Module 7
Part II The Carbon Cycle
Chapter 8
Carbon on Earth
The Chemistry of Carbon
 Biotic carbon
 Abiotic carbon
 Highly organized
molecules within
living things
 After life they become
disorganized goo –
called kerogen, or
humic acids
Terrestrial Planets
Venus, Earth, and Mars
All three planets had about the same amount of
carbon:
Venus has the carbon content in it’s very dense
atmosphere of carbon dioxide and sulfuric acid
Earth has the highest concentration of carbon in
limestone and rocks
Mars has it’s carbon locked up in the polar ice caps that
are carbon dioxide dry ice
Organic Carbon
 The backbone of life
 A means of storing energy
 Photosynthesis, carbon dioxide, water, and sunlight
produces plants that store energy as food
 The early plants were converted to fossil fuels – more
stored energy as fuel instead of food
Oxidation states, electron
bookkeeping
 Methane is totally reduced carbon, has an oxidation state
of -4
 To calculate oxidation states we assign the common states
to hydrogen and oxygen, then realize that the molecule
has to be neutral, so the leftover number is assigned to
carbon
 Hydrogen is +1, there are four of them in methane, so the
carbon must be -4
 This is fully reduced carbon
 Reduced carbon is easily oxidized
 CH4 + 2 O2 → CO2 + 2 H2O
Oxidized carbon
 CO2 is fully oxidized
 The oxidation number for carbon is +4
 We calculate this by assigning -2 to each oxygen (Group 16 in the
periodic table, needs two more electrons)
 Oxygen is -4, so carbon must be +4
 Oxidized carbon is stable, low energy, and the preferred state for
carbon
 Oxidized carbon will not become reduced carbon without a great
deal of effort
 In between is the carbohydrates, where carbon has a zero oxidation
state CH2O formaldehyde, is the simplest carbohydrate. O is -2, H is
+1(x2) so C must be in the 0 oxidation state
Carbon forms
 Photosynthesis uses oxidized carbon to reduce
the carbon to carbohydrates
 We use carbohydrates as fuel and oxidize the
carbohydrate back to CO2 when we exhale during
respiration
Animals are not the only organisms to breathe!
The Land Breathes
 The land inhales CO2 in the summertime
growing season and exhales during the
winter months
 Reversed in the Southern Hemisphere where
there is less land
 The land breathes on an annual cycle
The Ocean Breathes
 The carbon is inorganic, and stable, it
involves the carbonate buffer system that
we will study in chapter 10, this is called
dissolved inorganic carbon
 The ocean effects atmospheric CO2 on time
scales of centuries
 The glacial-interglacial cycles were amplified
somehow by the ocean carbon cycle.
The Rocks Breathe
 The sedimentary rock carbon pool is larger than the
ocean, land or atmospheric pools
 Carbon in the solid Earth exists as limestone CaCO3,
and to a lesser extent, organic carbon
 Most of the organic carbon in sedimentary rocks is
kerogen
 Kerogen is useless as a fossil fuel because it is too
diluted
 The solid Earth is the largest but slowest breathing
of the carbon reservoirs
The Atmosphere is the Grand Central
Station for the CO2 Cycles
Glacial-Interglacial Cycles
The beat of the ice-age rhythm apparently originates
from variation in the Earth’s orbit around the sun
The orbit varies through three main cycles, and the
orbital variations drive climate by changing the
distribution of sunlight at the Earth’s surface
 1. Precession Cycle
 2. Obliquity Cycle
 3. Eccentricity Cycle
Precession
 The axis of rotation spins like a wobbly top
 Called the precession of season, or the precession of
the equinoxes
 Completes the entire circle in 20,000 years
 Solar heat influx variability comes from precession
 Seasonal cycle in the North is weakened and in the
South it is intensified because the Earth is closest to
the sun in the winter season in the northern
hemisphere
Precession orbital cycle
Obliquity
 The angle of the pole of rotation relative to
the plane of Earth’s orbit
 Varies between 22 and 25.5 degrees
 Angle of tilt is currently 23.5 degrees
 Cycle time is 41,000 years
 The impact of obliquity on solar heating is
strongest in the high latitudes
Obliquity of Earth’s Orbit
Eccentricity
 The third cycle involves how elliptical the orbit of the
Earth is
 The eccentricity of the orbit has cycles of 100,000 and
400,000 years
 At present the orbit of Earth is nearly circular
 The orbital cycles affect climate by redistributing the
energy from one place to another and from one
season to another
Milankovitch cycles
The CO2 Thermostat
 At the cool surface of the Earth, oxidized carbon wants to
be calcium carbonate – limestone
 In the hot interior of the Earth, oxidized carbon wants to
be free, as CO2
 The CO2 thermostat regulates atmospheric CO2 and climate
on geologic time scales of hundreds of thousands of years
 It is possible to change the set point of the thermostat,
creating a hot house world like that of the dinosaurs, or an
icy world like today
 The thermostats of Venus and Mars are broken
Take home points of chapter 8
 1. the most stable form of carbon on Earth is oxidized.
