The Carbon Cycle (incl. synoptic links with Water Cycle) (copy)

Earth’s Major Stores of Carbon: Lithosphere

Size: <100 million GtC (marine sediments/sedimentary rocks), 4,000 - 10,000 GtC (fossil fuels)

Distribution:

• Inorganic (non-living or previously living): Carbonate-based sedimentary deposits (e.g. limestone)

• Organic: Litter, Organic matter (e.g. peat), Humic substances found in soils, Coal, Oil, Gas, Oil shale

Earth’s Major Stores of Carbon: Hydrosphere (incl. process of fossilisation & vertical distribution)

Size: 38,000 - 40,000 GtC

Distribution:

• Surface layer (930 GtC): rapid exchange with atmosphere

• Deep ocean (37,100 GtC): carbon stays for centuries

• Vertical Distribution: polar waters colder > more soluble > holds more CO2

Process of fossilisation: Organisms die = dead cells/shells sink into deep water = forms layers of sediments which turn into rock (estimated 100 million GtC)

• Primarily stored: dissolved CO2 and bicarbonate ions

Earth’s Major Stores of Carbon: Biosphere

Size: 3,170 GtC

Distribution:

• Living vegetation: 19% (amount of carbon varies depending on location e.g. high-latitude > low latitude, & vegetation type)

• Reason: low temperatures, waterlogging, permafrost > decomposition slower

• Plant litter: leaf tissues = 70% of forest litter (so rapid CO2 release)

• Soil humus: 69% = soil, 31% = biomass

• TRF: main biomass store, tundra: main soil store e.g. peat/organic matter

• Fact: carbon stored (soil) > carbon stored (vegetation above it)

Earth’s Major Stores of Carbon: Atmosphere

Size: 800-850 GtC (+4-5/year via humans)

Distribution:

• More industrial activity, more land mass, more biomass > higher in North

• CO2: 720-800 GtC & 0.04% of atmosphere

• CO2: (3,000 - 9,000) to 400 ppm over last 500 million years

Earth’s Major Stores of Carbon: Cryosphere

Size: 1,600 GtC

Distribution:

• Frozen organic matter in permafrost of Arctic (Siberia, Canada, Alaska), trapped for thousands of years

Risk: warming > CH4 release (25x more potent than CO2) > positive feedback

Sere (definition & how it stores carbon)

A stage in ecosystem succession (when species/habitat changes over time)

Steps:

• Ecosystem develops from bare ground to climax community > carbon store increases (via carbon sequestration)

• Plants decompose > builds up humus (which stores carbon)

• However: forest fire/disease > opposite effect

Example: solidified lava fields of Eldgjá in Iceland

Photosynthesis (fast):

- Phytoplankton in euphotic (sunlight) zone/terrestrial plants/photosynthetic algae & bacteria = carbon turned into organic matter

- Uses energy from sunlight to combine CO2 with H2O to form carbohydrates (CH2O), which store energy (whilst releasing O2)

Respiration (fast):

- Plants use stored carbohydrates as energy source

- Some remain as biomass (which consumers get energy from)

- O2 + CH2O liberates stored energy (with H2O & CO2 as additional by-products)

• Transfers occur at various scales, such as the plant scale or within a sere

• Microbial respiration in the soil (pedosphere), which releases CO2 as decomposers break down organic matter

• Fast: rate of repsiration changes drastically according to diurnal/seasonal cycles

• Acts as a carbon source (if it exceeds photosynthesis)

Decomposition (fast):

- Transforms organic matter into stabler forms (& releases CO2, CH4 if oxygen deficient)

- Physical mechanisms: animals/wind/other plants/leaching/transport

- Chemical mechanisms: oxidation/condensation

- Biological mechanisms: feeding/digestion aided by enzymes

- Decomposers: break down cells/tissues from dead organisms from biomolecules into smaller molecules (which allows recycling into soil)

- Lack of decomposition = lack of plant growth

Combustion (fast):

- When organic material burned with O2 present to produce CO2/H2O/energy

- Biomass combustion: burning of living and dead vegetation

- Happens most in:

- Boreal (northern) forests (e.g. Alaska)

- African savannah grasslands

- Tropical forests

- Temperature forests (e.g. US)

- Agricultural waste after harvests (e.g. South/East Asia)

- Fires (which burn 3-4 million km^2/year & release >8 billion tonnes of CO2/year): releases/oxidises only 10-20% of carbon (& allows organic layer of soil to accumulate storing carbon)

Burial (slow)/Compaction (slowest):

- Oceans absorb carbon = goes into shells/skeletons of marine creatures (as CaCO3)

- Creatures die = sink to bottom

- Compacting down = creates sedimentary rocks

- Under heat and pressure: carbon (from organic matter) = trapped in sediment & converted into hydrocarbons

