Carbon Sequestration: Methods, Role, and Challenges

Carbon Sequestration refers to the process of capturing and securely storing atmospheric carbon dioxide (CO₂) to mitigate or delay the impacts of climate change. This mechanism is pivotal in reducing greenhouse gas concentrations and maintaining the global carbon balance. The following are key methodologies utilized in carbon sequestration:

1. Natural Carbon Sequestration

• Forests, Urban Greenery, and Parks: Forest ecosystems and urban greenery act as significant carbon sinks. Trees absorb CO₂ during photosynthesis and store it within their biomass, which includes trunks, branches, roots, and soil. Forest conservation, reforestation, and urban greenery, such as parks and green roofs, contribute to carbon sequestration by capturing CO₂. Urban parks also enhance air quality, reduce the urban heat island effect, and provide green spaces that support biodiversity. Initiatives to increase tree cover and green infrastructure are crucial in mitigating carbon emissions both in natural and urban environments.

• Soil: Soils are a major reservoir of carbon, with organic matter being a primary storage medium. Agricultural practices such as no-till farming, cover cropping, and crop rotation are essential to enhancing soil carbon sequestration by increasing soil organic carbon (SOC) levels.

• Oceans: Oceans represent the largest natural carbon sink, absorbing a significant proportion of anthropogenic CO₂ emissions. Marine organisms, particularly phytoplankton, absorb CO₂ during photosynthesis, and upon death, these organisms facilitate carbon transfer to deeper ocean layers through biological sequestration processes. Additionally, ocean buffering plays a crucial role in mitigating atmospheric CO₂ levels by chemically neutralizing absorbed carbon dioxide, helping to maintain oceanic pH balance.

2. Artificial Carbon Sequestration

• Alternative Energy from CO₂: Technologies are being developed to convert captured CO₂ into alternative energy products, contributing to sustainable energy solutions. For more details, see: Alternative Energy Initiative.

• CO₂ to Usable Energy: Technologies are being developed to convert captured CO₂ into usable energy products, contributing to a circular carbon economy. For more details, see: CO₂ to Usable Energy.

• Sustainable Jet Fuel Production: Neste has expanded its refinery operations to establish a sustainable jet fuel supply chain at Changi Airport. This initiative is part of broader efforts to reduce carbon emissions from aviation by providing an alternative fuel source that generates significantly lower life-cycle greenhouse gas emissions compared to conventional jet fuel. For more details, see: Neste's Sustainable Jet Fuel Initiative.

• Tires Created from CO₂: Innovative technologies are being used to create tires using captured CO₂, which helps reduce the reliance on fossil fuels in tire manufacturing and contributes to carbon utilization efforts.

• Carbon Capture and Storage (CCS): CCS involves the capture of CO₂ emissions from point sources such as industrial facilities, followed by transportation and injection into geological formations, such as depleted oil and gas reservoirs, for long-term storage. CCS is a technologically mature solution for mitigating industrial carbon emissions.

• Direct Air Capture (DAC): DAC technology involves the extraction of CO₂ directly from ambient air using chemical solvents or filters. The captured CO₂ can then be compressed and sequestered or repurposed for applications such as synthetic fuel production. DAC offers potential for negative emissions but is currently limited by high energy and operational costs.

• Bioenergy with Carbon Capture and Storage (BECCS): BECCS integrates bioenergy production with carbon capture, wherein biomass is cultivated to absorb atmospheric CO₂, which is subsequently captured and stored during biomass combustion. BECCS has been proposed as a means to achieve negative emissions, contingent upon sustainable biomass sourcing.

3. Geological and Innovative Approaches

• Mineralization: Mineral carbonation involves the reaction of CO₂ with alkaline earth metals to form stable carbonate minerals. This process occurs naturally over geological timescales but can be accelerated artificially to enhance carbon sequestration. Mineralization offers a permanent solution for CO₂ storage, though scalability remains a challenge.

• Biochar: Biochar is produced via pyrolysis of organic material in an oxygen-limited environment. It is a stable form of carbon that can be applied to soils to enhance soil fertility and sequester carbon for extended periods. Biochar production also provides a means to valorize agricultural waste while contributing to carbon sequestration.

• Blue Carbon: Coastal and marine ecosystems, such as mangroves, seagrasses, and salt marshes, play a critical role in sequestering "blue carbon." These ecosystems capture CO₂ and store it in both their biomass and sediment, providing long-term sequestration while also supporting biodiversity and coastal resilience.

