Transcript Carbon cycle

Conditions for life
Our solar system
Venus
Mercury
Mars
Earth
Saturn
Jupiter
Neptune
Uranus
http://photojournal.jpl.nasa.gov
Earth, our home
http://visibleearth.nasa.gov
Earth: Goldilocks Zone
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Earth’s position (“third rock from the Sun”) is in the
“Goldilocks Zone” (0.9 – 1.4 AU)
 that is, in a position that is not too hot and not too cold
(“just right”)
 Venus is too hot, Mars is too cold, Earth is just
right
 note: 1 Astronomical Unit (AU) = 149,598,000 km
http://www.dailymail.co.uk
Venus
Earth
Mars
Distance from
Sun
0.72 A.U.
1 A.U.
1.52 A.U.
Mass
4.87 x 1024 kg
5.98 x 1024 kg
6.42 x 1023 kg
Density
5.25 g cm-3
5.52 g cm-3
3.94 g cm-3
Gravity
0.88 Earth
gravity
1 Earth gravity
0.38 Earth
gravity
Radius
6052 km
6378 km
3397 km
Atmospheric
pressure
90.9 atm
1 atm
0.069 atm
Surface
temperature
460 C
15 C
-59 C
Atmospheric
CO2
96%
0.0389%
95%
Atmospheric N2
3.5%
77%
2.7%
Water vapour
0.01%
1%
0.03%
Oxygen
0%
21%
0.13%
Venus
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.html
moon
Venus!
http://blog.thomaslaupstad.com/2007/04/12/moon-venus-and-earthshine/
Mars
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mars.html
Earth, Venus, and Mars
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Earth actually has similar composition with Venus and
Mars but water is not stable or not present in Venus or
Mars
Venus is very hot (460 C) – hot enough to melt lead
 has a dense atmosphere, mainly composed of CO2
 atmosphere is shrouded with clouds of sulfuric acid
and water droplets
 because of thick cloud cover, Venus surface receives
only 44% of solar radiation that Earth does
 but heat from surface is nearly completely absorbed
by clouds and atmosphere
 runaway greenhouse warming
Runaway greenhouse warming in Venus
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Venus has no way of removing CO2
Perhaps early in Venus history, Venus had plate
tectonics and oceans to control climate
But as the Sun became hotter, ocean boiled away
Nearer Sun, hotter. H2O readily evaporates from surface
Because of its proximity to the Sun, there is no cold trap
(for water to condense into ice)
 water vapor rises to high altitudes, where it is more
easily destroyed by solar UV
 H2O dissociates to H and O.
 H, being light, escapes, but O reacts with other
molecules to form other molecules
 means increasingly more water is being lost
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Absence of cleansing action by H2O precipitation permits
CO2 atmosphere to grow
Absence of water means rock weathering also ceases to
capture CO2 from the atmosphere and lock it to form
carbonate rocks
CO2 traps infrared radiation from surface => temperature
rises => more liquid water evaporates => enhances
Greenhouse further
 this positive (amplifying) feedback produces a
runaway greenhouse effect
All water is eventually removed from the atmosphere
 yields a massive, very hot, and dry CO2 atmosphere
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Continuous volcanic out gassing of materials like sulfur
dioxide without water cleansing produces sulfuric acid
clouds
 without its abundant water, Earth would probably be
like Venus
Venus is hotter than Earth because of its greenhouse
gases rich atmosphere
 without these gases, Venus would actually be -20 C
and any water would be frozen
 hotter than Earth not so much because Venus is
closer to the Sun
… and the problem with Mars
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Mars is very cold (-59 C) and has a very thin CO2
atmosphere
Mars may have once had water in a very large northern
basin, Oceanus Borealis
Mars is 90% lighter than Earth
 lower gravity on Mars
 means Mars cannot hold onto its early atmosphere
Mars is also too small
 to have sustained plate tectonics
 rocks cannot return CO2 into the atmosphere
 geochemical cycle has stopped
 so Mars loses heat easily
Earth’s shields
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Earth supports life because it has abundance of water
and water exists primarily as liquid (not as vapour or ice)
Earth is also shielded from UV by the ozone layer in the
stratosphere
Earth’s magnetic fields shield us from
 the Sun’s solar wind (flux of electrons, protons, and
charged helium nuclei) that travel several hundreds of
kilometers per second
 can kill a human
 cosmic rays (protons and heavier nuclei particles)
travelling at near light speeds
 rays come from extra solar activities such as
supernova explosions
Water
The water molecule
E.A. Mathez, 2009, Climate Change: The Science of Global
Warming and Our Energy Future, Columbia University Press.
