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Title: Lecture 5 ??????


1
Lecture 5 ?????? Understanding the global
carbon cycle
  • What is Biogeochemistry?
  • Biogeochemistry and Carbon Cycle
  • The Breathing of Gaia
  • Carbon Cycling

2
BioGeoChemistry
  • life processes on earth are, in essence, carbon
    chemistry.
  • The carbon cycle, movement of carbon atoms
    through various places of storage on earth
    (reservoirs), is tied to life processes.
  • In studying the carbon cycle, biology and
    geochemistry merge to form a new scientific
    discipline biogeochemistry.

3
BioGeoChemistry
  • The all-important role of life processes in
    maintaining Earth's environments was stressed by
    the Russian mineralogist, Vladimir Vernadsky
    (1863-1945), the father of biogeochemistry.
  • The American geochemist G. Evelyn Hutchinson
    (1903-1991) first outlined the principles.
  • The basic elements of biogeochemistry have been
    popularized by the James Lovelock (1919 -), under
    the label of Gaia Hypothesis.?
  • Gaia Hypothesis a concept that life processes
    regulate the radiation balance of Earth to keep
    it habitable.

4
BioGeoChemistry
  • Biogeochemists study the carbon cycle and its
    interactions with the cycles of other elements
    involved in life processes nitrogen, oxygen,
    phosphorus, sulfur and iron, etc.
  • It is the hydrological cycle that helps drive the
    carbon cycle, and this is where the climate and
    carbon cycle are most intimately connected.
  • Biogeochemistry studies the history of the great
    carbon reservoirs in the crust of Earth (e.g.
    limestone rocks coal deposits) and distribution
    of nitrate and phosphate in oceans.

5
BioGeoChemistry
  • Biogeochemistry seeks to explain the composition
    of the atmosphere as a result of bacterial action
    and photosynthesis.
  • It records the exchange of matter at the
    interfaces
  • (1) decay of organic matter in soils and
    resulting gases released into the air
  • (2) the uptake of oxygen by oceans and its
    utilization at depth
  • (3) leaching of nutrients from soil and their
    transport into ocean

6
Biogeochemistry and carbon cycle
  • Carbon cycle is the core of biogeochemistry. It
    describes the movement of carbon atoms through
    the life-support systems on the surface of the
    planet.
  • Models of the carbon cycle consist of
    "reservoirs" of carbon and the "fluxes" between
    these reservoirs.
  • Reservoirs include ocean, atmosphere, biosphere,
    soil carbon, carbonate sediments, and organic
    carbon sediments.
  • Fluxes describe the rate at which atoms move from
    one reservoir into another. E.g., flux could be
    the rate of movement of carbon between organic
    matter produced in ocean surface and the
    sediments in the ocean floor.

7
A sketch of carbon cycle illustrating fluxes and
reservoirs (From SeaWIFS project)
8
Biogeochemistry and carbon cycle
  • The crucial questions concern the mechanisms that
    control the fluxes, and how these controls change
    as the planet is warming.
  • What controls the productivity of the ocean, and
    what controls the proportion of the matter
    produced that reaches the ocean sediment? How
    does the amount of plankton change with a warming
    ocean, and how does the flux of organic matter to
    the seafloor change as a result?
  • As for future projection, we first must
    understand what has happened in the past and what
    has happened so far.

9
Reservoirs of carbon (in GtC) and fluxes between
reservoirs (arrows)
Reservoirs differ greatly in size and in their
ability to respond to changes, a property called
reactivity.? Large reservoirs with small fluxes
in and out are not very reactive. Small
reservoirs with relatively large fluxes in and
out are very reactive - as far as carbon is
concerned, the atmosphere is such a Reservoir.
Fortunately, the atmosphere is closely coupled to
the ocean, a large Reservoir that can offset this
problem and stabilize the atmosphere.
Unfortunately, the atmosphere's dependency on the
ocean has a drawback if the ocean reacts to
climate change by giving off a small proportion
of its CO2, the atmosphere, with its low
concentrations of CO2, greatly amplifies the
effect. In other words, what seems a small
adjustment for the ocean results in a big change
in the atmosphere.
10
Why So Little Carbon in our Atmosphere?
  • Plants, algae and shell-making organisms are
    responsible for the large-scale solidification of
    CO2 within carbonate minerals (in limestone) and
    organic materials. Making coal and other organic
    matter has also led to splitting the carbon from
    the oxygen, with much of the oxygen staying in
    the air. This has produced an atmosphere
    fundamentally different from those of Venus and
    Mars.
  • Earth would be chemically out of balance and
    therefore "unsustainable" were it not for Earths
    ongoing life processes.
  • The low CO2 in atmosphere are a result of the
    biologically-mediated movement of CO2 from
    reactive reservoirs (the atmosphere and ocean) to
    much less reactive reservoirs (limestones and
    organic matter).
  • Although these long-term reservoirs can be heated
    (through subduction by plate tectonics),
    rereleasing the CO2 into atmosphere, weathering
    and life processes then cycle them back into the
    long-term storage, continuously keeping the
    values low.

