The Carbon Cycle

Question 1

Which are the 2 grand feedbacks systems in the carbon cycle?

Answer 1

1. The “weathering CO2 thermostat” that stabilizes temperature
2. The “calcium carbonate pH-stat” that stabilizes pH in the ocean

Question 2

Which carbon reservoirs are there? which are bigger and which are smaller?

Answer 2

Atmosphere
Land
Ocean
Rocks
Fossil fuels
(all connected equally to the atmosphere (see box figure in notes)

Atmosphere < Land < Fossil fuels &laquo_space;Ocean &laquo_space;Rocks

Question 3

Describe the rock carbon cycle.

Answer 3

(fig in notes, volcano to sea to sea bottom)

• chemical weathering of rocks is a pathway for carbon to exit the biosphere and return to the solid earth.
• CO2 from atmosphere -> to rock -> to lithosphere
• Flux ≈ 0,1 PgC/yrs
• Cooler temperature -> less rainfall -> lower freshwater runoff -> lower weathering rates -> slower CO2 removal -> weathering carbon sink is stronger on a warmer planet
• has an important role, but takes 100 000 years

Question 4

How is the fossil fuel carbon pool made?

Answer 4

• ancient sunlight harvested by photosynthesis - thus storage of reduced C have been protected from oxidation by buildup of organic debris or burial - subjected to slow increasing pressure - millions of years -> increased ratio of C & H over O which leads to concentrated energy.

Coal:
dead vegetation protected from oxidation = peat –> compressed to lignite –> compressed to higher C concentration coal –> anthracite most pure coal

Oil:
dead plankton sink to sea floor –> pressure –> rock trap for oil

Question 5

Which are the ocean carbon pools? Which are bigger and which are smaller?

Answer 5

(figure in notes)

Dissolved inorganic C (in surface ocean)
Living biota
Dissolved organic C
Dissolved inorganic C (in intermediate and deep ocean)
Ocean floor sediments

Question 6

What is the air-sea gas exchange?

Answer 6

Atmosphere CO2 (gas) ocean CO2 (aqueous)

• CO2 (aq) is proportional to its atmospheric partial pressure
• increasing atmospheric CO2 level increases amount of ocean C
• increase in water temperature leads to decrease in ocean C & increase in atmospheric C

Question 7

What is the ocean C cycle response to CO2 emissions and temperature increase?

Answer 7

• decreasing CO2 solubility with temperature increase leads to continued warming will lead to outgasing of CO2 to the atmosphere
• ocean CO2 uptake will continue to be large, but the share of emissions will decrease.

Question 8

Describe the ocean acidification.

Answer 8

When adding Co2 to ocean almost all react with CO3^(2-)
CO2* + CO3^(2-) + H2O 2HCO3-
- this consumption of CO3^(2-) and adding CO2* decrease the equilibrium pH –> net effect is decrease of CO3^(2-) and increase in H+

Question 9

What are the effects on ecosystems of ocean acidification?

Answer 9

The effect of ocean acidification is negative on ecosystems. CaCO3 in water is calcite and aragonite. Aragonite is less stable and dissolves more easily because of higher [CO3^(2-)] content than calcite. As CO3^(2-) levels drop because uptake of CO2, ocean organisms made of arognite will be the first to dissolve, and before that the growth of shell organisms is inhibited.

Question 10

What land carbon pools are there?

Answer 10

Living plants
Dead plants
Peatland soils
Mineral soils
Permafrost soils

Dead plants < Living plants = Peatland soils < Permafrost soils < Mineral soils

Question 11

What factors controll the photosynthesis productivity?

Answer 11

Photosynthesis rates (per area and time)
Increases with temperature, sunlight, water, nutrients availability and atmospheric CO2 levels

• govenrs and limits the productivity on land
• hte incease in photosynthesis due to an individual factor typically levels off because other factors limit the photosynthesis
• higher photosynthesis rates do not necissarily lead to higher plant C density, eg faster growing trees not always larger

Question 12

How can the higher temperature due to climate change alter the land vegetation?

Answer 12

• if annual rainfall is large enough (and temperatures high enough) forest vegetation will prevail
• BUT, under a long dry period a fire could burn down all forest. The grass recovers first and once in grassland state fires spread more quickly and continue to burn everything. But if a lot of rain there can be more forest quicker and fires don’t spread as wide.
• –> climate change and deforestation could trigger a switch from forest to stable grassland state.

Question 13

How is carbon stored in soils?

