The Geological Carbon Cycle and Energy Security

The Geological Carbon Cycle

Definition: The natural carbon cycle involves the continuous movement and storage of carbon between the land, oceans, and the atmosphere.
Forms of Carbon: * Inorganic: Found in rocks as bicarbonates and carbonates. * Organic: Found in plant material and living organisms. * Gaseous: Found as atmospheric $CO_2$ and $CH_4$ (methane).
System Balance: Generally, there is an equilibrium between production (sources) and absorption (sinks). This equilibrium can be disrupted by events like volcanic eruptions, taking a long time to restore.
Terminology: * Stores: Locations where carbon is held (terrestrial, oceanic, atmospheric). * Fluxes: The movement or transfer of carbon between these stores. * Carbon Sink: A store that absorbs more carbon than it emits (e.g., intact tropical rainforest). * Carbon Source: A store that emits more carbon than it stores (e.g., damaged tropical rainforest).
Spheres of Storage: * Atmosphere: Stored as $CO_2$ and methane. * Hydrosphere: Stored as dissolved $CO_2$ . * Lithosphere: Stored as carbonates in limestone and fossil fuels (coal, gas, oil). * Biosphere: Stored in living and dead organisms.

Main Carbon Stores

Marine Sediments and Sedimentary Rocks (Lithosphere - Long-term): * The largest store: contains approximately $66,000 - 100,000$ million billion metric tons of carbon. * Recycled via the rock cycle and continental drift over thousands or millions of years.
Oceans (Hydrosphere - Dynamic): * The second largest store: contains $38,000$ billion metric tons. * Highly active; carbon is utilized by marine life, lost to the lithosphere, or gained from rivers and erosion.
Fossil Fuel Deposits (Lithosphere - Long-term/Dynamic): * Contains roughly $4000$ billion metric tons. * Traditionally changed slowly, but human exploitation has made this store more dynamic.
Soil Organic Matter (Lithosphere - Mid-term): * Stores $1500$ billion metric tons. * Can hold carbon for over a hundred years; currently affected by deforestation and agriculture.
Atmosphere (Dynamic): * Stores $750$ billion metric tons. * $CO_2$ levels have increased by around $40\%$ since the Industrial Revolution due to human activity.
Terrestrial Plants (Biosphere - Mid-term/Very Dynamic): * Stores $560$ billion metric tons. * Highly vulnerable to climate change and deforestation.
Distribution Factors: * Ocean storage is larger in the Southern Hemisphere. * Biosphere storage is land-focused, particularly in the tropics and Northern Hemisphere.

Fluxes of the Carbon Cycle

Carbon Sequestration: The transfer of carbon from the atmosphere to other stores (can be natural or artificial).
Photosynthesis: Plants convert carbon dioxide and water into oxygen and glucose using light energy. This process removes $CO_2$ from the atmosphere. * Formula: $Carbon\,Dioxide + Water \rightarrow Light\,Energy \rightarrow Oxygen + Glucose$
Respiration: The reverse of photosynthesis where organisms convert oxygen and glucose into energy, releasing water and $CO_2$ . * Formula: $Oxygen + Glucose \rightarrow Carbon\,Dioxide + Water$ * Daily variations: Plants are net absorbers of $CO_2$ during the day but release $CO_2$ at night when photosynthesis stops.
Combustion: Burning fossil fuels or organic matter (trees/wildfires) releases stored carbon into the atmosphere.
Decomposition: Decomposers (bacteria, detritivores) break down dead organisms, respiring and returning $CO_2$ to the atmosphere while adding organic matter to the soil.
Diffusion: The ocean absorbs $CO_2$ . Increased levels since pre-industrial times have led to a $30\%$ increase in ocean acidity.
Sedimentation: Marine shell fragments (calcium carbonate) or decaying vegetation become compacted over time to form limestone or fossil fuel deposits.
Weathering and Erosion: Carbonation weathering occurs when $CO_2$ in the air creates carbonic acid with rainwater, eroding rocks like limestone. The carbon enters the ocean via the water cycle.
Metamorphosis: Extreme heat and pressure create metamorphic rock, resulting in the release or trapping of carbon.
Volcanic Outgassing: $CO_2$ pockets in the Earth's crust are released during eruptions or via fissures.