11
Seafloor Spreading Rate Hypothesis is also known
as BLAG Hypothesis to denote its initial authors,
the geochemists Robert Berner, Antonio Lasaga,
And Rober Garrels. It proposes that the
tectonic-scale climate changes are driven by
variations in the global average rate of seafloor
spreading that leads to the variations of
volcanic and in turn could alter the amount of
CO2 emitted into the atmosphere.
12
Initial Forcings
Negative Feedback Loop
Initial Forcings
Negative Feedback Loop
13
Chemical Weathering
HCO3- Bicarbonate
14
Negative Feedback From Chemical Weathering
  • The chemical weathering works as a negative
    feedback that moderates long-term climate change.
  • This negative feedback mechanism links CO2 level
    in the atmosphere to the temperature and
    precipitation of the atmosphere.
  • A warm and moist climate produces stronger
    chemical weathering to remove CO2 out of the
    atmosphere ? smaller greenhouse effect and colder
    climate.

(from Earths Climate Past and Future)
15
BLAG Carbon Cycle
  • On Land CaSiO3 CO2 -gt CaCO3 SiO2
  • Subduction CaCO3 SiO2 -gt CaSiO3 CO2

On tectonic timescale
BLAG hypothesis provides a long-term regulatory
mechanism to the climate system by moving a
roughly constant amount of total carbon back and
forth between the rocks and the atmosphere.
16
Uplift (Weathering) Hypothesis
Maureen Raymo and her colleagues (1986) proposed
a secondary hypothesis to explain how the plate
tectonic activity might moderate the amount of
atmospheric CO2 level. The uplifting of mountains
and plateaus (mainly caused by the collision of
continents) inevitably results in several
processes favoring/accelerating the chemical
weathering to remove atmospheric CO2 level gt
17
????????chemical weathering ????? BLAG???chemical
weathering ????????????????????????CO2???,???????
???? ? ?????????? ?????,???? ? uplifting??????che
mical weathering ?????????????,??????????????????
????????????? ?
18
  • The weathering on land (CaSiO3 CO2 -gt CaCO3
    SiO2) was first proposed by Harold Urey in 1950s
    to understand the fundamental process of removing
    CO2 from atmosphere.
  • According to Ureys model, the amount of
    atmospheric CO2 is regulated by the presence
    hydrologic cycle.

19
  • Is this really valid?
  • Atmospheric CO2 also comes out of volcanoes.
  • The rate at which this happens is presumably
    independent from the surface reactions described
    in Ureys proposal.
  • After entering the atmosphere, some of CO2 is
    concentrated in the soil by the action of plants
    (and bacteria, fungi).

20
  • Is this really valid?
  • The reactions of CO2 with silicate minerals
    within the soil, therefore, do not proceed
    according to the concentration of atmospheric
    CO2. In addition, the rate of dissolution of
    rocks is contingent not only on the presence of
    water, but also the presence of microscopic
    organisms on the surface of the rocks.
  • Moreover, the precipitation of the carbonate and
    silica is made possible not only by inorganic
    processes but also by organisms (algae, corals,
    and foraminiferans produce carbonate and diatoms
    and sponges make silica).

