MET 112 Global Climate Change - Lecture 9
The Carbon Cycle
Dr. Craig Clements
San José State University
Goals
 We want to understand the difference
between short term and long term carbon
cycle
 We want to understand the main
components of the long term carbon cycle
The Earth’s history can be
characterized by different
geologic events or eras.
An Earth System Perspective
 Earth composed of:
– Atmosphere
– Hydrosphere
– Cryosphere
– Land Surfaces
– Biosphere
 These ‘Machines’ run the Earth
Hydrosphere
 Component comprising all liquid water
– Surface and subterranean (ground water)
 Fresh/Salt water
 Thus…lakes, streams, rivers, oceans…
 Oceans:
– Oceans currently cover ~ 70% of earth
– Average depth of oceans: 3.5 km
– Oceans store large amount of energy
– Oceans dissolve carbon dioxide (more later)
– Circulation driven by wind systems
– Sea Level has varied significantly over Earth’s history
– Slow to heat up and cool down
Land Surfaces
 Continents
 Soils surfaces and vegetation
 Volcanoes
 Climate:
– Location of continents controls
ocean/atmosphere circulations
– Volcanoes return CO2 to atmosphere
– Volcanic aerosols affect climate
Biosphere
 All living organisms; (Biota)
 Biota- "The living plants and animals of a
region.“ or "The sum total of all organisms alive
today”
– Marine
– Terrestrial
 Climate:
 Photosynthetic process store significant amount
of carbon (from CO2)
The Earth’s history can be
characterized by different
geologic events or eras.
Interactions
 Components of the Earth System are linked by
various exchanges including
 Energy
 Water (previous example)
 Carbon
 In this lecture, we are going to focus on the
exchange of Carbon within the Earth System
Carbon: what is it?
 Carbon (C), the fourth most abundant element
in the Universe,
 Building block of life.
– from fossil fuels and DNA
– Carbon cycles through the land (bioshpere),
ocean, atmosphere, and the Earth’s interior
 Carbon found
– in all living things
– in the atmosphere
– in the layers of limestone sediment on the
ocean floor
– in fossil fuels like coal
Carbon: where is it?
 Exists:
– Atmosphere:
–CO2 and CH4 (to lesser extent)
– Living biota (plants/animals)
–Carbon
– Soils and Detritus
–Carbon
–Methane
– Oceans
–Dissolved CO2
–Most carbon in the deep ocean
Carbon conservation
 Initial carbon present during Earth’s formation
 Carbon doesn’t increase or decrease
globally
 Carbon is exchanged between different
components of Earth System.
The Carbon Cycle
 The complex series of reactions by which carbon
passes through the Earth's
– Atmosphere
– Land (biosphere and Earth’s crust)
– Oceans
 Carbon is exchanged in the earth system at all time
scales
- Long term cycle (hundreds to millions of years)
- Short term cycle (from seconds to a few years)
The carbon cycle has different
speeds
Short Term Carbon Cycle
Long Term Carbon Cycle
Short Term Carbon Cycle
 One example of the short term carbon cycle involves plants
 Photosynthesis: is the conversion of carbon dioxide and
water into a sugar called glucose (carbohydrate) using
sunlight energy. Oxygen is produced as a waste product.
 Plants require
 Sunlight, water and carbon, (from CO2 in atmosphere or
ocean) to produce carbohydrates (food) to grow.
 When plants decay, carbon is mostly returned to the
atmosphere (respiration)
 Global CO2
Short Term Carbon Cycle
 One example of the short term carbon cycle involves plants
 Photosynthesis: is the conversion of carbon dioxide and
water into a sugar called glucose (carbohydrate) using
sunlight energy. Oxygen is produced as a waste product.
 Plants require
 Sunlight, water and carbon, (from CO2 in atmosphere or
ocean) to produce carbohydrates (food) to grow.
 When plants decay, carbon is mostly returned to the
atmosphere (respiration)
 During spring: (more photosynthesis)
 atmospheric CO2 levels go down (slightly)
 During fall: (more respiration)
 atmospheric CO2 levels go up (slightly)
Carbon exchange (short term)
 Other examples of short term carbon
exchanges include:
 Soils and Detritus:
- organic matter decays and releases carbon
 Surface Oceans
– absorb CO2 via photosynthesis
– also release CO2
Short Term Carbon Exchanges
Long Term Carbon Cycle