Photosynthesis stores energy from the sun by producing
organic carbon, which is the backbone of life
 2. There is less carbon in the atmosphere that any other
carbon reservoir. These other reservoirs tug on
atmospheric CO2 seasonally for the land, and on glacial
interglacial 100,000 year time scales from the ocean
 3. The weathering of igneous rocks on land controls the
partial pressure of CO2 in the atmosphere on million year
time scales. The thermostat is broken on Venus because no
water, and on Mars because there is no volcanic activity
left.
Chapter 9
Fossil Fuels and Energy
Energy
All energy comes from the stars,
Mostly from our sun
Previous definition: watts = joules/second
 terawatts = 1012 watts, written TW
 1,000,000,000,000 watts
Energy sources
 Wind (Denmark)
Hydroelectric
(2% globally)
 Solar
 Biomass energy
Energy sources
Renewable


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

Geothermal
Solar
Wind
Wood
Waste electric power
Non-renewable
 Fossil fuels
 Radioactive elements
Fossil Fuels
“Only a small fraction of the
buried organic carbon is in a
convenient form for fuel”
Traditional fossil fuels
 Largest reservoir is coal: it was produced in swamps
where the organic material was protected from the
atmosphere by water
 Freshwater has less sulfur, burns “cleaner”
 Saltwater swamps contains sulfur, burns to forms
aerosols and produce acid rain as sulfuric acid
Coal
 Begins as plant material (carbon based)
Carbon
Peat
Coal
By a pressure and temperature process that takes
millions of years.
The oldest coal is the cleanest coal.
“Coal is the most abundant
fossil fuel, and the future of
the Earth’s climate depends
mostly on what happens to
that coal”
Coal in power plants
 Coal fired power plants are established
 They produce cheap energy
 Would be very expensive to replace with a cleaner
fuel source until the necessity arises
“Oil is probably the most
convenient but the least
abundant of the fossil
fuels, so it is the most
expensive.”
Source of oil
 Organic rich sediments buried 2-5 km
 50 – 150 ° C
 Temperature and pressure converts some of the
organics to oil
 Higher temperatures produce natural gas, mostly
methane
 Only a tiny fraction of the oil and gas produced can
be harvested
Oil is the most expensive
 Oil fuels the transportation industry
 More energy per weight than any battery (so
far)
 Convenient liquid form as opposed to:
 Coal, not used in transportation since the steam
engine
 Natural gas which must be a pressurized container
Sources
Traditional:
 Oil fields – pumped from under ground or water
 largest fields in Saudi Arabia, and in Kuwait
Non-traditional:
 Oil shales – low grade fuel for power plants, Estonia
produces about 70%
 Tar sands – requires steam (Canada)
How long will it last?
We have differing opinions here: The oil industry has been saying
forty years for a long time but new sources and initiatives keep
adding time.
“There is enough oil to keep pumping for
decades, but the peak rate of oil
extraction could be happening right
now.”
Natural gas
Coal – solid
Oil – liquid
Natural gas – gas usually in the form
of methane CH4
Energy of methane
“Methane carries more energy per carbon
that the others because methane is the
most chemically reduced form of carbon.”
Reduced form + oxygen → oxidized form + water
Along with a release of energy (the ability to do work).
Global sources of Energy in
2001
Biggest users of energy







China
India
Brazil
U.S.
France
Denmark
Japan
Energy consumption
per dollar GPD (Gross
Domestic
Productivity).
Energy Consumption per person
 U.S.
 Japan
 France
 Denmark
 Brazil
 China
 India
Source?





U.S.
Japan
France
Denmark
Brazil
 China
 India
petroleum, gas, coal
petroleum, gas, coal
petroleum, gas, coal
petroleum, gas, coal
petroleum, gas, coal
coal, petroleum, gas
coal, petroleum, gas
New coal plants
China and India are building new coal fired
plants at an alarming rate.
http://ingienous.com/?page_id=8399
Bottom Line
“Coal is the form of fossil fuel with
the potential of increasing the
temperature past the turning point
of 2° C. The future of the earth
depends most on what happens to
that coal.”