The Role of Carbon in Climate Change

The role of carbon dioxide (CO₂) in climate change is well-established and supported by an extensive body of scientific evidence. Here is a summary of the level of certainty regarding carbon's role in climate change:

Scientific Consensus

The Intergovernmental Panel on Climate Change (IPCC), which synthesizes the latest climate research, consistently reports high confidence that the increase in atmospheric CO₂ is primarily responsible for global warming. CO₂, along with other greenhouse gases like methane (CH₄) and nitrous oxide (N₂O), traps heat in the atmosphere, which leads to increased surface temperatures.

Paleoclimate Records

Historical climate data, derived from ice cores, tree rings, and sediment layers, indicate a strong correlation between atmospheric CO₂ levels and global temperature changes. CO₂ concentration has been shown to play a crucial role in regulating Earth's climate over millennia.

Radiative Forcing

CO₂ is known to be a potent greenhouse gas due to its ability to absorb infrared radiation emitted by the Earth. The concept of radiative forcing—a measure of the effect of CO₂ on the energy balance of the Earth—confirms the significant warming impact of rising CO₂ levels.

Human Contribution

The rise in CO₂ concentration since the Industrial Revolution is mainly attributed to human activities such as the burning of fossil fuels, deforestation, and land-use changes. These activities have led to an increase in atmospheric CO₂ levels from pre-industrial values of approximately 280 ppm (parts per million) to over 420 ppm today.

Carbon Isotope Analysis

Analysis of carbon isotopes (such as the ratio of carbon-12 to carbon-13) confirms that the additional CO₂ in the atmosphere is largely of anthropogenic origin, coming from fossil fuel combustion and deforestation.

Certainty in Climate Models

Climate models have consistently demonstrated that increasing CO₂ leads to a rise in global average temperatures. Estimates of climate sensitivity—how much the Earth's temperature will increase in response to a doubling of CO₂—provide robust evidence that higher CO₂ concentrations drive significant warming.

Observational Data

Contemporary observations, including temperature records, ocean heat content, and changing weather patterns, align with predictions made by climate models that account for increasing CO₂ concentrations. These observations strengthen confidence in the causal role of carbon in driving current climate changes.

Example A-Level Biology Exam Question: Carbon Sequestration

Question

2020 / SP / H2 P3

2. Trees remove carbon dioxide from the atmosphere and store this carbon in their biomass in the long term. This long-term storage of carbon is called carbon sequestration.

(a) The carbon sequestered in a tree is not in the form of carbon dioxide.
Identify three biomolecules in which carbon may be sequestered in a tree.

1. ---

2. ---

3. ___________________________ [3]

(b) Singapore has a high density of urban trees and plants, earning it the description 'garden city.' It also has a busy road network.
The rain tree, Samanea saman, is the commonest urban tree species in Singapore.

Table 3.1 shows the number of rain trees located along roads and in parks in Singapore and the rate at which these trees sequester carbon in tonnes per year. One tonne is equivalent to 1000 kg. The trees sequester carbon in tonnes per year. One tonne is equivalent to 1000 kg.

Location	Number of rain trees	Rate of carbon sequestration (tonnes year⁻¹)
Roads	26,226	487
Parks	2,356	44

For the rain trees located along roads and in parks in Singapore, calculate the mean mass of carbon sequestered by a single rain tree in one year.
Show your working and give your answer in kilograms (kg) to three significant figures. [2]

(c) (i) In total, Singapore's urban trees sequester 3,872 tonnes of the element carbon per year.
This helps offset carbon dioxide emissions from human activities like burning fossil fuels in the engines of cars.
There are about 600,000 cars in Singapore.
Each car is estimated to emit 4.04 tonnes of carbon dioxide per year.
27% of the mass of carbon dioxide is carbon.

Use the data provided to calculate the percentage of the element carbon released into the atmosphere by cars in Singapore that is offset by the urban trees.
Show your working. [3]

(ii) With reference to your answer to (c)(i) and any other relevant aspects, comment on the benefits to the environment of planting urban trees in Singapore. [3]

Suggested Answers

(a) The carbon sequestered in a tree may be found in the following biomolecules:

1. Cellulose

2. Lignin

3. Starch

(b) Mean mass calculation
For roads:

For parks:

Overall mean mass

(c) (i) Carbon offset calculation
Total carbon emissions from cars:

Mass of carbon in CO₂:

Percentage offset by trees:

(ii) Benefits of planting urban trees

• Trees improve air quality by absorbing pollutants and sequestering carbon, reducing the net CO₂ emissions.

• Urban trees help to mitigate the urban heat island effect, providing cooling benefits and reducing the need for air conditioning.

