Carbon Science Encyclopedia

Carbon: Overview

Carbon is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent, meaning four electrons are available to form covalent bonds.¹ Carbon is the 15th most abundant element in the Earth's crust and the fourth most abundant element in the universe by mass, after hydrogen, helium, and oxygen.

The name "carbon" derives from the Latin carbo, meaning coal or charcoal. In Japanese, carbon is known as tanso (炭素), combining the characters for charcoal and element.² Carbon's unique ability to form long chains and complex structures makes it the basis of organic chemistry and, by extension, all known life.

Allotropes of Carbon

Carbon exists in several allotropic forms, each with distinct physical and chemical properties. The best known allotropes are diamond, graphite, and amorphous carbon.³

Diamond -- Each carbon atom is bonded to four others in a tetrahedral arrangement (sp³ hybridization), creating an exceptionally hard, transparent crystal with a refractive index of 2.417.

Graphite -- Carbon atoms form hexagonal layers with sp² bonding. Weak van der Waals forces between layers allow them to slide, giving graphite its lubricating properties and electrical conductivity along the basal plane.

Graphene -- A single layer of graphite, graphene exhibits remarkable mechanical strength (130 GPa tensile strength), electron mobility (200,000 cm²/Vs), and thermal conductivity (5,000 W/mK).⁴

Fullerenes -- Spherical or tubular molecules, including buckminsterfullerene (C₆₀) discovered in 1985, and carbon nanotubes with aspect ratios exceeding 10,000:1.

The Carbon Cycle

The carbon cycle describes the biogeochemical movement of carbon between Earth's atmosphere, oceans, biosphere, and geosphere. This cycle operates across multiple timescales, from rapid biological exchanges (days to years) to slow geological processes (millions of years).⁵

Atmospheric CO₂ concentration has risen from pre-industrial levels of approximately 280 ppm to over 421 ppm in 2026, primarily due to fossil fuel combustion and land-use change. The ocean absorbs roughly 26% of anthropogenic CO₂ emissions, while terrestrial ecosystems absorb approximately 31%.⁶

Key carbon reservoirs include the deep ocean (~37,000 GtC), sedimentary rocks (~66,000,000 GtC), terrestrial biosphere (~2,000 GtC), and the atmosphere (~870 GtC). Annual anthropogenic emissions total approximately 10.5 GtC/year.⁶

Climate Impact

Carbon dioxide is the primary greenhouse gas driving anthropogenic climate change. The radiative forcing from CO₂ alone accounts for approximately 2.16 W/m² of additional energy retention in Earth's climate system since 1750.¹

The equilibrium climate sensitivity (ECS) -- the long-term temperature response to doubling CO₂ -- is estimated at 2.5-4.0 C (best estimate 3.0 C), according to IPCC AR6.¹ Current warming since pre-industrial levels stands at approximately 1.2 C as of 2026.

Methane (CH₄) contributes an additional 0.54 W/m² of radiative forcing. Although methane has a shorter atmospheric lifetime (~12 years) compared to CO₂ (300-1000 years), its global warming potential is 80x that of CO₂ over 20 years.⁷

Carbon Capture Technology

Carbon capture and storage (CCS) technologies aim to remove CO₂ from industrial emissions or directly from the atmosphere. Three primary approaches exist:

Post-combustion capture uses chemical solvents (typically amine scrubbing) to extract CO₂ from flue gases after fuel combustion. Current efficiency: 85-95% capture rate.⁸

Direct Air Capture (DAC) removes CO₂ directly from ambient air using solid sorbents or liquid solvents. Costs have decreased from over $600/tCO₂ in 2020 to below $400/tCO₂ in 2026, with projections targeting $100-150/tCO₂ by 2035.⁸

Bioenergy with CCS (BECCS) combines biomass energy generation with carbon capture, potentially achieving net-negative emissions. Global BECCS potential is estimated at 2-5 GtCO₂/year by 2050.⁹

Carbon Policy Framework

The Paris Agreement (2015) established the global framework for limiting warming to 1.5-2.0 C above pre-industrial levels. As of 2026, 194 parties have submitted Nationally Determined Contributions (NDCs) outlining emissions reduction targets.¹⁰

Carbon pricing mechanisms operate in 73 jurisdictions worldwide, covering approximately 23% of global greenhouse gas emissions. Prices range from below $1/tCO₂ (Poland) to over $130/tCO₂ (EU ETS peak, 2025).¹⁰

Article 6 of the Paris Agreement established rules for international carbon market cooperation, enabling transfers of mitigation outcomes between nations. The voluntary carbon market reached a value of approximately $2.3 billion in 2025.¹⁰

References

1. [1] IPCC, "Climate Change 2021: The Physical Science Basis," Working Group I contribution to the Sixth Assessment Report, Cambridge University Press, 2021.
2. [2] Greenwood, N.N. and Earnshaw, A., "Chemistry of the Elements," 2nd Edition, Butterworth-Heinemann, 1997.
3. [3] Hirsch, A., "The era of carbon allotropes," Nature Materials, vol. 9, pp. 868-871, 2010.
4. [4] Novoselov, K.S. et al., "Electric field effect in atomically thin carbon films," Science, vol. 306, pp. 666-669, 2004.
5. [5] Ciais, P. et al., "Carbon and Other Biogeochemical Cycles," in IPCC AR5 WG1, Cambridge University Press, 2013.
6. [6] Friedlingstein, P. et al., "Global Carbon Budget 2025," Earth System Science Data, 2025.
7. [7] Myhre, G. et al., "Anthropogenic and Natural Radiative Forcing," in IPCC AR5 WG1, 2013.
8. [8] IEA, "CCUS Technology Report 2026," International Energy Agency, Paris, 2026.
9. [9] Fuss, S. et al., "Negative emissions -- Part 2: Costs, potentials and side effects," Environmental Research Letters, vol. 13, 2018.
10. [10] UNFCCC, "Paris Agreement Status of Ratification and NDC Registry," 2026.