Take home points, Chapter 9
 Ultimately, the energy available to humankind
includes instantaneous solar energy, which is
abundant but spread out; stored solar energy is in
the form of fossil fuels; and stored solar energy
from stellar explosions in the form of radioactive
elements.
 Of the fossil fuels, coal is the most abundant. Oil
may run out in the coming decades, and the peak
rate of oil extraction may be upon us even now.
continued….
 We can project energy demand in the
future as the product of population,
economic growth, and energy
efficiency.
Chapter 10
The Perturbed Carbon Cycle
The atmosphere ain’t what it
used to be!
Ozone
 Three oxygen atoms
 Very reactive
 O2 bonds break with UV-c, forming O
free radical, recombines with an O2 to
form O3
 Stratospheric O3 absorbs (filters) UV-b
radiation, forming O2
Montreal Protocol 1987
 Phased out production and release of
chlorofluorocarbons because it breaks
down stratospheric ozone (Freon,
aerosol propellants, refrigerants)
 Asthma and allergy suffers feel it, plant
leaves get burned and scarred
Stratospheric Ozone
 Is a Good thing
 CO2 in the stratosphere sheds heat as IR
to space
 The ozone depletion causes cooling in
the stratosphere
 Result: the stratosphere is cooling
Surface/tropospheric Ozone
Tropospheric ozone comes from several sources.
Biomass burning and industrial activity produce carbon
monoxide (CO) and volatile organic compounds (VOCs)
which are oxidized to form ozone. Nitrogen oxides
(NOx) from industrial processes, biomass burning,
automobile exhaust and lightning also form
tropospheric ozone. A small amount of tropospheric
ozone also comes from the stratospheric ozone layer.
http://earthobservatory.nasa.gov/Features/Aura/Images
/TroposphericOzone_HiRes.jpg
Ozone
Ozone Hole
 Ozone hole located over Antarctica is a different
problem than the ozone as a greenhouse gas
 HNO3 acid clouds react with chlorine, which in turn,
consumes the ozone
 Satellite was programmed to throw out data that
violated common sense, so the hole was a surprise
 Revisiting discarded satellite data revealed that the
hole had been growing for some time
Methane
Natural Sources
Human Sources
 Wetland degradation
 Termites
 Organic carbon in
freshwater swamps
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



Energy emissions
Landfills – “swamp gas”
Ruminant animals
Rice agriculture
Biomass burning
Methane Clathrates – Fire Ice
 http://www.youtube.com/watch?v=U46XOoU0DrM
 Overall human impact has doubled since pre-human
levels
 CH4 is responsible for 25% of anthropogenic
greenhouse heat trapping
Carbon Dioxide
 Methane is transient, but CO2 accumulates
 Background levels were around 280 ppm until
~ 1750, coinciding with the New World,
“pioneer effect”
 Deforestation for agriculture and
development is one source of atmospheric
CO2
 The second source is fossil fuel combustion
CO2 and CH4, 1000 years
Atmospheric CO2
 CO2 is complicated, and the atmosphere is the
exchange place for the three remaining
carbon reservoirs
 Land cycles annually
 Oceans cycle by centuries or more
 Rock cycles by millennia or more
The Missing Sink
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Deforestation releases about 1.5 Gtons C /year
Fossil fuels release about 8.5 Gtons C /year
Release is about 10 Gtons C /year
Atmospheric levels are rising by about 4 Gtons C /year
Mathematically we are missing about 6 Gtons C /year
 There is a missing carbon sink – about 6 Gtons /year
Terrestrial Carbon Sink
 The measurements are variable
 The research indicates that the land is taking
up the missing carbon
 Studies conclude that the “missing sink” is
located in the high latitudes of the northern
hemisphere
CO2 Fertilization
 Higher concentrations of CO2 encourages
plants to grow faster (greenhouses)
 The growth is an initial spurt, and tends to
level off
 Higher CO2 concentrations fro plants means
less water loss when plants open the stomata
to take in the CO2
Respiration in Soils
 As organic carbon is oxidized to CO2,
the soil releases the CO2
 Warmer soils decompose faster
 Tropical soils contain very little carbon
 The permafrost is full of carbon
 As soils warm, the CO2 emissions get
higher
Ocean uptake CO2
 Ultimately the fossil fuel CO2 will be
cleaned up by the oceans
 60 years ago, scientists thought it would
be a quick process
 50x more CO2 in the ocean
 70% of the Earths surface, average 4 km
deep
But…
 The surface of the ocean limits the contact
between the atmosphere and the deep
ocean
 The ocean uptake of fossil fuel carbon
depends on circulation
 Ocean ventilation – at high latitudes the
cold water sinks and takes gases with it – it
takes centuries to make the loop
Also…
 The thermocline is a few hundred
meters deep, and the ventilation to the
atmosphere is a few decades
 The surface ocean mixed layer (driven
by the wind) is about 100 meters deep
and ventilation to the atmosphere is
annually
Buffer chemistry of inorganic carbon
 In seawater, freshwater lakes, rivers, reservoirs,
swimming pools and human blood
 The major ions in seawater are Na+, Mg2+, Ca2+, K+,
Sr2+, Cl-, SO42- (sulfate), HCO3- (bicarbonate), Br-,
B(OH)3 (boric acid), and F-. Together, they account
for almost all of the salt in seawater.
