2.3B Carbon cycle Notes

Organic and Inorganic Carbon Stores

1. Carbon exists in both organic and inorganic forms across different environmental reservoirs.
2. These carbon stores play a crucial role in the carbon cycle, regulating atmospheric CO₂ levels and maintaining ecosystem balance.

Organic Carbon Stores:

1. Organic carbon is stored in living organisms (plants, animals, microorganisms) and in fossilized remains such as crude oil, coal, and natural gas.
2. These stores represent carbon that was once part of biological material and can be released back into the system through decomposition, combustion, or consumption.

Inorganic Carbon Stores:

1. Inorganic carbon exists as dissolved CO₂, carbonates, and bicarbonates in water, air, and soil.
2. It does not directly form part of living organisms.

Equilibrium in Carbon Stores

1. A carbon store is in equilibrium when the rate of carbon absorption equals the rate of carbon release.
2. This dynamic balance ensures that the carbon content of the store remains relatively stable over time.
   1. Absorption occurs when carbon moves into a store (e.g., photosynthesis removes CO₂ from the atmosphere, oceans absorb CO₂).
   2. Release occurs when carbon exits a store (e.g., respiration, decomposition, volcanic activity, fossil fuel combustion).
   3. Equilibrium ensures that atmospheric CO₂ remains stable, supporting a balanced climate and ecosystem function.

Residence Time of Carbon

Different stores have vastly different residence times, depending on how quickly carbon moves in and out of them.

Carbon Flows in Ecosystems

1. Carbon moves between organic and inorganic stores through various biological and physical processes.
2. These carbon flows include both transfers (where carbon changes location but not form) and transformations (where carbon changes its chemical form).

Transfers

1. Feeding: carbon moves up trophic levels (e.g., herbivores eating plants).
2. Defecation: carbon is returned to the soil as organic waste.
3. Death and decomposition: carbon moves from organisms to decomposers and soil.

Transformations

1. Photosynthesis: converts inorganic CO₂ + H₂O into organic glucose using light energy.
2. Cellular respiration: releases CO₂ and H₂O from organic molecules to provide energy.
3. Dissolution: atmospheric CO₂ dissolves into ocean water.
4. Combustion: converts organic carbon in biomass/fossil fuels into CO₂.
5. Fossilization: partially decomposed organic matter is converted to fossil fuels over geological time.

Creating a Systems Diagram of the Carbon Cycle

A typical systems diagram of the carbon cycle should include arrows between the atmosphere, producers, consumers, decomposers, soils, and fossil fuel stores, labeled with these key processes.

1. Identify the Stores like the Atmosphere:
   1. Biosphere: Plants, animals, and decomposers.
   2. Lithosphere: Fossil fuels and sedimentary rocks.
   3. Hydrosphere: Dissolved carbon in oceans.
2. Add the Flows:
   1. Photosynthesis: $CO_2$ → Plants.
   2. Feeding: Plants → Animals.
   3. Respiration: Plants/Animals → $CO_2$.
   4. Decomposition: Dead matter → Soil/Atmosphere.
   5. Fossilization: Organic matter → Fossil fuels.
   6. Combustion: Fossil fuels → $CO_2$.

Carbon sequestration

It plays a critical role in mitigating climate change by reducing the amount of CO₂ in the atmosphere.

Natural Sequestration

1. Photosynthetic organisms (e.g., trees, seagrasses, algae) absorb atmospheric CO₂ and convert it into biomass, locking carbon in living tissues.
2. Over geological timescales, organic matter is buried and transformed into coal, oil, and natural gas, representing long-term geological sequestration.

Artificial Sequestration

1. Carbon Capture and Storage (CCS) technologies capture CO₂ from industrial sources and store it underground in geological formations.
2. These are increasingly being explored as part of climate change mitigation strategies.

The Role of Carbon Sequestration in Climate Mitigation

1. Carbon sequestration is a critical tool for addressing climate change, but it is not a standalone solution.
2. It must be combined with efforts to reduce emissions, such as transitioning to renewable energy, improving energy efficiency, and adopting sustainable land management practices.