Answer 13

soil organic carbon is partially decomposed plant matter and soil organism mass - decomposition though respiration by microbes

Soil holds much more CO2 than atmosphere, so a tiny increase in soil C would reduce CO2 atmospheric substansially

Continued global warming will lead to very large losses of soil carbon and thus CO2 are added to atmosphere

Question 14

How much carbon in the soil is dead vs living carbon on land?

Answer 14

85% land carbon is dead carbon residing from organic matter. 80% percent of the living carbon on land exists in forests.

Question 15

How come carbon can be stored for so long in soil?

Answer 15

Organic carbon persists in soil as C and not CO2. This is because of the lack of accessibility of soil C for decomposting microbes.

It is inaccessible in 2 ways, which corresponds to lack of O2.

1. Long term protection through special inaccessibility from microbes.
2. Long term protection through low interaction with surface and metals.

Question 16

What does the amount of soil carbon depend on?

Answer 16

• rate of input of organic C and rate of decay in soil
• Input depends on:
  
  • photosynthesis rate
  • plant and vegetation type (large roots)
  • harvest (removal of plant matter)
• Decay rate is proportional to soil carbon content, but varies with:
  
  • soil temperature
  • soil water (O2 rate)
  • soil type

(figure on atmosphere C to plants to soil in notes)

Question 17

Define net primary production.

Answer 17

Net primary production (NPP) is the origin of soil which determines the soil carbon levels
Higher NPP –> higher C stock

Carbon from plants above ground and roots below ground.

Question 18

Describe how temperature is a decay factor for C in soil.

Answer 18

There is a threshold at which C storage is filled. The temperature of soil determines the soil carbon level. There is higher livels of soil decay with higher temperatures. In cooler climates there is relatively more C in the soil.

Question 19

How does carbon storage in peatlands work?

Answer 19

Peatlands store much reduced carbon. There is slow O2 diffusion in water which means slow decay of dead plant matter. Thus carbon stays as carbon and doesn’t turn into CO2.

Question 20

What are some human influences on carbon stocks on land?

Answer 20

• mineral fertilizers leads to larger soil stocks (increased NPP/ha –> increased C input to soil)
• after a harvest it takes 50-200 years for a forest carbon stock to recover
• Land use is a source of CO2 because of deforestation driven by increasing food production fpr a growing population.

Question 21

Describe the carbon opportunity cost.

Answer 21

The Carbon Opportunity Cost (COPC) is the lost potential of carbon storage caused by land use. The lost carbon storage means increase in CO2 in atmosphere.

COPC = (one-off emissions in ton of CO2 per ha)/(accumulative harvest over T years with discounting)

Question 22

Why use discounting?

Answer 22

Discounting is used to value emission reduction today more than future reductions.

Question 23

What are the main causes of methane emissions?

Answer 23

40% energy, gas leaks

40% food, mainly beef (ruminants with microbes in their stomache) and dairy

Question 24

Describe the anoxic and oxic chemical production equations

Answer 24

Anoxic: 2*CH2O –> CO2 + CH4
Oxic: CH2O + O2 –> CO2 + H2O

Much less methane being produced when oxygen is present.

Question 25

Describe the 4 land carbon feedback mechanisms due to increased CO2 levels and climate change, and if they have a cooling or warming (negative or positive) effect.

Answer 25

1) Increased photosynthesis - due to increase in CO2 and temperature. This increases the land C pools and keeps down the atmospheric CO2 levels. —> cooling response
2) increased decomposition of mineral soil C - due to increasing temperatures. The increasing temperatures increases the decomposition rate and thus decreases the land carbon pools which in turn increases the atmospheric CO2 levels. –> warming effect
3) Thawning of permafrost - soil C decomposting because of increasing temperatures. –> warming effect
4) Forest diebacks - due to hotter draughts there is more fires and less trees grow back becuase of the increasing temperatures. Also because of the human deforestation. –> warming effect

Question 26

TF: Explain the mechanisms by which weathering of rocks control atmospheric CO2 levels on time scales of millions of years. Make a drwing to illustrate.

Answer 26

Chemical weathering of rocks is a pathway for carbon to exit the biosphere and return to solid earth. It goes: CO2 from atmosphere - to rock - to earth.
In steady state, the weathering carbon sink is balanced by the degassing source, and has a flux of ≈ 0.1 PgC/year.
It is dependent on rainfall and therefore also temperature. Cooler temperatures means less rainfall. That means lower freshwater runoff and thus lower weathering rates and so slower CO2 removal.

(figure important!)

