Carbon Dioxide (CO₂)
It is the gas the entire field is named for. “Carbon” footprints, “carbon” accounting, “carbon” markets — all take their name from a single molecule that is, per tonne, the weakest of the greenhouse gases yet by far the most consequential.
Carbon dioxide is the reference point of climate accounting: the gas against which every other is measured, and the one whose cumulative build-up sets the carbon budget.
Carbon dioxide (CO₂) is a colourless gas and the main greenhouse gas released by human activity, mostly from burning fossil fuels. It is the reference gas for global warming potential (GWP 1), so other gases are expressed as CO₂e. Its concentration reached about 427 ppm in 2025.
What carbon dioxide is
Carbon dioxide (CO₂) is a colourless greenhouse gas, made of one carbon and two oxygen atoms, that is released when carbon-containing materials are burned or break down. It is the largest single contributor to human-caused global warming, chiefly because of the vast quantities emitted by burning fossil fuels.
CO₂ is one of the several gases counted in a greenhouse gas inventory, but it holds a special status: it is the yardstick. Its warming effect is the baseline to which every other gas is compared, which is why the common unit of climate accounting — the carbon dioxide equivalent — carries its name. Atmospheric CO₂ has risen from a pre-industrial level of around 280 parts per million (ppm) to an annual average of 427 ppm at Mauna Loa in 2025, higher than at any time in at least 2 million years, and the primary driver of the observed warming.
| Chemical formula | CO₂ (one carbon atom, two oxygen atoms) |
|---|---|
| Molar mass | 44.01 g/mol |
| Carbon to CO₂ | Carbon is 27.3% of CO₂ by mass, so 1 tonne of carbon becomes 3.664 tonnes of CO₂ (converter) |
| Global warming potential | 1 (by definition) |
| Concentration, 2025 | 427.35 ppm annual mean at Mauna Loa (NOAA, preliminary); about 280 ppm before industrialisation |
| Fossil CO₂ emissions, 2025 | 38.1 billion tonnes, a record (Global Carbon Budget projection), plus 4.1 billion tonnes from land-use change |
| Taken up by land and ocean | About 56% of human CO₂ emissions each year, on average over six decades (IPCC AR6) |
Sources: NOAA Global Monitoring Laboratory, Mauna Loa annual means (2025 value preliminary); Global Carbon Budget 2025 (13 November 2025); IPCC AR6 Working Group I, Summary for Policymakers. Molar mass from standard atomic weights (C 12.011, O 15.999).
This page defines the gas and its role; the emission factors that convert fuel use into CO₂, and the inventories that total it, are handled by the calculators and standards this entry links to.
The reference gas for CO₂e
Carbon dioxide anchors the whole system of comparing greenhouse gases. Its global warming potential is defined as exactly 1, and every other gas’s GWP expresses how much more heat it traps per tonne relative to CO₂. Multiplying each gas by its GWP converts it into carbon dioxide equivalent (CO₂e) — the common currency of emissions.
This makes CO₂ the weakest greenhouse gas per tonne but the reference for all the others. On the IPCC AR6 100-year basis, a tonne of methane traps 29.8 [GreenCalculus gwp.CH4_fossil.ar6_100 · IPCC AR6 · v2026.284] times as much heat as a tonne of CO₂, and a tonne of nitrous oxide 273 [GreenCalculus gwp.N2O.ar6_100 · IPCC AR6 WGI Ch 7 Table 7.SM.7 (2021) — AR6 GWP-100] times as much. CO₂ dominates emissions not because it is potent, but because it is emitted in overwhelmingly larger quantities.
Long lifetime and the carbon budget
What makes CO₂ uniquely important over the long run is its persistence. A significant fraction of the CO₂ emitted today remains in the atmosphere for centuries, and a portion for millennia — unlike methane, which breaks down in roughly a decade. CO₂ therefore accumulates: each year’s emissions add to a stock that barely diminishes on human timescales.
Because warming tracks the total CO₂ ever emitted, the climate responds to the cumulative stock, not the annual flow. This is the basis of the carbon budget — the finite total of CO₂ that can be emitted for a given temperature limit. It is also why reaching net zero for CO₂ is what ultimately halts CO₂-driven warming: only when additions stop does the stock stop growing.
| Point | ppm |
|---|---|
| 1960 | 317.0 ppm |
| 1970 | 326.0 ppm |
| 1980 | 339.0 ppm |
| 1990 | 354.0 ppm |
| 2000 | 370.0 ppm |
| 2010 | 390.0 ppm |
| 2020 | 414.0 ppm |
| 2025 | 427.0 ppm |
The curve only bends when net emissions reach zero. Each year of continued emissions lifts the line further, and because the gas is so long-lived, even a large annual cut merely slows the rise rather than reversing it — the level keeps climbing until additions stop. That is the physical reason a stable climate requires net-zero CO₂, not just lower CO₂.
Fossil vs biogenic CO₂
Not all CO₂ is treated the same in accounting. Fossil CO₂ comes from burning coal, oil, and gas — carbon locked away for millions of years, now added to the active carbon cycle. Biogenic CO₂ comes from recently living biomass — burning wood, crop residues, or biofuels — carbon that was absorbed from the atmosphere in the recent past.
Because biogenic CO₂ is part of a short-cycle exchange, GHG accounting typically reports it separately from fossil CO₂ rather than adding it to the main total, on the basis that the carbon was recently drawn down and may be re-absorbed. The distinction is an accounting convention with important caveats around timing and land use — but the headline is that fossil and biogenic CO₂ are not simply pooled together.