Ecosystems as Carbon Stores, Sinks, and Sources

1. Ecosystems play a crucial role in the carbon cycle by acting as stores, sinks, or sources of carbon.
2. The balance between carbon inputs (such as photosynthesis) and outputs (such as respiration, decomposition, or combustion) determines whether an ecosystem absorbs, stores, or releases carbon.

1. Carbon Sink

1. A carbon sink absorbs more carbon than it releases, leading to a net uptake of carbon dioxide (CO₂) from the atmosphere.
2. This happens when photosynthesis exceeds cellular respiration.
3. Over time, this stored carbon helps reduce atmospheric CO₂ levels, mitigating climate change.

2. Carbon Store

1. A carbon store holds carbon but does not significantly increase or decrease its total amount over time.
2. It has a near balance between photosynthesis and respiration.

3. Carbon Source

1. A carbon source releases more carbon than it absorbs, leading to an increase in atmospheric CO₂.
2. This occurs when respiration, decomposition, or combustion surpasses photosynthesis.
3. In a wildfire, trees burn and release CO₂ almost instantly.
4. In deforestation, trees are cut down and either burned or left to decay, both of which release stored carbon into the atmosphere.

Fossil Fuels as Carbon Stores and Sources

1. Fossil fuels represent vast, long-term carbon stores formed when past ecosystems acted as sinks millions of years ago.
2. However, when extracted and burned, they become major carbon sources, disrupting the natural carbon balance.

1. Formation as Carbon Stores

1. In past geological eras, ecosystems acted as carbon sinks, absorbing large amounts of atmospheric carbon through photosynthesis.
2. Instead of being released back into the atmosphere, much of this carbon became trapped in sediments and gradually transformed into fossil fuels under pressure and heat over millions of years.
3. These deposits locked away carbon, preventing it from cycling through the environment.

2. Fossil Fuels as Carbon Sources

1. When fossil fuels are burned, they shift from being long-term carbon stores to major carbon sources, releasing stored carbon back into the atmosphere as carbon dioxide (CO₂).
2. This rapid release significantly contributes to the greenhouse effect and climate change.
3. Unlike forests or soil, which can store carbon temporarily and release it over natural cycles, fossil fuels represent carbon that was effectively removed from the active carbon cycle for millions of years.

Agricultural Systems as Carbon Stores, Sources, and Sinks

1. Agricultural systems can influence the carbon cycle, either absorbing and storing carbon (carbon sinks), maintaining carbon levels (carbon stores), or releasing carbon into the atmosphere (carbon sources).
2. The impact depends largely on the farming techniques used.

Agricultural Systems as Carbon Sinks

1. Certain regenerative agricultural techniques can increase carbon sequestration in soils and biomass, effectively turning agricultural systems into carbon sinks.
2. These methods increase soil organic matter, reduce disturbance, and enhance photosynthesis.
3. Key sink-promoting practices include:
   1. Crop rotation: Alternating crops over time to improve soil health and reduce nutrient depletion.
   2. Cover cropping: Growing non-commercial crops (e.g., rye, clover) between planting seasons to protect soil from erosion and add organic matter.
   3. No-till farming: Planting crops without ploughing the soil, which preserves soil structure and minimizes carbon loss.

Agricultural Systems as Carbon Sources

1. Other intensive or unsustainable practices can accelerate carbon release, turning agricultural systems into carbon sources.
2. These practices typically disturb soil, reduce organic matter, or involve land conversion.
3. Key source-promoting practices include:
   1. Draining wetlands → exposes organic-rich soils to oxygen, causing rapid decomposition and CO₂ release.
   2. Monoculture → reduces soil biodiversity and organic matter replenishment, making soils more prone to degradation.
   3. Heavy tillage → breaks soil aggregates, exposing organic matter to oxygen and increasing microbial decomposition and carbon emissions.
   4. Deforestation for agriculture → releases stored biomass carbon and removes the photosynthetic sink.

The Role of Oceans in the Carbon Cycle

1. Oceans play a crucial role in regulating atmospheric carbon dioxide (CO₂) by acting as a carbon sink.
2. CO₂ from the atmosphere dissolves in seawater, where it can be stored for long periods.