Carbon sequestration (slow):

- Capturing CO2 from atmosphere & storing it long-term

- Geologic sequestration: CO2 captured at source & injected underground/oceans as liquid

- Terrestrial/biological sequestration: using plants/soil to capture CO2 & storing it as carbon (however forest fire/disease/infestation = carbon emitted back into atmosphere)

Weathering (slow):

- Carbonation (chemical weathering): CO2 + water vapour = weak carbonic acid (makes precipitation acidic)

- CaCO3 (in rocks) + acidic water = calcium bicarbonate (carried away via runoff/percolation, so lithosphere > hydrosphere)

Thesis (Natural Variations vs Human Activity)

• <1800s: natural variation dominated - maintained a dynamic equilibrium

• After: human activity “short-circuits” system by transferring carbon from slow cycle (lithosphere) to fast cycle (atmosphere) > irreversible positive feedback loops

• Pace: natural shifts take millennia (180 to 300 ppm), human shifts took 200 years (now >400 ppm)

Milankovitch cycles

• Changes in Earth's eccentricity/orbit (circular → elliptical every 100k years) > alters seasonal distribution/intensity of solar radiation (allows snow to survive in summer at high latitudes) > triggers (inter-)glacial periods > positive feedback via oceans

Wildfires

• Positive feedback: global warming via more CO2 = drier forests = more wildfire risk (worsened by slash & burn)

• Statistic: fires burn 3-4 million km^2 & release >8 billion tonnes of CO2

• However: fires release only 10-20% of carbon (& help build up the soil organic layer (long-term storage)

• Example: 1997 Indonesia (caused by El Nino) = <11.7 million hectares burnt & 1.17 GtC released (19% of global CO2 emissions that year)

Volcanic activity

• Short-term impact (cooling): releases SO4 = ↑ solar radiation reflected = cooler atmosphere

• Long-term impact: releases CO2 (negligible: <1% of human emissions)

• Example: 1815 Mt Tambora Indonesia - 100 million tonnes of CO2 released

Combustion of fossil fuels:

- 30% of CO2 absorbed by oceans = ocean acidifcation = kills phytoplankton (carbon sinks) = positive feedback

• Scale: transfers 100s of millions of years of stored carbon instantly

- Belchatow Power Station: <30 million tons of CO2 annually

Farming practices (incl. River Exe)

• >3 metre deep gullies/peat surface drying = oxygen can penetrate into peat = decomposition = CO2 release (1-10 million hectares/year globally)

• Conversely: >2,600 hectares of peatland restored (keeps carbon sequestered) = <9 tonnes of CO2 saved/year & <50% reduction in carbon leaving moorland via runoff

• Sheep & cattle compaction = high downward pressure “poaching pockets” (10-12 cm deep for cattle, 2-6 cm deep for sheep) > destroyed soil structure > weaker carbon pump

Deforestation

• Soil Mechanism: Loss of shade → drier soil → faster decomposition → CO2 release (as well as weakened carbon pump)

• Example: The Amazon Rainforest - Historically a sink (300m tons/year), but some areas are now becoming net sources due to clearing.

• Rate of Loss: ~10 million hectares/year (2015–2020). Roughly 274 km² lost daily.

Urbanisation

• Covers 2% of land but generates 97% of anthropogenic CO2

• Cement production: 8% of emissions

Radiative forcing

Difference between incoming solar energy absorbed by Earth & energy radiated back to space

Impact of Carbon on Atmosphere/Global Climate

• EGE: CO2/CH4 > traps more outgoing long-wave radiation, “radiative forcing” = 1.4 W/m²) > warming

• Thawing of permafrost > CO2/CH4 release > warming (positive feedback)

• Permafrost: 1,672 GtC (∴ if 10% thawed → could increase temperatures by 0.7*C by 2100)

• CO2 levels continue to rise at projected rate = nearly all ice cover melted by 2100

• Up to 20% of extra (pre-Industrial Revolution) CO2 may remain in atmosphere for thousands of years

Impact on Land

• More CO2 = more photosynthesis = more intensive agriculture/longer growing season e.g. Sahel desert (& negative feedback as more CO2 intake)

• However: baked soils/stressed plants/risk of dehydration > overall impact can be negative & carbon pump weakens (positive feedback)

Impact on Oceans (incl. eval.)