21
Lessons we learn are
  • The above thought analysis of Ureys approach
    point to the very importance of life in
    influencing the atmospheric CO2 levels.
  • The reactions that govern the long-term storage
    of carbon are rate-dependent and these rates are
    determined not only by the plate tectonics BUT
    ALSO by the life processes, factors not included
    in Ureys model.
  • Therefore, in foreseeing what will happen in
    humans timescale, the changes in our ecosystem
    talk.

gt The breathing of Gaia
22
Important indication of Keeling curve
CO2 changes seasonally over quite a large range.
In addition, continuing the measurements showed
that the values drift upward from one year to the
next. After these discoveries, the science of the
carbon cycle had changed forever. Since then, the
"Keeling curve" has become the symbol of the
ever-changing chemistry of the atmosphere and the
associated warming of our planet.
23
The breathing of Gaia
  • Is it the ocean with its large reservoir, warming
    and cooling? Or is it processes on land, having
    to do with plant growth indicated in Keeling
    curve?
  • The answer is actually land plants. Since most of
    the land is located in NH, the fluctuations are
    greatest here. (If the ocean were to blame, we
    should see a larger effect in SH.)
  • Gaia breathes?on an annual cycle.
  • Expect an equally vigorous exchange within the
    ocean? Yes, such an exchange does exist and it
    results in a rather short residence time of the
    carbon in the atmosphere, less than 10 years.

24
The Carbon cycling
25
The Carbon cycling
The exchange of carbon between the atmosphere and
the ocean/land takes place in several ways
  1. The physical carbon pump
  2. The biological carbon pump
  3. The marine carbon cycle
  4. The terrestrial carbon cycle

26
The physical carbon pump
  1. The most important mechanism is through physical
    mixing of the ocean (i.e. vertical deep mixing).
    When seawater is cooler it takes up more.
  2. Vertical circulation makes sure that CO2 is
    constantly being exchanged between ocean and
    atmosphere and is ultimately responsible for the
    fact that cold water fills the depths of the
    ocean.
  3. Vertical circulation acts as an enormous carbon
    pump, giving the ocean more carbon than if
    equilibrium with the surface ocean.

27
Sketch illustrating the concept of vertical deep
mixing
What will happen if the ocean become warmer (or
cooler)?
28
Warming the oceans A Thought Experiment
  1. Warming of ocean waters takes place from the top,
    so at first a little more CO2 is released into
    the air from below. The warm current is not as
    cool it used to be when it reaches high
    latitudes. It then takes up less CO2 than it
    would otherwise and, in addition, it does not
    sink as deeply.

29
Warming the oceans A Thought Experiment (cont.)
  1. The ocean also yields some of its own CO2 and
    slows its uptake of CO2 from the atmosphere. The
    deep cold water no longer participates very
    actively in the vertical circulation and tends to
    stagnate. Oxygen (O2) is used up while CO2 is
    being produced from organic matter on the sea
    floor and from organic matter still falling down
    from above. In places where O2 is entirely used
    up, nitrate (NO3) is used by the bacteria as an
    oxygen source instead. In this process, nitrous
    oxide (N20 a greenhouse gas) and molecular
    nitrogen (N2) are made while nitrate is being
    destroyed.

30
Warming the oceans A Thought Experiment (cont.)
  1. By warming the oceans and weakening the physical
    pump, we have created a deep ocean reservoir rich
    in CO2 and poor in nutrients. When this cold
    water returns to the surface, it will now bring
    CO2 back to the atmosphere, without the means to
    recapture it by photosynthesis (for which
    nutrients are needed). Such a process could have
    contributed to the pulsed nature of CO2 rise
    during deglaciation, as revealed by the ice cores.

31
Cooling the oceans Another Thought Experiment
  1. Cooling also takes place from the top by removing
    heat because of evaporation, freezing, and
    infrared radiated to the sky.
  2. As it cools, the water will uptake more CO2 and
    readily mix vertically (cold water is heavier
    than warm water), sinking to the depth level
    appropriate for the density of the sinking water.

32
Cooling the oceans Another Thought Experiment
(cont.)
  1. On the whole, the atmospheric CO2 is drawn down
    and the cooling process initiates further cooling
    due to the loss of greenhouse gas, a case of
    positive feedback. This might trigger the
    reglaciation.

33
Cooling the oceans Another Thought Experiment (a
corollary)
  1. A corollary to (1)-(3) is that the water column,
    after cooling, is quite well mixed, which was not
    necessarily the case (previous warm stage)
    before.
  2. If the mixing was slower before (during the
    previous warm stage), CO2 could have accumulated
    in intermediate waters within the subsurface
    layer of water (called the thermocline).