Water
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2 atoms of hydrogen and 1 atom of oxygen - H2O
One of the most unique and most important molecule on
Earth
 Ice (solid water) has a lower density than liquid water,
so ice floats
 other molecules: solid phase will sink (higher
density)
negative side
O
104.5
H
H
positive side
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Ice forms at the surface of water
 ice is now a heat insulator for the waters below
 below ice is liquid water (crucial if life is to continue in
cold weather)
Bipolar charge because of atom arrangement
 -ve on oxygen side & +ve on hydrogen side
 bipolarity charge makes water stable and solvent for
many substances
 many chemical reactions can take place in water
Bipolarity makes water stable means
 large amount of energy needed to evaporate water
 large amount of energy has to be removed for
freezing water
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Specific heat for water is among the highest
 amount of energy to raise 1 gram of substance by 1
C
 to heat 1 g water by 1 C requires 1 calorie or 4.186 J
 compare that to 1 g dry air (udara) which requires
1.006 J (about 1/4 less of that for water)
 this means water can absorb and release relatively
large amounts of heat with very little change in its
temperature
 high specific heat of water is one reason why oceans
are much slower to respond to the heating or cooling
of atmosphere
 also why seasonal change in temperature of
oceans are much less that that of the atmosphere
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Hottest and coldest temperature ever recorded:
 on land: 58 C (Libya desert) and -88 C (Antarctica)
 range = 146 C
 on ocean: 36 C (Persian Gulf) and -2 C (near poles)
 range = 38 C only (much less than that for land)
Ocean is a natural thermostat
 annual sea surface temperature variation
 2 C in tropics, 8 C in middle latitudes, 4 C in
polar regions
 global average ocean temperature 17 C
 releases and absorbs heat over decades to centuries,
whereas the atmosphere does the same but in days
to weeks
Why is water still on Earth?
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Earth’s atmosphere is layered
 troposphere (8-15 km) then stratosphere
Upper troposphere is very cold
 liquid water condenses into ice before it can reach
stratosphere, making the stratosphere very dry
 if water escapes into stratosphere and higher, UV
rays would dissociate water molecule into H and O
H, being light, would not be held down by gravity and
would escape into space
 eventually all water would be lost from Earth
 this “cold trap” is essential to trap water on Earth
Hydrological cycle
(T)
(E)
Capillary rise
Water entry into soil is called inflitration. Runoff is water flowing on the soil surface,
unable to enter into soil (unable to inflitrate into soil)
Environmental soil physics by Daniel Hillel, 1998, Academic Press
Water balance within the root zone
Run in
RI + R + I + CR =  + RO + ET + P
Balance looks deceptively simple, but some parameters are difficult to measure in
practice, such as RI, RO, CR, P, and ET (especially the T component)
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Water balance can be simplified by using some
assumptions
 no irrigation, so I = 0
 flat land or incoming water same as outgoing water by
runoff, so RI = RO
 deep water table (i.e., > 2 m), so CR = 0
 balance over long term (i.e., a year), so no change is
soil moisture between the period, so  = 0
Simplified equation:
 R = ET + P
 this equation, though much simpler, has to be used
with care because it uses a lot of assumptions which
may not be appropriate in some conditions
Carbon cycle
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Carbon cycle has a long term efffect on Earth’s climate
Carbon cycle has two cycles
 short-term cycle
 long-term cycle
Carbon exists mainly as
 gas CO2 in atmosphere
 dissolved bicarbonate ions (HCO3-) in oceans
 various organic compounds in soil
Carbon is a major component in all living organisms
 plants – 50%
 animals – 19%
Long term carbon cycle
Rocks (75,000)
rock
degassing
rock weathering
rock burial
Organic carbon rocks
rock burial
rock weathering
Carbonate rocks
Surface carbon reservoirs:
oceans (40)
atmosphere (0.75)
biota (0.6)
soil (1.6)
rock
degassing
figures in trillion tonnes or teratonnes
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Weathering (chemical breakdown) of rocks remove CO2
from the atmosphere
 CO2 reacts with water and silicate and carbonate
minerals to form, in water, Ca, Mg, bicarbonate ions,
and silicia:
4CO2 + 6H2O + CaSiO3 + MgSiO3
Ca2+ + Mg2+ + 4HCO3- + 2H4SiO4
or
atmospheric CO2 + water + Ca and Mg silicate minerals
ions and species dissolved in river water
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The dissolved ions wash into rivers which eventually
flows into the sea. In the ocean, organisms use the
dissolved Ca and bicarbonate ions to make shells:
Ca2+ + 2HCO3CaCO3 + CO2 + H2O
or
Ca and bicarbonate ions in seawater
calcite + CO2 and water
Dissolved silicate precipates to opal:
H4SiO4
SiO2.H2O + H2O
or
dissolved silica
opal + water
