Carbon Emissions
The world is on course to emit 10.4 billion tonnes of carbon from fossil fuels in 2025 — or 38.1 billion tonnes, depending on who you ask. Both figures are right. One counts the carbon atoms, the other counts the carbon dioxide they become, and the difference is a factor of 3.664.
“Carbon emissions” is shorthand, and before you can use a number you have to know which of three things it is short for.
Carbon emissions usually means carbon dioxide (CO₂) released by burning fossil fuels, making cement and clearing land. In company reporting it is often loosely used for all greenhouse gases in CO₂e. Science sometimes counts carbon alone: 1 t of carbon = 3.664 t CO₂.
What carbon emissions means
Carbon emissions are releases of carbon to the atmosphere, almost always as carbon dioxide. The term is the everyday name for the biggest part of human emissions: the CO₂ that comes from burning coal, oil and gas, from making cement, and from cutting down forests.
Strictly, “carbon” is an element and “carbon dioxide” is a molecule. When people say a factory emits carbon, they mean it emits carbon dioxide: carbon that was locked in fuel, combined with oxygen during combustion and released as gas. The shorthand is so widespread — carbon footprint, carbon price, carbon budget, low-carbon — that “carbon” has become the word for climate impact in general.
That convenience comes at a cost. The same phrase is used for CO₂ alone, for all greenhouse gases expressed as CO₂ equivalent, and in scientific work for the mass of carbon atoms only. The three differ by large factors, and the label on the number rarely says which one it is.
Definition at a glance
| Usual meaning | Carbon dioxide (CO₂) released to the atmosphere by human activity |
|---|---|
| Loose meaning | All greenhouse gases in tonnes of CO₂ equivalent (CO₂e) — as in “carbon footprint” |
| Scientific meaning | Mass of carbon only (tonnes C, GtC) — multiply by 44/12 = 3.664 for CO₂ |
| Main sources | Coal, oil and natural gas combustion; cement; land-use change |
| 2025 global total | 42.2 GtCO₂ (38.1 fossil + 4.1 land use), projected |
| Unit to look for | t CO₂, t CO₂e or t C — the unit tells you which meaning is in use |
The three meanings in use
| Meaning | Unit | Who uses it | What it includes |
|---|---|---|---|
| Carbon dioxide only | t CO₂ | Energy statistics, the Global Carbon Budget, most news reporting | CO₂ from fuels, cement and land use; no other gases |
| All greenhouse gases, CO₂-equivalent | t CO₂e | Corporate footprints, the GHG Protocol, ESG and climate reporting | CO₂ plus methane, nitrous oxide and F-gases, weighted by GWP |
| Carbon atoms only | t C, GtC | Carbon-cycle science, forestry, some older studies | The mass of carbon, whatever molecule it ends up in |
The first two are often confused, and the gap is large. Fossil CO₂ is about three-quarters of global greenhouse-gas emissions in CO₂e; the remaining quarter is mostly methane and nitrous oxide. For a dairy farm or a landfill, non-CO₂ gases can be most of the footprint, so a “carbon emissions” figure that counts only CO₂ can miss the bulk of it. When a company publishes a “carbon footprint”, it almost always means CO₂e — see CO₂e.
Carbon vs CO₂: the 44/12 conversion
A carbon atom weighs 12 atomic mass units; a CO₂ molecule, with two oxygen atoms added, weighs 44. So every tonne of carbon that is burned becomes 44 ÷ 12 = 3.664 tonnes of CO₂. The oxygen comes from the air, which is why the CO₂ weighs more than the fuel’s carbon.
t CO₂ = t C × 44/12 = t C × 3.664 · t C = t CO₂ × 12/44 = t CO₂ × 0.2727
The Global Carbon Budget publishes in both units: fossil emissions of 10.4 GtC in 2025 are the same as 38.1 GtCO₂. A figure in GtC compared with one in GtCO₂ will look 3.7 times smaller and lead to wrong conclusions. The carbon to CO₂ converter does the arithmetic.
The carbon unit is natural in carbon-cycle science, where carbon moves between the air, the oceans, soils and forests in different chemical forms; tracking the atom avoids changing units at each step. A forest that stores 100 tonnes of carbon has drawn 366 tonnes of CO₂ out of the atmosphere. Carbon credits, corporate inventories and almost all policy use tonnes of CO₂ or CO₂e.
Where carbon emissions come from
Fossil CO₂ — the largest component — comes from three fuels and one industrial process. In 2024 fossil emissions reached 10.3 GtC (37.8 GtCO₂), split as follows:
Global fossil CO₂ emissions by source, 2024. Source: Friedlingstein et al., Global Carbon Budget 2025, Earth System Science Data 18, 3211.
Coal leads because it has the most carbon per unit of energy. Burning natural gas releases a little over half as much CO₂ as coal for the same heat (about 56 against 95 tonnes per terajoule on IPCC default factors), which is why switching from coal to gas cuts carbon emissions — though methane leaking from gas supply chains can erode that advantage. Cement is the one large source that is chemistry rather than combustion: heating limestone releases CO₂ from the rock itself, so it is a process emission that clean fuel alone cannot remove.