Complex Carbon Processes and Ocean Sequestration

Ocean Role: The ocean is the largest carbon sink, storing 50 times more carbon than the atmosphere.
The Biological Carbon Pump: * Phytoplankton: Microscopic organisms that photosynthesize. They comprise $1\%$ of photosynthetic biomass but contribute nearly half of global primary production. * Food Chain: Carbon passes through the marine food web. Some organisms build shells/skeletons from sequestered carbon. Upon death, these sink to form limestone sediments.
The Physical Pump: * Surface layers absorb atmospheric $CO_2$ to form carbonic acid. * Thermohaline Circulation: This global system of ocean currents moves carbon-rich surface water into the deep ocean via vertical and horizontal circulation. * Temperature Influence: Cold water absorbs more $CO_2$ than warm water. Polar regions thus hold more carbon, while tropical waters may release $CO_2$ . * Vertical Stratification: $CO_2$ concentration is $10\%$ higher in the deep ocean than at the surface.
Thermohaline Circulation Mechanism: 1. Cold, saline, dense water sinks in the North Atlantic/Polar oceans. 2. The current is recharged near Antarctica by extra-cold, dense water. 3. The current divides, moving northward into the Indian and Pacific Oceans. 4. Branches warm and rise, looping back south and west. 5. Warmed surface waters eventually return to the North Atlantic to cool and restart the $1000$ -year cycle.

Terrestrial Sequestration and Soils

Primary Producers: Plants sequester carbon through photosynthesis. About $95\%$ of a tree's biomass consists of sequestered $CO_2$ converted to cellulose.
Consumers and Decomposition: When plants are eaten, carbon becomes fats and proteins. Decomposition by microorganisms is faster in tropical climates (high rainfall, heat, and oxygen).
Flux Variations: * Diurnally: Positive flux to ecosystems during the day; negative flux (carbon release) at night. * Seasonally: In the Northern Hemisphere winter, CO2 levels rise as plants die/decay; in spring, levels drop as growth begins.
Regional Trends: Forest area is declining in the tropics ( $5\,GtC$ loss in 25 years in Brazil/Indonesia) but increasing in Russia ( $0.3\,GtC$ ), USA ( $2.9\,GtC$ ), and China ( $2.3\,GtC$ ).
Soil Storage: * Soils store $20-30\%$ of global carbon. * Humus: A dark, rich substance formed by long-term decomposition, containing $60\%$ carbon. * Capacity Factors: Climate (growth/decay rates), soil type (clay-rich soils hold more than sandy soils), and land use (cultivation/disturbance).

The Greenhouse Effect and Anthropogenic Interference

Natural Greenhouse Effect: Shortwave solar radiation ( $100\%$ ) arrives at Earth: * $31\%$ is reflected by clouds/atmosphere. * $69\%$ is absorbed by the surface/oceans. * This $69\%$ is reradiated as longwave radiation, which is trapped by greenhouse gases to maintain stable temperatures.
Enhanced Greenhouse Effect: Since $1750$ , $CO_2$ and $CH_4$ concentrations have increased by more than $25\%$ . Since the $1980$ s, $75\%$ of emissions are from fossil fuels.
Human Impacts: * Land Use Change: Responsible for $1/10$ th of annual carbon release. * Farming: $70\%$ of Amazon deforestation is for cattle ranching; cattle release significant methane. * Deforestation: Accounts for roughly $20\%$ of global emissions; turns carbon sinks into sources. * Urbanisation: Urban areas cover $2\%$ of land but account for $97\%$ of human-caused emissions. Cement production alone accounts for $7\%$ of global $CO_2$ emissions. * Combustion: Fossil fuel use has added over $180\,Gt$ of carbon to the atmosphere.