34
Cooling the oceans Another Thought Experiment (a
corollary)
  1. With intensified mixing, the thermocline
    initially could release additional CO2 to the
    atmosphere, counteracting the positive feedback
    from cooling.
  2. This might help explain why during the initial
    phase of reglaciation, the atmospheric CO2 tend
    to stay high upon cooling as evidenced in ice
    cores.

35
Lessen learned
  • The above thought experiments illustrate how
    complicated things can get when considering the
    exchange of CO2 between ocean and atmosphere upon
    changing the climate.
  • Whether the scenarios outlined in the thought
    experiments have much resemblance to the reality
    is another matter (perhaps they do. Maybe they
    don't).
  • But it is this kind of thinking that needs to be
    exercised before going into the mathematical
    models to make them responsive to simulate
    climate change.

36
The biological carbon pump
Ocean gets a disproportionate share of the CO2
available to the ocean-atmosphere system (about
50 times larger).
37
The biological carbon pump
  1. The main reason CO2 readily reacts with water
    (H2O) to make soluble species of ions, the
    bicarbonate? (HCO3-).
  2. Another reason the physical pump described
    previously cold water holds more CO2 in solution
    than warm water. This cold, CO2-rich water is
    then pumped down by vertical mixing to depths.
  3. The last reason for the oceans big share of
    carbon is its?biological pump removing CO2 from
    the surface water of the ocean, changing it into
    living matter and transporting it to the deeper
    water layers.

38
The biological pump A Thought Experiment
  1. We start with a well-mixed ocean, dark and quite
    cold throughout.
  2. We then turn on the Sun and heat the ocean from
    above.
  3. A warm-water layer develops on top of the ocean,
    and since it is euphotic, green algae will now
    grow in this layer gt CO2 is being fixed into
    carbon compounds (photosynthesis, you know).
  4. Some of these particles of the algae (dead
    organic stuff) sink out of the euphotic zone into
    the deeper cold waters.
  5. Others could be re-mineralized decay by the
    action of bacteria, releasing CO2 back to the
    water.

39
The biological pump A Thought Experiment (cont.)
But how long can this process of carbon fixation
(item 3), carbon settling (item 4), and carbon
recycling (item 5) continue in our experiment?
Answer It can continue until all the nutrients
that are necessary for photosynthesis have been
used up.
Used up all the nutrients?
40
The sketch of oxygen profile with an oxygen
minimum zone (OMZ) at mid-depth (typically 1-km
below sea surface)
41
What about the recycling of nutrients
(phosphorous, sulfur, and nitrogen) through decay
of organic matter?
  • Yes, the decay of the organic particles not only
    recycles carbon, but also the nutrients.
  • However, the amount that is being recycled is
    diminished as the export of particles to deeper
    layers (and ocean bottom) continues.
  • At some point, the recycling (item 5)becomes
    negligible because all the nutrients have been
    exported to the cold layers below and nothing can
    grow anymore.

42
The biological pump A Thought Experiment (cont.)
  • Vertical profile of nutrients concentrations
    shows practically nothing in the warm layer, a
    maximum below the warm layer where bacteria have
    remineralized many of the particles received from
    above, and an exponential decay with depth, as
    there is less and less left for the bacteria to
    remineralize.
  • At the point of the nutrient maximum, right below
    the upper warm layer, there would also be an
    oxygen minimum zone (OMZ).
  • If we now add a slow upward movement of the water
    to simulate the process of deep circulation, we
    have a first-order model of the oxygen minimum in
    the oceans.

43
Oceanic biological pump
  • CO2 is fixed by photosynthesis,
  • 2) this organic matter sinks into deeper waters,
  • 3) bacterial decay releases CO2 and other
    nutrients, making them available to be used again
    by phytoplankton, until
  • 4) ultimately deposition locks away the carbon
    in sediments.

44
The Redfield Ratio
  1. Removing the nutrients from the surface layer,
    carbon also is being removed. The content of
    total dissolved carbon in the surface layer
    decreases.
  2. At the same depth as the nutrient maximum there
    is a maximum in total dissolved carbon as well.
  3. How much carbon is exported from the surface
    layer in the process of losing all the nutrients?
    To estimate this amount, one must know the ratio
    of nutrient atoms to carbon atoms within the
    organic matter settling out of the euphotic zone.