On top of fossil emissions, land-use change — mainly deforestation — added a projected 4.1 GtCO₂ in 2025, giving total human CO₂ emissions of about 42.2 GtCO₂. Roughly half of each year’s emissions is absorbed by the oceans and by plants on land, the natural carbon sinks; the rest accumulates in the atmosphere, where it drives warming and uses up the remaining carbon budget.
Carbon emissions are a flow and CO₂ in the air is a stock. Emissions only have to rise for the stock to grow — they have to fall to zero for it to stop.
How carbon emissions are calculated
Almost nobody measures carbon emissions at the chimney. They are calculated from the carbon in the fuel: every tonne of carbon burned produces 3.664 tonnes of CO₂, adjusted for the small share that does not fully oxidise. Published emission factors package that chemistry per unit of fuel, so the calculation becomes fuel used × factor.
On current UK government factors, burning a litre of diesel releases 2.58354 kg CO₂e [GreenCalculus fuels.gbr.diesel_average_biofuel_blend.litre · DEFRA 2026 'Fuels'!D72 · v2026.237], a litre of petrol 2.075 kg CO₂e [GreenCalculus fuels.gbr.petrol_average_biofuel_blend.litre · DEFRA 2026 'Fuels'!D96], a kilowatt-hour of natural gas 0.18231 kg CO₂e [GreenCalculus fuels.gbr.natural_gas.kwh_gcv · DEFRA 2026 'Fuels'!D42], and a tonne of industrial coal 2415 kg CO₂e [GreenCalculus fuels.gbr.coal_industrial.tonne · DEFRA 2026 'Fuels'!D131]. These factors are almost entirely CO₂; the small methane and nitrous oxide share is already folded in as CO₂e, which is why fuel factors serve both the narrow and the loose meaning of “carbon emissions” nearly equally well.
Biomass is the exception that trips people up. Burning wood releases CO₂ too, but that carbon was taken from the air recently, so the GHG Protocol reports it separately as biogenic CO₂ rather than inside the scope totals.
Worked micro-example
A tonne of steam coal contains roughly 0.66 tonnes of carbon (an illustrative carbon content; it varies by grade). Burned completely, the carbon becomes CO₂.
| Expressed as | Calculation | Result |
|---|---|---|
| Carbon | 1 t coal × 0.66 | 0.66 t C |
| Carbon dioxide | 0.66 t C × 44/12 | 2.42 t CO₂ |
| CO₂e (UK factor, includes CH₄ and N₂O) | 1 t × 2,415 kg CO₂e/t | 2.42 t CO₂e |
The same tonne of coal can be reported as 0.66, 2.42 or 2.42 “tonnes of carbon emissions”. The last two agree because coal combustion is almost pure CO₂; for a landfill or a herd of cattle, where methane dominates, the CO₂-only and CO₂e figures would be far apart. Always read the unit.
Common mistakes
- Comparing tonnes of carbon with tonnes of CO₂. They differ by 3.664. Convert before comparing.
- Assuming “carbon emissions” includes every gas. Energy statistics usually mean CO₂ only; corporate footprints usually mean CO₂e. Check the unit.
- Leaving out non-CO₂ gases where they matter. In agriculture, waste and refrigeration, methane, nitrous oxide and F-gases can dominate.
- Forgetting land use. Global fossil CO₂ excludes deforestation, which added about 4.1 GtCO₂ in 2025.
- Adding biogenic CO₂ into scope totals. Report it separately under the GHG Protocol.
- Treating “carbon” as soot. Carbon emissions here means CO₂, an invisible gas — not black carbon particulates, which are an air pollutant with their own climate effect.
Convert between carbon and CO₂, or calculate the carbon emissions from your own fuel use.
Frequently asked questions
Carbon emissions are releases of carbon dioxide (CO₂) to the atmosphere, mainly from burning coal, oil and natural gas, making cement and clearing forests. The term is also used loosely for all greenhouse gases expressed in CO₂ equivalent, as in a company’s carbon footprint. Global CO₂ emissions were projected at 42.2 billion tonnes in 2025.
No. Carbon is an element; carbon dioxide is a molecule of one carbon and two oxygen atoms. In everyday use “carbon emissions” means CO₂, but when a figure is given in tonnes of carbon (t C or GtC), multiply by 44/12, or 3.664, to get tonnes of CO₂. Ten billion tonnes of carbon is about 36.6 billion tonnes of CO₂.
Coal. In 2024 it produced 41% of global fossil CO₂ emissions, followed by oil at 32%, natural gas at 21% and cement at 4%, according to the Global Carbon Budget. By sector, the power sector is the largest source of greenhouse-gas emissions (26% in 2023), followed by transport (15%), according to UNEP figures cited in the UN’s 2025 SDG report.
Strictly, carbon emissions are CO₂ only, while greenhouse-gas emissions also include methane, nitrous oxide and fluorinated gases, converted to CO₂ equivalent. CO₂ is about three-quarters of the global greenhouse-gas total. In corporate reporting the two terms are often used interchangeably, so check whether a figure is in t CO₂ or t CO₂e.
Mostly by calculation, not measurement. The carbon content of a fuel determines how much CO₂ it releases when burned, so emissions are estimated as the amount of fuel used multiplied by a published emission factor. A litre of diesel, for example, releases about 2.6 kg CO₂e on UK government factors.
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