Implications for Climate and Ecosystems

Temperature: Rising $CO_2$ drives increased average temperatures. Europe expects higher-than-average increases, especially in the East and North (winter) and South (summer).
Precipitation: Northern Europe expects increased precipitation; Southern Europe expects decreases. The ITCZ Model explains how convectional rainfall shifts.
Ecosystems: * Arctic: Warming twice as fast as the global average. Thawing permafrost releases methane, creating a positive feedback loop. Tundra species like the Red Fox are shifting north, competing with the Arctic Fox. * Marine: Ocean acidification threatens coral reefs. When algae leave due to heat stress, coral bleaching occurs, impacting the $25\%$ of marine species that rely on reefs.
Hydrological Cycle: Increased evaporation, melting sea ice/glaciers, and increased risks of winter flooding and summer droughts.

Energy Security and the Global Energy Mix

Energy Security: Uninterrupted availability at an affordable price. * Long-term: Investments to meet economic/environmental needs. * Short-term: Ability to react to sudden supply/demand changes.
Supply Aspects: Availability, Accessibility, Affordability, and Reliability.
Measuring Consumption: Measured via Kgoe/yr (equivalent kg of oil), GJ/yr, or Mwh/yr.
Energy Intensity: Energy used per unit of GDP; decreases as countries develop and become more efficient.
Energy Mix Breakdown: * Coal: $27\%$ of production; declining in China. * Petroleum: $32\%$ of production; usage increasing. * Natural Gas: $22\%$ of production; half the emissions of coal. * Uranium (Nuclear): $4\%$ of production; low carbon footprint. * Renewables: Wind, solar, and geothermal are increasing. Solar usage grows as technology costs drop. Tidal and wave power remain expensive with few schemes (e.g., the abandoned Swansea Bay scheme).

Key Energy Players and Path Challenges

Energy Players: TNCs (BP, Shell, ExxonMobil, Saudi Aramco, Petrobras, PetroChina, Gazprom); IGOs like OPEC; National Governments; and Consumers.
TNC Advantages: Massive economic value, ability to bypass political tensions, and investment in local infrastructure.
OPEC: Controls $81\%$ of discovered oil reserves; aims to stabilize markets via quotas. Between $2012-2016$ , they flooded the market to compete with US fracking.
Geopolitics: Energy supply is used as a political tool. Pathways like pipelines and tankers depend on international cooperation.
Choke Points: Transport bottlenecks like Ukraine, where disruption leads to price spikes and insecurity.

Unconventional Fossil Fuels and Alternatives

Shale Gas (Fracking): * Process: Pumping water, chemicals, and sand to break shale and release hydrocarbons. * Pros: Less polluting than coal, economic boost. * Cons: Groundwater pollution, low-magnitude earthquakes, flammable tap water (USA).
Tar Sands: Extracting bitumen from sands. * Pros: High value, creates rural jobs. * Cons: Extremely water/energy intensive; produces chemical-heavy tailing ponds.
Biofuels: Theoretically carbon-neutral but associated with deforestation (oil palms in Malaysia, maize in USA). * The Tortilla Riots (2001): Competition between biofuel production and food supply in Mexico led to unaffordable corn prices.
Carbon Capture and Storage (CCS): Technology to capture $CO_2$ from power plants and store it underground. Potentially reduces emissions by $19\%$ but is currently very expensive.
Hydrogen Fuel Cells: Convert chemical energy from hydrogen into electricity, producing only water as a by-product.

Climate Tipping Points and Feedback Mechanisms

Negative Feedback: Dampens the original process.
Positive Feedback: Amplifies the original process (e.g., permafrost melting releasing methane, leading to more warming).
Peatlands: Accumulations of decayed vegetation. A $4^\circ C$ increase leads to a $40\%$ loss of organic carbon in shallow peat and $86\%$ in deep peat.
Critical Thresholds (Tipping Points): * Forest Die-back: When levels of tree loss in the Amazon prevent moisture recycling, causing irreversible forest death. In Boreal forests, summer stress causes trees to stop absorbing $CO_2$ . * Thermohaline Slowdown: Melting ice sheets release fresh water (less dense), disrupting the circulation. Some scientists fear this could lead to another ice age if the circulation stops.
Climate Change Uncertainties: Future emissions are hard to predict due to variables in economic growth, population change ( $1$ billion more consumers by $2050$ ), and globalization versus technological advancements.