1. Absorption of Carbon Dioxide

1. CO₂ enters the ocean through a process called dissolution, where gas from the atmosphere dissolves in seawater.
2. Once dissolved, it undergoes several chemical reactions:
   1. Some CO₂ remains as dissolved gas.
   2. Some reacts with water to form carbonic acid (H₂CO₃), which further breaks down into bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions.
   3. Marine organisms, such as corals and shell-forming species, use carbonate to build calcium carbonate (CaCO₃) shells and skeletons.

2. Release of Carbon Dioxide

1. CO₂ is also released from the ocean back into the atmosphere when water warms or when ocean currents bring deep, carbon-rich water to the surface.
2. Warmer waters hold less CO₂, meaning climate change can reduce the ocean’s ability to absorb carbon.

Human Impacts on Oceanic Carbon Uptake

1. The rapid increase in atmospheric CO₂ due to fossil fuel use has exceeded the ocean’s natural buffering capacity.
2. Although oceans currently absorb more CO₂, this comes with consequences:
   1. Ocean acidification due to excess dissolved CO₂.
   2. Reduced absorption efficiency as surface waters warm.
   3. Potential weakening of the ocean carbon sink over time.

Ocean Acidification

1. The ocean absorbs about 25% of the carbon dioxide ( CO₂) emitted by human activities each year.
2. While this helps reduce atmospheric CO₂ levels, it comes at a cost: ocean acidification.

How Does Ocean Acidification Occur?

1. When CO₂ enters the ocean, it reacts with water to form carbonic acid ( H₂CO₃).
2. This weak acid dissociates into hydrogen ions ( H⁺) and bicarbonate ions ( HCO₃⁻), increasing the ocean's acidity.

The Impact on Marine Organisms

1. Corals:
   1. Coral reefs are built by tiny coral polyps that secrete calcium carbonate skeletons.
   2. Acidification slows this process, weakening reefs and making them more susceptible to erosion and bleaching.
2. Mollusks:
   1. Shellfish like oysters, clams, and mussels struggle to form shells in acidic conditions.
   2. This affects their survival and the livelihoods of communities that depend on them.
3. Plankton:
   1. Some plankton species, such as coccolithophores, form calcium carbonate shells.
   2. These organisms are foundational to marine food webs, so their decline can have cascading effects on entire ecosystems.

Ecosystem-Level Consequences

1. Coral reefs, which support ~25% of marine biodiversity, become structurally weaker, leading to habitat loss for thousands of species.
2. Food webs are altered as planktonic base species are affected.
3. Fisheries and coastal communities reliant on shellfish or reefs face economic and food security challenges.

Measures to Alleviate Human Impact on the Carbon Cycle

1. Addressing the impacts of human activity requires a combination of technological, ecological, and policy-based measures to reduce emissions and enhance carbon sinks.
2. To mitigate the effects of human activities on the carbon cycle, a range of measures is required to reduce carbon emissions and enhance the carbon absorption capacity of ecosystems.

1. Low-Carbon Technologies

1. Low-carbon technologies aim to reduce the amount of CO₂ released during energy production, transportation, and industrial processes.
2. These technologies are essential for transitioning away from fossil fuels and mitigating climate change.

2. Reducing Fossil Fuel Burning, Soil Disruption, and Deforestation

1. Fossil fuels: Limiting combustion directly reduces the largest anthropogenic CO₂ flux to the atmosphere.
2. Soil management: Conservation tillage and reduced soil disturbance maintain soil carbon stocks.
3. Forests: Preventing deforestation and promoting sustainable forest management maintains large carbon sinks.

3. Carbon Capture through Reforestation and Artificial Sequestration

1. Reforestation and afforestation increase the uptake of CO₂ via photosynthesis, turning degraded land into carbon sinks.
2. Artificial carbon capture involves technologies such as Carbon Capture and Storage (CCS), which capture CO₂ from point sources and inject it into geological formations for long-term storage.

4. Enhancing Oceanic Carbon Uptake

1. Ocean fertilization and other geoengineering methods aim to increase biological productivity in oceans, encouraging phytoplankton growth to enhance carbon uptake.
2. However, these approaches are experimental and controversial, with uncertain long-term impacts.