45
The Redfield Ratio (cont)
  1. Typical numbers describing the composition of
    phytoplankton are CNP 106161. Whenever 106
    carbon atoms are fixed into organic matter (by
    photosynthesis), 16 nitrogen atoms are fixed
    (taken from nitrate, NO3- , and ammonia, NH3), as
    well as one phosphorus atom. This sequence of
    numbers is called the "Redfield Ratio" after
    American oceanographer Alfred Redfield (1934).

46
The biological pump
  • Oceanic upwelling attempts to bring both carbon
    and nutrients back to the surface.
  • However, the biologic activity in the surface
    layer (aided by sunlight) keeps removing the
    nutrients and causing them to settle back down,
    together with the appropriate amount of carbon
    (determined by the Redfield Ratio).
  • This is a way of pumping nutrients and carbon
    down, against the upward movement of upwelling,
    and hence the term "biological pump. It aids to
    hide some of the carbon into sediment reservoir.

47
The biological pump
  • If the biological pump were turned off,
    atmospheric CO2 would rise to about 550 ppm
    (compared to the current 375 ppm).
  • If the pump were operating at maximum capacity
    (that is, if all the oceanic nutrients were used
    up) atmospheric CO2 would drop to 140 ppm.
  • Thus, if we change the overall concentration of
    nutrients in the ocean there is a net effect on
    carbon cycle.

48
The Marine Carbon Cycle (MCC)
  • The "physical carbon pump" and the "biological
    carbon pump" illustrate the mixing of the ocean
    and the biological processes in the sunlit zone
    of the ocean.
  • They are of prime importance in controlling the
    carbon budget of the sea and the exchange with
    the atmosphere.
  • Also, we have mentioned the ways in which carbon
    is stored in sediments and recycled.
  • Together, these concepts define the marine carbon
    cycle.

49
The Marine Carbon Cycle (MCC)
  • MCC involves the production and recycling of two
    types of carbon-rich materials organic matter
    and carbonate (CaCO3). The latter processes about
    four times more carbon atoms than the former.
  • The production of solid CaCO3 (so called
    carbonate precipitation?) occurs in the surface
    waters, both
  • organically - by organisms that build their
    shells from CaCO3, AND
  • inorganically according to the chemical
    equilibrium in the oceans
  • Ca 2 2HCO3- ? CaCO3
    CO2 H2O

50
The Marine Carbon Cycle (MCC)
  • Surprisingly, the deposition of large quantities
    of calcium carbonate actually tends to raise the
    atmospheric CO2.
  • However, carbonate precipitation is closely
    coupled to the "real" organic biological pump
    (discussed earlier).
  • The net effect the carbonate cycle (NOT carbon
    cycle) acts as a dragging force on the biological
    pump.
  • The amount of drag can be modified by changing
    the ratio of the number of carbon atoms that are
    involved in the carbonate cycle to those
    partaking in the organic biological cycle.

51
The Marine Carbon Cycle (MCC)
Bad guy
Good guy
Typical marine phytoplanktons diatoms (left) and
Coccolithophores (right)
52
The Marine Carbon Cycle (MCC)
  • In ocean, this is done mainly by changing the
    amount of silicate (SiO4).
  • Marine organisms called diatoms grow rapidly in
    the presence of silicate. They fix carbon into
    organic matter and take much of it down to deep
    waters (at the end of their life cycle).
  • If silicate is little, organisms called
    coccolithophores?(???) grow more readily than
    diatoms. They precipitate lots of carbon into
    carbonate. But they remove calcium carbonate from
    surface waters by precipitation, which makes
    these waters reject CO2 and thus tend to raise
    the atmospheric CO2.

53
The Marine Carbon Cycle (MCC)
  • Therefore, any process favoring the growth of
    organisms made from silicate (e.g. diatoms), over
    organisms made from carbonate (e.g.
    coccolithophorids) will tend to lower the
    atmospheric CO2, and vice versa.
  • Factors controlling the diatoms vs.
    coccolithophorids species include temperature,
    nutrient levels, and light. More subtle indirect
    factors, however, are not yet understood.

54
The Marine Carbon Cycle (MCC)
  • Blooms of carbonate-fixing plankton, like
    coccolithophores and coral, would have the net
    effect of bringing CO2 from surface waters to the
    atmosphere.
  • What precisely causes the blooms of
    coccolithophores and whether their population is
    increasing or decreasing as the planet warms
    remain unclear at present.