It includes:

• Volcanic CO2 degassing
• CO2 in atmosphere with flux 0.1 PgC/yr over volcano
• arrows showing runoffs from earth with equation over the sea: CaSiO3 + CO2 –> CaCO3 + SiO2
• ocean: solid shells sinking to sea floor: CaCO3 + SiO2
• Ocean floor: CaCO3 + SiO2, sliding down towards volcano
• Metamorphic decarbonization: CaCO3 + SiO2 –> CaSiO3 + CO2

Question 27

TF: The ocean has absorbed about 30% of human CO2 emissions since 1750, and is likely to be a net sink of future emissions. How does the ocean take up this additional CO2? Describe the mechanisms involved and some of their key characteristics, such as timescale.

Answer 27

By dissolving CO2 in seawater.

1. Atmosphere to sea-surface transport:
   CO2 (gas) CO2 (aq)
   timescale ≈ 1 year
2. Carbonate buffering (seawater CO3 consumption)
   CO2* + CO3^(2-) + H2O 2HCO3- (CO2* = sum CO2 and H2CO3)
   timescale ≈ 1000 years
3. Downward diffusion of carbon and mixing of surface water with deep water
   timescale ≈ 1000 years
4. CaCO3 compensation
   CaCO3 + CO2* + H2O Ca(2+) + 2HCO3-
   timescale ≈ 1000-10 000 years

Question 28

Explain the mechanisms of the “soft tissue pump” in the ocean. Make a drawing to illustrate. Also explain how this pump influences the CO2 concentration in the ocean and in the atmosphere.

Answer 28

Soft tissue pump = plankton in ocean (figure in notes important!)

Photosynthesis by plankton causes uptake of CO2
When the plankton die they sink downwards, exporting the C to deeper waters

Atmosphere CO2 (gas) - surface ocean -> CO2 (aq) - photosynthesis -> Plankton - Intermediate ocean -> Dead plankton

1. -> decomposition -> CO2 -> upwelling CO2 to surface ocean
2. -> sinking biota (very little) to deep ocean -> ocean floor sediments

Influence on CO2 concentration in:

1. Ocean - photosynthesis of the “soft tissue pump” contributes to lower CO2 concentration in the surface ocean
2. Atmosphere - ocean keeps dissolving CO2 and thus keeps the atmospheric CO2 levels lower

Question 29

TF: Methane emissions from ruminants

Why and how does ruminants produce methane in their feed digestion? Describe the mechanisms behind the emissions. Make a drawing to illustrate.

Answer 29

Ruminants have an anoxic incubator organ called “rumen”. The rumen has to microbes that enable digestion of the cellulose in the eaten plants. Some of the microbes that exploit the energy in the plant material produce methane. –> methane is exhausted from the animals

Hydrolysis and fermentation by anaerobic bacteria and funghi which gain energy from partial oxidation of carbon in organic matter to fatty acids, H2 and CO2. Degredation of fatty acids, H2 and Co2 by “archaea” leads to anoxic energy gain through:
CO2 + 4H2 –> CH4 + 2H2O because lack of oxygen
acefate + H+ –> CH4 + CO2 archaea gain energy through reactions.

Question 30

TF: Carbon buffer chemistry in sea water

Describe the change in carbon buffer chemistry in sea water in response to addition of CO2. Why does adding CO2 make seawater more acid?

Answer 30

Carbonate buffering:
CO2* + CO3(2-) + H2O 2HCO3-
- this is the chemistry of absorbing CO2 to water by C diffusion and water mixing.

The consumption of CO3(2-) and adding of CO2* decreases the equilibrium pH, as the reaction of CO2* with water also produces hydrogen ions, H+, which by definition makes for an acid.
CO2* + H2O HCO3- + H+

The net effect is a decrease in CO3(2-) and an increase in H+ which leads to acidification.

Question 31

TF: What is the ocean carbonate system?

Answer 31

The ocean carbonate system is the chemical process that allows for inorganic carbon to dissolve in the ocean in 3 fully oxidized forms, connected through chemical equilibria.

CO2 (aq) + H2O H2CO3
CO2* + H2O H+ + HCO3-
HCO3- H+ + CO3(2-)

Question 32

TF: In what way is the carbonate system important to the ocean C cycle and ocean ecosystem functioning? Describe 2 important aspects.

Answer 32

1) The carbonate system is important because it governs the pH in the ocean.
- increase of CO2* -> decrease in pH
- increase of CO3(2-) -> increase in pH

2) Due to the carbonate system the ocean can take up much more carbon than otherwise (so is needed for other ecosystems to function)