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Carbon Cycle / Greenhouse Gases

Carbon dioxide

Carbon dioxide (chemical formula CO2) is a colorless, odorless gas composed of one carbon atom covalently double-bonded to two oxygen atoms. It occurs naturally in the Earth's atmosphere as a trace gas, plays a central role in the global carbon cycle, and is the most significant long-lived greenhouse gas contributing to anthropogenic climate change.

From tanso.wiki, the open carbon-science encyclopedia Reviewed by the Atmospheric Sciences working group

Overview #

Carbon dioxide is generated by all aerobic organisms when they metabolize organic compounds, and is consumed by photosynthetic organisms during the biosynthesis of carbohydrates. It is also released by the combustion of carbonaceous fuels, by volcanic activity, and by the slow weathering of carbonate rocks. Although it occupies only about 0.04% of the atmosphere by volume, CO2 exerts a disproportionately large influence on the Earth's energy balance through its absorption of outgoing infrared radiation.

Definition

Carbon dioxide (CO2) — an inorganic compound consisting of one carbon and two oxygen atoms, existing as a gas at standard temperature and pressure, and serving as the primary long-lived radiative forcing agent in Earth's atmosphere.

Within the carbon cycle, CO2 moves through reservoirs of widely differing residence times: a few years in vegetation, decades in surface oceans, and millennia in the deep ocean and sedimentary rocks. Anthropogenic emissions perturb this cycle by releasing carbon stored over geologic timescales on timescales of decades.

Physical properties #

CO2 is a linear, centrosymmetric triatomic molecule with two equivalent C=O double bonds of length 116.3 pm. Its symmetric stretching mode is infrared inactive, but the antisymmetric stretch and bending modes are strongly active and underpin its role as a greenhouse gas.

Molecular structure #

The molecular geometry is linear with a bond angle of 180°. The two C=O bonds are polar, but the linear geometry results in a net dipole moment of zero. The electronic ground state is X ¹Σg+, with the first excited singlet state lying ~5.7 eV above it.

Phase behavior #

At atmospheric pressure CO2 sublimes directly from solid (dry ice) to gas at −78.46 °C. Liquid CO2 exists only above the triple point pressure of 518 kPa. Above the critical point (31.0 °C, 7.39 MPa) CO2 becomes a supercritical fluid, a state widely used as a green solvent.

Selected phase parameters of CO2.
Parameter Value Conditions
Triple point−56.6 °C, 518 kPaSolid–Liquid–Gas equilibrium
Critical point31.0 °C, 7.39 MPaSupercritical onset
Sublimation point−78.46 °C1 atm
Heat of sublimation25.13 kJ/molat 195 K
Specific heat (gas)0.839 J/(g·K)25 °C, constant pressure

Sources and sinks #

The global carbon budget partitions emissions and removals across natural and human-driven fluxes. The latest assessments place the total anthropogenic emission flux at roughly 40.6 GtCO2/yr, of which approximately half is taken up by land and ocean sinks while the remainder accumulates in the atmosphere.

Anthropogenic emissions #

Fossil fuel combustion, cement manufacture, and land-use change dominate human-driven CO2 release. The historical cumulative emission since 1750 is estimated at ~2,500 GtCO2, with more than half emitted after 1990.

  • Coal: ~40% of fossil-fuel CO2, dominant in electricity generation.
  • Oil: ~32%, primarily transportation and petrochemicals.
  • Natural gas: ~21%, heating and combined-cycle power.
  • Cement & industry: ~7%, from process emissions and high-temperature heat.

Natural sinks #

The terrestrial biosphere absorbs roughly 11±3 GtCO2/yr through net primary productivity exceeding ecosystem respiration, while the oceans absorb a further 10±2 GtCO2/yr through the physical solubility pump and biological pump.

Solubility pump

A process by which CO2 dissolves preferentially into cold, dense polar surface waters that sink to depth, sequestering carbon away from the atmosphere on centennial-to-millennial timescales.

Role in climate #

Carbon dioxide accounts for roughly two-thirds of the additional radiative forcing from long-lived greenhouse gases since 1750. As of 2025, CO2 contributes approximately 2.2 W/m2 of forcing relative to pre-industrial levels. 

radiative_forcing(CO2) = 5.35 * ln(C / C0)   // W/m^2
where:
  C  = current concentration (ppm)
  C0 = pre-industrial baseline (~278 ppm)

Climate sensitivity is conventionally expressed as the equilibrium temperature response to a doubling of CO2. The IPCC AR6 assessed likely range is 2.5–4.0 °C, with a best estimate near 3 °C.

Measurement #

Atmospheric CO2 is monitored via a global network of surface stations, aircraft profiles, and satellite remote sensing. The Mauna Loa record, initiated in 1958, is the longest continuous in-situ measurement and forms the canonical reference series.

Representative CO2 monitoring platforms.
Platform Type Coverage Since
Mauna Loa ObservatoryIn-situ NDIRHawaii (NH baseline)1958
NOAA GML NetworkFlask + towerGlobal, ~70 sites1968
TCCONGround FTIRTotal column, 30+ sites2004
OCO-2 / OCO-3Satellite SWIRGlobal, XCO22014
GOSAT-GWSatellite SWIR/TIRGlobal, multi-gas2024

Mitigation pathways #

Limiting warming consistent with the Paris Agreement requires reaching net-zero CO2 emissions globally near mid-century. Mitigation strategies fall into four broad categories:

  1. Decarbonized supply: renewables, nuclear, and abated fossil generation with carbon capture.
  2. End-use efficiency: electrification of transport, heating, and industry.
  3. Negative emissions: afforestation, soil carbon, BECCS, and direct air capture.
  4. Demand-side change: material efficiency, mode shift, and dietary change.

Net zero

A state in which anthropogenic CO2 emissions are balanced by anthropogenic removals over a specified period, such that the atmospheric stock no longer increases from human activity.

References #

  1. IPCC AR6 Working Group I. The Physical Science Basis. Cambridge University Press, 2021.
  2. Friedlingstein P. et al. “Global Carbon Budget 2024.” Earth System Science Data, 2024.
  3. NOAA GML. “Trends in Atmospheric Carbon Dioxide.” Boulder, CO, 2025.
  4. Keeling C. D. “The Concentration and Isotopic Abundance of Carbon Dioxide in the Atmosphere.” Tellus, 1960.
  5. Joos F. et al. “Carbon dioxide and climate impulse response functions.” ACP, 2013.

Further reading #

  • Archer, D. The Long Thaw. Princeton University Press.
  • Pierrehumbert, R. Principles of Planetary Climate. Cambridge University Press.
  • Le Quere, C. et al. Decadal trends in the global carbon budget.