Emissions
Nobody has ever weighed a year of the world’s emissions. There is no meter on the sky. The figure every climate target is written against — tens of billions of tonnes a year — is built up from fuel bills, cattle counts, factory output and a published factor for each, added together line by line.
An emission is simply something released into the air, and almost every emissions number you will ever read was calculated, not measured.
An emission is a substance released into the air. In climate accounting, emissions means greenhouse gases — mainly CO₂, methane and nitrous oxide — from fuel, industry, farming and waste, estimated as activity × an emission factor and reported in CO₂e.
What emissions means
In everyday English an emission is the act of sending something out, or the thing that is sent out. In climate and environmental work the word has narrowed to one meaning: gases and particles released into the atmosphere by a process, a machine, a building, an animal or a whole economy.
The word has older, broader senses that still turn up. Physicists talk about the emission of light, heat or radio waves from a source; a lamp emits photons and a star emits radiation. Car owners in many countries take their vehicle for an “emissions test”, which checks the exhaust for pollutants such as carbon monoxide, nitrogen oxides and unburnt hydrocarbons. All of these share the same core idea — something leaving a source and entering its surroundings — but when a company report, a government target or a news story says “emissions”, it almost always means emissions to air, and usually greenhouse-gas emissions in particular.
That last distinction matters more than it looks. “Emissions” on its own can mean the whole set of substances discharged to air — greenhouse gases and conventional pollutants together — or it can be shorthand for the greenhouse gases alone. Climate reporting uses the second meaning; air-quality regulation uses the first. Reading a figure correctly starts with knowing which one the writer meant.
Emissions are a flow, not a stock. They are counted per period — tonnes per year, grams per kilometre, kilograms per kilowatt-hour — and they add to the stock of gas already in the atmosphere. The stock is what warms the planet; the flow is what a business, a city or a country can actually change. That is why every inventory, target and carbon budget is expressed in emissions per year rather than in atmospheric concentration.
Definition at a glance
| Plain meaning | Something released into the air — gases or particles leaving a source |
|---|---|
| Climate meaning | Greenhouse gases released by human activity (anthropogenic emissions) |
| Main gases | Carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), fluorinated gases (HFCs, PFCs, SF₆, NF₃) |
| Unit | Tonnes of CO₂ equivalent (tCO₂e) per year, after weighting each gas by its global warming potential |
| How it is found | Mostly calculated: activity data × emission factor. Direct measurement is the exception (large stacks, some leak surveys) |
| Who it belongs to | Split into scope 1 (direct), scope 2 (purchased energy) and scope 3 (the rest of the value chain) |
| Opposite | Removals — CO₂ taken out of the atmosphere by a sink, such as a forest or a storage reservoir |
That figure, from the European Commission’s EDGAR 2025 report, excludes land use, land-use change and forestry. The UN Environment Programme’s Emissions Gap Report 2025 puts the 2024 total at 57.7 GtCO₂e, up 2.3%, because it counts land-use change as well. Both are correct; they answer slightly different questions. Any global emissions number is only comparable with another one built on the same boundary.
How the standards define it
GHG Protocol Corporate Standard (glossary): emissions are “The release of GHG into the atmosphere.” A GHG source is “Any physical unit or process which releases GHG into the atmosphere.”
The same standard splits emissions by ownership. “Direct GHG emissions are emissions from sources that are owned or controlled by the company.” “Indirect GHG emissions are emissions that are a consequence of the activities of the company but occur at sources owned or controlled by another company.”
IPCC: the IPCC glossary treats anthropogenic emissions as greenhouse gases, their precursors and aerosols released by human activities — burning fossil fuels, deforestation, land-use change, livestock, fertiliser, waste and industrial processes. National inventories follow the sector structure of the IPCC 2006 Guidelines: energy; industrial processes and product use; agriculture, forestry and other land use; and waste.
US EPA, on air quality: ground-level ozone “is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC).” The emission is the NOx and VOC; ozone is what they become.
Notice what the GHG Protocol definition leaves out. It says nothing about intent, legality or whether the gas is captured later. A tonne of CO₂ that leaves a chimney is an emission even if the same company plants trees the same year. Those trees are a removal, counted separately. Keeping the two apart is the foundation of gross versus net reporting.