Sources and sinks
CO₂ moves through the carbon cycle between sources that release it and sinks that absorb it. The human sources that matter for accounting are:
- Fossil-fuel combustion — energy, industry, and transport; by far the largest source.
- Industrial process emissions — chemically released CO₂, notably from cement and lime production.
- Land-use change — deforestation and soil disturbance releasing stored carbon.
Against these, natural sinks — the oceans and land vegetation — have taken up about 56% of human CO₂ emissions each year over the past six decades (IPCC AR6); the remainder accumulates in the atmosphere. The Global Carbon Budget finds that climate change is now weakening those sinks. Carbon dioxide removal aims to enhance or engineer sinks to draw CO₂ back out.
Carbon dioxide in a GHG inventory
In a company’s inventory, CO₂ is usually the largest line by mass and appears across all scopes: direct combustion and process emissions in Scope 1, the CO₂ embedded in purchased electricity in Scope 2, and value-chain CO₂ throughout Scope 3. It is calculated by multiplying activity data — litres of fuel, tonnes of clinker — by a CO₂ emission factor, then reported in CO₂e (where, for CO₂ itself, the CO₂e figure equals the CO₂ mass because its GWP is 1).
Because CO₂’s GWP is 1 by definition, it is the one gas whose mass and CO₂e value are identical — a useful sanity check when reading an inventory. The other gases only reach comparability once multiplied up by their GWPs, which is why a footprint dominated by methane or nitrous oxide can be far larger in CO₂e than its raw gas mass suggests.
Worked micro-example
Carbon dioxide is calculated by multiplying activity data by an emission factor. A business that burns 10,000 litres of 100% mineral diesel in a year, using the UK government’s DEFRA 2026 factor:
| Step | Value |
|---|---|
| Diesel burned | 10,000 L |
| Emission factor (DEFRA 2026) | × 2.66155 kg CO₂e / L |
| Emissions | ≈ 26,600 kg = 26.6 t CO₂e |
Almost all of that is carbon dioxide: the carbon in the diesel leaves the exhaust as CO₂, and the small amounts of methane and nitrous oxide from combustion add only a little on a CO₂e basis. For the CO₂ itself, mass and CO₂e are the same number, because its GWP is 1.
Common mistakes
- Equating carbon dioxide with all greenhouse gases. CO₂ is one gas; a full footprint also counts methane, nitrous oxide, and F-gases, each converted to CO₂e.
- Confusing CO₂ with CO₂e. CO₂ is the gas; CO₂e is the common unit that expresses all gases in CO₂-equivalent terms. They coincide only for CO₂ itself.
- Pooling fossil and biogenic CO₂. They are accounted separately; adding biogenic CO₂ into the fossil total misstates the inventory.
- Focusing on annual flow, not cumulative stock. Because CO₂ persists for centuries, it is the total ever emitted that drives warming — the logic behind carbon budgets.
- Assuming sinks will keep pace. Land and ocean sinks take up only a little over half of emissions; the rest accumulates, so emissions must fall to near zero to stabilise concentrations.
Put a tonne of CO₂ in perspective with relatable equivalents.
Quick converters: move between atmospheric concentration and mass with ppm CO₂ to Gt carbon, ppm CO₂ to Gt CO₂ and ppm to ppb.
Frequently asked questions
CO₂ is the gas carbon dioxide. CO₂e (carbon dioxide equivalent) is the common unit that expresses all greenhouse gases in terms of the amount of CO₂ that would cause the same warming, using each gas’s global warming potential. For carbon dioxide itself the two are equal, because its GWP is 1.
No — per tonne it is the weakest of the accounted gases, with a GWP of 1. Methane, nitrous oxide, and the fluorinated gases trap far more heat per tonne. Carbon dioxide dominates warming because it is emitted in vastly greater quantities and persists in the atmosphere for centuries.
Fossil CO₂ comes from burning coal, oil, and gas — carbon locked away for millions of years and now added to the atmosphere. Biogenic CO₂ comes from recently living biomass, such as wood or biofuels. Because biogenic carbon was recently absorbed from the air, GHG accounting typically reports it separately from fossil CO₂ rather than in the main total.
Because a large share of CO₂ stays in the atmosphere for centuries, it accumulates, and warming tracks the total ever emitted rather than any single year. This is the basis of the carbon budget — the finite amount of CO₂ that can be emitted for a given temperature limit — and why net zero for CO₂ is what ultimately halts CO₂-driven warming.
The annual average at Mauna Loa was 427.35 parts per million in 2025 (NOAA, preliminary), against about 280 ppm before industrialisation. The Global Carbon Budget puts the 2025 global average at 425.7 ppm, 52% above pre-industrial levels.
Multiply by 3.664. Carbon makes up 27.3% of the mass of CO₂ (12.011 of 44.01 g/mol), so 1 tonne of carbon, once burned, becomes 3.664 tonnes of CO₂. Use the carbon to CO₂ converter.
CO2 is carbon dioxide, a colourless gas made of one carbon and two oxygen atoms. It is the main greenhouse gas released by human activity, mostly from burning coal, oil and gas, and the reference gas for global warming potential (GWP 1). Its concentration reached about 427 ppm in 2025, up from around 280 ppm before industrialisation.
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