Carbonate/bicarbonate buffer
 Atmospheric CO2 dissolves in seawater and is hydrated to
form carbonic acid, H2CO3. Carbonic acid is diprotic; that is,
it can undergo two de-protonation reactions to form
bicarbonate (HCO3-), and carbonate (CO32-). The coexistence of these species in seawater creates a chemical
buffer system, regulating the pH and the pCO2 of the
oceans. Most of the inorganic carbon in the ocean exists as
bicarbonate (~88%), with the concentrations of carbonate
ion and CO2 comprising about 11% and 1%, respectively.
http://oceancolor.gsfc.nasa.gov/SeaWiFS/TEACHERS/CHEMISTRY/
What does that mean?
 pH reactions, CO2 reacts with H2O to form carbonic
acid (carbonated soda drinks)
CO2 + H2O  H2CO3
 Carbonic acid loses a hydrogen, forms an acidic
proton and bicarbonate (hydrogen carbonate)
H2CO3  H+ + HCO3 Hydrogen carbonate loses the second acidic proton
and forms more acid and the carbonate ion
HCO3-  H+ + Co32-
Lets Assume
 We can ignore the tiny input of the Hydrogen ion and
recombine the equations to show and easier
illustration of le Châtelier’s principal
CO2 + CO32- + H2O  2 HCO31%
11%
88%
 Any additional CO2 is reacted with the carbonate ion
to produce the hydrogen carbonate ion
pH Chemistry
 A bucket of seawater can absorb or release more CO2
because of the pH chemistry
 The buffer stabilizes the pH and the concentrations of
the CO2
 The amount of CO2 that can be absorbed depends on
the concentration of the carbonate
 It is about 11% and CO2 is about 1%, so it works well
 This buffer system also keeps your blood pH in
balance
Perturbation
If you perturb, stress, or change the
system, it will react in such a way to
relieve the perturbation, stress, or
change in the system – it will reach
a new equilibrium
le Châtelier’s Principle
 Le Châtelier's Principle states that if a dynamic
equilibrium is disturbed by changing the conditions,
the position of equilibrium moves to counteract the
change.
 In other words, look at the equation, if you add
products, it will shift to reactants
 If you take away reactants, it will shift to reactants
 It will shift to overcome the stress
Seawater pH
 The relative concentrations of carbon dioxide and
carbonate ion in seawater determine its pH
 Fossil fuel CO2 makes seawater more acidic
 The buffer helps resist the change in pH
 Life forms in the ocean that make their shells out of
CaCO3 will suffer at lower pH
 Think of putting baking soda (sodium bicarbonate)
into vinegar (a weak acid) and watch the CO2 fizz out
Equilibrium Models
 Eventually after a long period of time, the CO2 will
spread out among the carbon reservoirs of the
atmosphere, ocean and land surface
 Models indicate that the atmospheric levels of CO2
will be higher than before the CO2 was released
 Eventually the budget for dissolved CaCO3 in the
ocean has to balance
 As the buffer chemistry recovers, atmospheric CO2
drops
Recovery
The climate cycle will ultimately
recover from the fossil fuel era when
the carbon returns to the solid Earth
as a result of the silicate weathering
CO2 thermostat from Chapter 8.
How long? First we have to stop
adding CO2 to the atmosphere.
The longevity of the global warming
climate event stretches out into time
scales of glacial – interglacial cycles,
time scales that are longer than the
age of human civilization.
Take home points, chapter 10
 The ozone hole problem is not the same as global
warming. They are different issues.
 Methane has about a 10 year lifetime in the
atmosphere, so its concentration reaches an
equilibrium after about this long.
 The land surface and the ocean are absorbing some
of our fossil fuel CO2, but this could slow or reserve in
a changing climate.
 Releasing fossil CO2 to the atmosphere will affect
climate for hundreds of thousands of years – as far as
we are concerned, forever.