• They contribute to biodiversity, providing habitats for birds and other wildlife in urban environments.

Relevant Physics, Chemistry, and Biology Concepts in Carbon Sequestration

Physics Concepts

• Energy Transformations: The process of photosynthesis, which captures solar energy and converts it into chemical energy, involves key energy transformation concepts. This is fundamental to understanding how plants sequester carbon in their biomass.

• Thermodynamics: Radiative forcing, which measures the impact of greenhouse gases like CO₂ on Earth's energy balance, involves thermodynamic principles. CO₂ absorbs and re-emits infrared radiation, which plays a significant role in regulating Earth's temperature.

• Fluid Dynamics: Carbon Capture and Storage (CCS) involves transporting CO₂, often as a supercritical fluid, through pipelines to storage sites. Understanding the behavior of CO₂ under various pressures and temperatures involves fluid dynamics.

• Diffusion: The movement of CO₂ in soil and ocean layers is governed by diffusion. The dissolution of CO₂ in ocean buffering and its movement through soil are explained by mass transport and diffusion principles.

• Mechanical and Structural Properties: Geological storage of CO₂ depends on the mechanical properties of geological formations, such as permeability and porosity, which are relevant concepts in mechanics and material science.

Chemistry Concepts

• Chemical Reactions: Carbon sequestration often involves chemical reactions, such as the conversion of CO₂ to stable carbonates in mineralization processes. The chemical interaction between CO₂ and minerals helps in the long-term storage of carbon.

• Ocean Buffering Chemistry: The absorption of CO₂ by oceans involves a series of chemical reactions, including the formation of bicarbonate and carbonate ions. These reactions help to buffer ocean pH levels and maintain equilibrium in marine environments.

• Carbon Compound Formation: The formation of organic molecules, such as cellulose and lignin in plants, involves complex biochemical reactions where carbon is incorporated into stable structures, effectively sequestering it over the long term.

Biology Concepts

• Photosynthesis: Plants, algae, and some bacteria absorb CO₂ from the atmosphere and convert it into glucose and other organic molecules through photosynthesis. This process is the foundation of natural carbon sequestration.

• Respiration and Decomposition: Both plant respiration and microbial decomposition play roles in the carbon cycle. While photosynthesis sequesters carbon, respiration releases some of it back into the atmosphere. Understanding this balance is key to optimizing carbon storage.

• Ecosystem Dynamics: Forests, grasslands, and marine ecosystems all play roles in carbon sequestration. The biological interactions within these ecosystems, such as nutrient cycling, growth rates, and species diversity, influence the efficiency of carbon capture and storage.

• Energy Transformations: The process of photosynthesis, which captures solar energy and converts it into chemical energy, involves key energy transformation concepts. This is fundamental to understanding how plants sequester carbon in their biomass.

• Thermodynamics: Radiative forcing, which measures the impact of greenhouse gases like CO₂ on Earth's energy balance, involves thermodynamic principles. CO₂ absorbs and re-emits infrared radiation, which plays a significant role in regulating Earth's temperature.

• Fluid Dynamics: Carbon Capture and Storage (CCS) involves transporting CO₂, often as a supercritical fluid, through pipelines to storage sites. Understanding the behavior of CO₂ under various pressures and temperatures involves fluid dynamics.

• Diffusion: The movement of CO₂ in soil and ocean layers is governed by diffusion. The dissolution of CO₂ in ocean buffering and its movement through soil are explained by mass transport and diffusion principles.

• Mechanical and Structural Properties: Geological storage of CO₂ depends on the mechanical properties of geological formations, such as permeability and porosity, which are relevant concepts in mechanics and material science.

Benefits of Carbon Sequestration

• Climate Stabilization: Mitigates atmospheric CO₂ concentrations, contributing to climate stabilization.

• Agricultural Productivity: Enhances soil fertility, leading to improved agricultural productivity and resilience.

• Ecosystem Services: Supports ecosystem services, including biodiversity conservation and the maintenance of ecosystem function.

Nuclear Energy as a Sustainable Option: Nuclear energy is experiencing a global renaissance as a reliable source of low-carbon energy. For more information on this trend and its potential impact on sustainable energy solutions, see: Nuclear Energy Global Renaissance.

Challenges

• High Costs and Energy Demands: Artificial carbon capture technologies often involve high costs and significant energy requirements.

• Risk of Leakage: Geological storage sites pose a risk of CO₂ leakage, necessitating rigorous monitoring and management.

• Knowledge Gaps: Limited understanding of the long-term ecological and environmental impacts of some emerging sequestration technologies underscores the need for continued research and development.