Acidification:

• 30% of anthropogenic CO2 diffused into oceans = more acidic

• Impact: shells of coral/plankton = thinner/more fragile = carbon pump weakens (positive feedback)

• Tipping points: coral reefs loss: food risk for 500 million people

Physical Impacts:

• Thermal expansion (water molecules move apart > 3.5 mm/year eustatic change since 1990s)

• Last 35 years: Arctic has retreated by 40% > dilutes salt content (lower concentration) > more buoyant surface layer > disrupts Thermohaline Circulation (where salty/dense water sinks = heat transported from tropics to Poles) > hotter at tropics/cooler at Poles (overall: warmer > positive feedback)

• Positive feedback: warmer oceans > reduced phytoplankton (optimal conditions are cooler) = reduced carbon sequestration (although increased CO2 = increased phytoplankton, nutrient limitation overrides benefit)

Evaluation:

• Kiribati/Maldives face “existential threats”, HICs can afford sea walls

How Water & Carbon Support Life & Climate

• Carbon: 18% of body, 50% of tree’s biomass

• Essential for all biological reactions incl. photosynthesis

• No greenhouse gases > no natural greenhouse effect > -18*C global temp.

Atmospheric relationship between Water & Carbon: Acid Rain

Carbon + water vapour > carbonic acid (acid rain), transferring carbon from atmosphere > hydrosphere > lithosphere

Soil organic carbon

Organic constituents in the soil: tissues from dead plants/animals/products produced as these decompose/soil microbial biomass

Positive feedback (111122224): water & carbon cycle

Start:

• Human activity =

• More CO2 released into atmosphere =

• Global warming (via e.g.e.)

1A:

• Global warming =

• Tundra/polar regions warms/melts (40% ice cover lost in 35 years) =

• More CO2/CH4 released into atmosphere =

• Increased rate of global warming (via e.g.e.)

1B:

• Global warming =

• Melting permafrost =

• Organic matter starts to decompose =

• Bacteria produce CO2 & CH4 =

• Increased rate of global warming (via e.g.e.)

1C:

• Global warming =

• Tundra/polar regions warms/melts =

• Darker land/ocean surfaces revealed (with lower albedo & greater heat absorption) =

• Further melting =

• Increased rate of global warming (via e.g.e.)

1D:

• Global warming =

• Tundra/polar regions warms/melts =

• Dilutes salt content (lower concentration) =

• More buoyant surface layer =

• Disrupts Thermohaline Circulation (where salty/dense water sinks = heat transported from tropics to Poles) =

• Warmer oceans (use “2B” to finish your loop)

2A:

• Global warming =

• Increased ocean temperatures =

• Increased energy for evaporation in oceans =

• More H2O released into atmosphere

• Increased rate of global warming (via e.g.e.)

2B:

• Global warming =

• Increased ocean temperatures =

• Oceans less soluble =

• Decreased CO2 absorbed by oceans =

• Increased rate of global warming (via e.g.e.)

2C:

• Global warming =

• Increased ocean temperatures =

• More acidic =

• Thinner shells/corals

• Decreased CO2 absorbed by oceans =

• Increased CO2 in atmosphere =

• Increased rate of global warming (via e.g.e.)

2D: Global warming =

• Increased ocean temperatures =

• Reduced phytoplankton (optimal conditions are cooler) =

• Reduced carbon sequestration (although increased CO2 = increased phytoplankton, nutrient limitation overrides benefit) =

• Increased CO2 in atmosphere =

• Increased rate of global warming (via e.g.e.)

4. Global warming =

• Wildfires/baked soils/stressed plants/risk of dehydration =

• Reduced photosynthesis =

• Increased CO2 in atmosphere =

• Increased rate of global warming (via e.g.e.)

Negative feedback: water & carbon cycle

1.

• Global warming =

• Increased photosynthesis (& CO2 intake) =

• Decreased CO2 atmospheric levels =

• Decreased rate of global warming

2.

• Global warming =

• Increased evaporation =

• Increased cloud formation =

• Reduction in amount of solar radiation reaching Earth’s surface =

• Decreased rate of global warming

3. Arctic has retreated by 40% =

• Dilutes salt content (lower concentration) =

• More buoyant surface layer =

• Disrupts Thermohaline Circulation (where salty/dense water sinks = heat transported from tropics to Poles) =

• Hotter at tropics/cooler at Poles

Impacts of Global Warming (for Life on Earth)

3 Main Impacts:

• Extreme weather

• Water shortages

• Biodiversity Loss (incl. ocean acidification)

Other Impacts:

Social:

• Warmer winters = disease-carrying vectors to migrate poleward (e.g. Italy)

Economic:

• “Winner”: UK wine industry (2004-2021: expanded by 400% & “intermediate-climate” wine region by 2040)

• “Loser”: Sub-Saharan Africa: maize yield declines <20%

• Increased heat stress = reduced labour productivity (negative feedback loop)

• Another negative feedback loop: lower yields = lower incomes = reduced ability to invest in adaptation technologies

Impact of Deforestation on Carbon Cycle

• No photosynthesis/respiration/decomposition

• Increased soil carbon content via ash (created by deforestation)

• Carbon dissolved in stream via soil erosion

• If land used for farming: methane

• Increased peatland exposure to wind/rain = increased decomposition rate = more CO2 released