Mg is removed from seawater mainly by reacting with hot
rocks to form clay minerals
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The carbonate shells accumulate at the ocean bottom
and eventually form carbonate-bearing sedimentary
rocks, such as coquina and chalk
With burial, these rocks heat up and compress, and the
carbonate minerals break down to release CO2
 the CO2 percolates out of the crust and escape into
the atmosphere, completing the cycle
SiO2 + CaCO3
CaSiO3 + CO2
or
silicate minerals + carbonate minerals
calcium silicate minerals + carbon dioxide
Coquina, a limestone composed of fossilized shell debris cemented together by calcite
http://gccweb.gccaz.edu/earthsci/imagearchive/chemical1.htm
http://geology.about.com/od/more_sedrocks/ig/sedrocksgallery/coquina.--2t.htm
Chalk, composed of fossilzed shells of microscopic organisms such as foraminifera
http://www.hunstantonfossils.co.uk/Hunstanton-Fossils-Geology/geology-guide.htm
Role of photosynthesis
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Another route for CO2 to return to atmosphere:
 removal of CO2 from the atmosphere by
photosynthesis
 then the burial of organic matter (OM) to make
organic-rich rocks, primarily coal and carbonaceous
shale
 CO2 + H2O
(CH2O)n + O2
 where (CH2O)n represents carbohydrates, starches,
and other organic compounds in plants
 oxidation of sedimentary rocks as they are exposed
by erosion or other physical breakdown returns CO2
into the atmosphere, completing the cycle
 (CH2O)n + O2
CO2 + H2O
Coal
Shale
http://www.geologytimes.com
http://gccweb.gccaz.edu/earthsci/imagearchive/chemical1.htm
Plate tectonics
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CO2 can also be released from activities of Earth’s plate
tectonics
 plate tectonics allow degassing of CO2 from rocks into
the atmosphere
 without plate tectonics, very difficult to return CO2 into
the atmosphere
 Earth would be frozen over
Plate tectonics is caused by the convection in Earth’s
mantle
 this convection is, in turn, caused by the decay of
radioactive elements, mainly potassium (isotope
40K), thorium (Th), and uranium (U)
Plate tectonics akin to a jigsaw puzzle
http://www.geography-site.co.uk/pages/physical/earth/tect.html
MANTLE
http://www.crystalinks.com/platetectonics.html
http://facstaff.gpc.edu/~pgore/Earth&Space/GPS/platetect.html
Carbon regulation of climate
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As CO2 increases, temperature increases due to
greenhouse effect, so weathering of rocks increases.
 Why?
 higher temperature may lead to higher rainfall
(higher ET), so higher rate of weathering
 higher CO2 or temperature may increase plant
photosynthesis, so plants produce more organic
acids and other compounds to increase rock
weathering
 As rate of rock weathering increases, more CO2 is
removed from atmosphere, so this leads to cooling
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But as Earth cools, the rate of rock weathering now
slows down, and CO2 builds up in the atmosphere
because of degassing by solid Earth
Almost all carbon on Earth is sequestered in rocks
 6.5 x 1016 tons of C are in rocks
 but only 4.1 x 1013 tons of C are in other surface
reservoirs (1000 times less than in rocks)
 this balance is important: it keeps Earth from being
too cold (too much CO2 locked up in rocks) or too hot
(too much CO2 released into atmosphere)
Short term carbon cycle
Marine photosynthesis
Atmosphere
carbon
Terrestrial
photosynthesis
Biota
carbon
Gas exchange
Terrestrial
respiration
Degassing
Litter fall, root decay,
calcification
Marine respiration
Ocean
carbon
River
transport
Soil
carbon
http://www.sheepdrove.com/312.htm
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Short term carbon cycle refers to the circulation of
carbon among the surface reservoirs (oceans,
atmosphere, soil, and biota)
Photosynthesis removes carbon from atmosphere, and
respiration returns it
Oceans absorb carbon from the atmosphere and
releases it in smaller quantities
 colder the ocean, carbon can be absorbed easier
 warmer the ocean, harder to hold on to the carbon
 analogy: cold Coke drink
Soil adds carbon via degassing
 decay of organic matter (higher the temperature,
faster decaying rate)
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Flow of carbon from one reservoir to another is well
established
 but the amount and mechanisms of transport is not
well established yet
Human activities, through burning of fossil fuels, add 6.3
Gt (gigatonnes or billion tonnes) carbon per year
Carbon imbalance:
Where’s the rest of
1.6 Gt C?
NET CHANGE =
INPUT – OUTPUT
3.2
=
(6.3 + 1.6) – (1.4 + 1.7)
3.2
=
4.8 ??!!
http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/carbon_cycle/Archive/carbon_cycle_2004.html
The case of the “missing” carbon
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Not all the amount of C added by human activities is
found in the atmosphere
The ocean plays a critical role in determining amount of
CO2 in the atmosphere on a long term time scale
Some of the anthropogenic C is stored in oceans and
biosphere
Critical to know all the sinks if to accurately predict the
expected climate change