55
The terrestrial carbon cycling
56
The terrestrial vs. oceanic biosphere
  • Carbon on land is locked up in (1) soils (soil
    carbon) and (2) in trees (biosphere reservoir).
  • Mass of oceanic biosphere is small compared with
    that of carbon in wood.
  • Plants on land appear, for some reasons, to be
    about twice as efficient in fixing carbon during
    photosynthesis than organisms in the ocean.

57
The terrestrial vs. oceanic biosphere
  • It is not easy to make a direct comparison
    between ocean and land carbon reservoirs. On land
    (carbon mainly moves through wood), we can
    measure "productivity" fairly simply the mass of
    carbon in trees divided by their average age.
  • In contrast, measurements of oceanic productivity
    are much more difficult. One reason is because
    many of the carbon-fixing organisms are extremely
    short-lived.
  • So, is there even a purpose in comparing the
    fixation of carbon by photosynthesizing bacteria
    and other phytoplankton in the ocean with the
    fixation of carbon in wood on land? What do you
    think?

58
Changing CO2 and terrestrial response
  • There are two carbon cycles of interest on land
  • (a) The cycle involving annual growth and
    decay, and (b) the cycle involving long-term
    storage of carbon in wood, remains in soil, and
    near-surface organic deposits.
  • Both cycles have the atmosphere as intermediary.

59
Changing CO2 and terrestrial response
Q1 How will the terrestrial biosphere and soil
carbon (has plenty of bacteria) respond to
global warming? Q2 How will its feed back
into the climate?
60
Annual growth and decay
  • Decays return CO2 to the air, a reservoir from
    which CO2 can be extracted for renewed growth.
  • The sensitivity of atmosphere to land plant
    growth and decay is evident from the Keeling
    curves Upon close inspection of the annual
    cycles, the amplitude of the annual cycles is
    found to increase with time.

What is your Interpretation here ?
61
Annual growth and decay
  • The favored interpretation terrestrial biosphere
    is growing and decaying at an increasing rate
    (particularly true in NH).
  • It is difficult to see how tropical forests could
    be expanding because they are burned and
    disappearing and because they are in the tropics
    lacking seasonal variation.
  • It is thus concluded that most of the observed
    biosphere expansion comes from temperate and
    northern forests.

62
Increased plant growth in the northern forests
Figure shows the increased rate of green
vegetation during the growing season (May
September) between 1982 and 1990 (from Myneni et
al. Nature, 1997)
63
Annual growth and decay
  • A puzzle Under human interference, why does the
    figure above show that the terrestrial biomass is
    expanding?
  • Something is disguising the observed trend of
    deforestation, or
  • there is some compensating process making it
    appear as if the biosphere is getting bigger, or
  • Maybe it is more vigorous growth (and decay) of
    annuals, deciduous trees, and bushes that are
    responsible for the increase in amplitude of the
    Keeling curve.

64
Plant Growth Factors and Greening
  • High CO2 indeed stimulate plant growth.
  • Plant has to balance its need for letting CO2
    into its photosynthetic factories without letting
    water inside the plant escape, a result of the
    plant opening its pores (called stomata) during
    photosynthesis.
  • If more CO2, the pores on leaves do not need to
    open as much to get the same amount and water can
    be retained better within the plant.
  • Plant thus grows more vigorously in places where
    water is a limiting factor (e.g. blooming in arid
    area).
  • Increased precipitation due to warming further
    favors an overall increase in annual growth and
    decay.

65
Plant Growth Factors and Greening
  • However, the above story is not complete and
    there is a downside.
  • At higher latitudes as the seasons change in
    response to warming, the programming of the
    various trees (time to shed leaves when the days
    are short) will be out of synch.
  • Opportunistic shrubs (not so programmed) that
    tend to hang on until it gets too cold will take
    an advantage.
  • Thus plants adapted to warmer climates very well
    but trees do not spread across landmasses very
    fast.
  • Thus, quite likely, we will see an increased
    turmoil in the plant world weakened tree stands
    increasingly susceptible to infestation of fire.

66
The Soil Cycle
  • Plant debris are deposited and buried in the
    soil.
  • Global warming is expected to increase the rate
    at which bacteria and fungi digest the deposited
    organic material.
  • This is true for the portion of soil carbon that
    have always been frozen or close to freezing,
    like the vast areas of tundra and peat deposits
    of high northern latitudes.
  • Scientists thus worry that the response of soil
    carbon will be a positive feedback, making our
    climate even warmer.

67
End of Lecture 4
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