Carbon is slowly and continuously being transported
around our earth system.
– Between atmosphere/ocean/biosphere
– And the Earth’s crust (rocks like limestone)

The main components to the long term carbon cycle:
Long Term Carbon Cycle

Carbon is slowly and continuously being transported
around our earth system.
– Between atmosphere/ocean/biosphere
– And the Earth’s crust (rocks like limestone)

The main components to the long term carbon cycle:
1. Chemical weathering (or called: “silicate to
carbonate conversion process”)
2. Volcanism/Subduction
3. Organic carbon burial
4. Oxidation of organic carbon
Where is most of the carbon today?
 Most Carbon is ‘locked’ away in the earth’s crust (i.e.
rocks) as
– Carbonates (containing carbon)
 Limestone is mainly made of calcium carbonate
(CaCO3)
 Carbonates are formed by a complex geochemical
process called:
– Silicate-to-Carbonate Conversion (long term carbon
cycle)
Silicate to carbonate conversion –
chemical weathering
One component of the long term
carbon cycle
Granite (A Silicate Rock)
Limestone (A Carbonate Rock)
Silicate-to-Carbonate Conversion
1. Chemical Weathering Phase
• CO2 + rainwater  carbonic acid
• Carbonic acid dissolves silicate rock
2. Transport Phase
• Solution products transported to ocean by
rivers
3. Formation Phase
• In oceans, calcium carbonate precipitates
out of solution and settles to the bottom
Silicate-to-Carbonate Conversion
Rain
2. Acid
Dissolves
Silicates
(carbonic
acid)
Land
1. CO2 Dissolves in
Rainwater
3. Dissolved Material Transported
to Oceans
4. CaCO3 Forms in
Ocean and Settles to
the Bottom
Calcium carbonate
Changes in chemical weathering
 The process is temperature dependant:
– rate of evaporation of water is temperature
dependant
– so, increasing temperature increases weathering
(more water vapor, more clouds, more rain)
 Thus as CO2 in the atmosphere rises, the planet
warms. Evaporation increases, thus the flow of carbon
into the rock cycle increases removing CO2 from the
atmosphere and lowering the planet’s temperature
– Negative feedback
Earth vs. Venus
 The amount of carbon in carbonate minerals (e.g.,
limestone) is approximately
– the same as the amount of carbon in Venus’
atmosphere
 On Earth, most of the CO2 produced is
– now “locked up” in the carbonates
 On Venus, the silicate-to-carbonate conversion process
apparently never took place
Subjuction/Volcanism
Another Component of the Long-Term
Carbon Cycle
Subduction
Definition: The
process of the
ocean plate
descending
beneath the
continental plate.
During this
processes, extreme
heat and pressure
convert carbonate
rocks eventually
into CO2
Volcanic Eruption
Eruption injected
(Mt – megatons)
17 Mt SO2,
42 Mt CO2,
3 Mt Cl,
491 Mt H2O
Can inject large amounts of
CO2 into the atmosphere
Mt. Pinatubo (June 15, 1991)
Organic Carbon Burial/Oxidation of
Buried Carbon
Another Component of the Long-Term
Carbon Cycle
Buried organic carbon (1)
 Living plants remove CO2 from the atmosphere
by the process of
– photosynthesis
 When dead plants decay, the CO2 is put back
into the atmosphere
– fairly quickly when the carbon in the plants is
oxidized
 However, some carbon escapes oxidation
when it is covered up by sediments
Organic Carbon Burial Process
O2
CO2
Removed
by PhotoSynthesis
CO2 Put Into
Atmosphere by
Decay
C
C
Some Carbon
escapes
oxidation
C
Result: Carbon into land
Oxidation of Buried Organic Carbon
 Eventually, buried organic carbon may be
exposed by erosion
 The carbon is then oxidized to CO2
Oxidation of Buried Organic Carbon
Atmosphere
Buried Carbon
(e.g., coal)
Oxidation of Buried Organic Carbon
Atmosphere
Erosion
Buried Carbon
(e.g., coal)
Oxidation of Buried Organic Carbon
Atmosphere
CO2
O2
C
Buried Carbon
Result: Carbon into atmosphere (CO2)
The (Almost) Complete Long-Term
Carbon Cycle
 Inorganic Component
– Silicate-to-Carbonate Conversion
– Subduction/Volcanism
 Organic Component
– Organic Carbon Burial
– Oxidation of Buried Organic Carbon