compared with oceanic sequestration of 74.8 PgC/ year
Tectonic processes may bring limestone rocks into contact with extreme heat causing chemical changes and
releases CO2 into the atmosphere
Additionally, at subduction zones, constructive plate boundaries or intra-plate hotspots, they may release CO2
into the atmosphere and is also common in geothermal areas like Iceland
Outgassing and degassing takes place through the main vent and carbon is recycled at subduction zones with
carbonate rocks dragged into the mantle, creating an upper mantle carbon concentration of 50 – 220 pm
This rate has increased over the last years due to biological evolution and deposition of organic sediment.
2. Chemical weathering of rock
Carbonation occurs where rainwater is a carbonic acid and absorbs CO2 from the air. It then dissolves rock
minerals to form new minerals like calcium carbonate. Rivers then transport these minerals to the see where they
are deposited and buried leading to sedimentation and the formation of new rocks
Metamorphosis also occurs where extreme heat and pressure forms metamorphic rock, during which some
carbon is released, and some becomes trapped.
Terrestrial processes
1. Terrestrial primary producers
Autotrophs sequester and store sun energy through photosynthesis to change CO2 into water and oxygen
2. Terrestrial consumers
Heterotrophs (animals) are known as consumers as they respire. Animals produce CO2 and herbivorous animals
emit methane as a by-product
Energy is passed from one consumer to another through food chains
3. Soil carbon cycle
Decomposition of plants
Soil biota digest dead plant material and release nutrients. Some CO2 is emitted into the atmosphere. Anaerobic
decomposition occurs emitting methane into the atmosphere and digestion by termites
Biocarbon storage is in the humus in soil as carbon structures are broken down
Higher humus means more carbon is stored
Biological processes
, 1. Biological carbon pump
Phytoplankton are mostly microscopic single celled plants found in warmer surface waters of oceans
They consume CO2 from the atmosphere during photosynthesis and store it in their bodies as carbohydrate and
turn it into organic matter
When they die, the phytoplankton transfer carbon to deeper ocean stores forming calcium carbonate when they
sink to seabed or shallow layers if eaten by zooplankton which transfer carbon to deeper waters when they die
2. Marine carbonate pump
Phytoplankton are at the base of the marine food web so when eaten by zooplankton, carbon is passed through
the food chain and CO2 is released back into the water as these organisms respire
Some organisms like zooplankton sequester CO2 turning the carbon into their hard-outer shells
When these organisms die, some of their shells dissolve into the ocean meaning the carbon becomes part of deep
ocean currents
Any dead organisms which sink to the seafloor become buried and compressed eventually undergoing
sedimentation to form limestone sediment. Over a long period, these can turn into fossil fuels
Carbon is taken from the atmosphere and chemically transferred to the seabed to become sedimentary
3. Thermohaline circulation 4. Physical pump (ocean currents)
Carbon compounds are transported between The Southern Ocean shows there exists a
the world’s oceans along the deep ocean physical pump involving the upwelling and
conveyor downwelling of currents which moves dissolved
Parts of the circulation helps transfer CO2 from CO2
equatorial zones to polar zones Ocean currents transfer heat energy, with large
Warm, salty ocean currents transfer heat energy gyres moving warm tropical water towards the
from tropical areas towards the poles. This poles and colder water towards the equator
gradient causes the large-scale circulation of sea These flows are linked to the thermohaline
water circulation, which starts in deep ocean areas and
Cold water can hold more gas meaning driven by differences in density between warm
Antarctica is an important carbon sink and cold water. Colder denser water sinks while
Estimated the Southern Ocean accounts for 25% warm water experiences upwelling
of the diffusion of CO2 from atmosphere to
oceans 5. Oceanic and terrestrial sequestration
Carbon is necessary for biosynthesis and
photosynthesis
Carbohydrates are created in the cells of
autotrophs (plants) by combined CO2 and water
and light
Respiration balances photosynthesis by
producing energy when combing oxygen
From the atmosphere To the atmosphere
1. Rock weathering (0.4 PgC/ year) 1. Freshwater outgassing (1 PgC/ year)
2. Photosynthesis (123 PgC/ year) 2. Volcanic eruptions (0.1 PgC/ year)
3. Atmosphere to ocean (ocean sequestration) (80 3. Land use change (1.1 PgC/ year)
PgC/ year) 4. Fossil fuel and cement production (7.8 PgC/ year)
5. Respiration and fire (118.7 PgC/ year)
6. Ocean to atmosphere (78.4 PgC/ year)
Stores
1. Sedimentary rock store (83M PgC)
2. Ocean store (37,100 PgC)
3. Permafrost (1700 PgC)
4. Terrestrial ecosystem (550 PgC)
Human processes
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