Greenhouse gases vs air pollutants
The same exhaust pipe or chimney releases two quite different families of substance. They are governed by different rules, harm people in different ways and are counted in different units — which is why “cutting emissions” can mean two different projects.
| Question | Greenhouse-gas emissions | Air-pollutant emissions |
|---|---|---|
| Main substances | CO₂, methane, nitrous oxide, F-gases | Particulate matter, nitrogen oxides, sulphur dioxide, carbon monoxide, lead, volatile organic compounds |
| The harm | Trap heat and warm the climate | Damage lungs, hearts and ecosystems; form smog and acid rain |
| Where the harm happens | Everywhere — a tonne has the same effect wherever it is released | Mostly local and regional — near the source and downwind |
| How long it lasts | Decades to millennia (CO₂ persists for centuries) | Hours to weeks |
| Common unit | Tonnes CO₂e per year | Tonnes per year of each pollutant; concentrations in µg/m³ |
| Typical rulebook | GHG Protocol, ISO 14064-1, CSRD/ESRS E1, emissions trading schemes | Clean air acts, vehicle emission standards, industrial permits |
The two families overlap at the source but not always in the solution. A diesel particulate filter cuts particulates sharply and does almost nothing to CO₂. Switching a boiler from coal to gas cuts sulphur dioxide and CO₂ at once. When someone says a technology “reduces emissions”, ask which family they mean.
The US EPA regulates six “criteria” air pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter and sulphur dioxide. Ozone is on the list but is never emitted directly — it forms in the air from other emissions. Emissions accounting tracks what leaves a source; air-quality monitoring measures what ends up in the air people breathe.
Which gases make up the total
Climate reporting covers the seven gases of the Kyoto basket. They are added together by converting each into CO₂ equivalent using its global warming potential. On the IPCC AR6 100-year basis a tonne of fossil methane counts as 29.8 tonnes of CO₂e [GreenCalculus gwp.CH4_fossil.ar6_100 · IPCC AR6 WGI Ch 7 Table 7.SM.7 (2021) — AR6 GWP-100 · v2026.237], and a tonne of nitrous oxide as 273 tonnes of CO₂e [GreenCalculus gwp.N2O.ar6_100 · IPCC AR6 WGI Ch 7 Table 7.SM.7 (2021) — AR6 GWP-100].
Share of global greenhouse-gas emissions by gas, 2024, in CO₂e, excluding land use. Source: EDGAR 2025 report, European Commission Joint Research Centre.
Three-quarters of the total is fossil CO₂, which is why “carbon emissions” is often used loosely for all of it. The other quarter is not a rounding error. Methane alone is nearly a fifth of global warming emissions, mostly from livestock, rice, landfill and leaks from oil and gas systems. The F-gases are tiny by mass but run to thousands of times CO₂’s warming per tonne — sulphur hexafluoride is 25200 [GreenCalculus gwp.SF6.ar6_100 · IPCC AR6 WGI Ch 7 Table 7.SM.7 (2021) — AR6 GWP-100] on the AR6 100-year scale.
Two things are deliberately left out. Water vapour is the largest natural greenhouse gas, but its concentration follows temperature, so it is treated as a feedback rather than an emission (see greenhouse gas). CO₂ from burning biomass is reported separately as biogenic, outside the scope totals, because the carbon was drawn out of the air recently.
How emissions are calculated
For almost every organisation, emissions are calculated. Continuous stack monitors exist on large power plants and some industrial sites; leak-detection surveys measure methane at others. Everyone else uses the same equation, which underpins national inventories and corporate footprints alike:
Emissions = activity data × emission factor
Activity data is how much of something happened — litres of fuel burned, kilowatt-hours of electricity bought, tonnes of waste sent to landfill, kilometres flown. The emission factor is how much greenhouse gas each unit releases. Multiply, convert each gas to CO₂e, and sum.
The factors come from published datasets. In the UK the government publishes them every year; on current figures, burning a litre of diesel releases 2.58354 kg CO₂e [GreenCalculus fuels.gbr.diesel_average_biofuel_blend.litre · DEFRA 2026 'Fuels'!D72], a kilowatt-hour of natural gas 0.18231 kg CO₂e [GreenCalculus fuels.gbr.natural_gas.kwh_gcv · DEFRA 2026 'Fuels'!D42], and a kilowatt-hour of UK grid electricity 0.131 kg CO₂e [GreenCalculus grid.gbr.electricity.location_based · DEFRA 2026 'UK electricity'!E25]. Every one of those numbers is revised as fuel blends, grid mixes and methods change, which is why an emissions figure is only as good as the factor behind it and the year that factor applies to.
This has a practical consequence that surprises people: two organisations with identical operations can report different emissions because they chose different factors, different GWP bases or different boundaries. Comparing emissions figures without checking those three things is comparing apples with estimates of oranges.
Whose emissions: scopes 1, 2 and 3
An emission happens at one physical source, but several organisations can have a reason to count it. The GHG Protocol settles who reports what with three scopes. Every tonne in a corporate GHG inventory sits in exactly one of them.
| Scope | What it covers | Direct or indirect | Typical sources |
|---|---|---|---|
| Scope 1 | Sources the company owns or controls | Direct | Boilers, furnaces, company vehicles, refrigerant leaks, process chemistry |
| Scope 2 | Generation of electricity, heat, steam and cooling the company buys | Indirect | The power station supplying the grid |
| Scope 3 | Everything else in the value chain, upstream and downstream | Indirect | Purchased goods, freight, business travel, commuting, use and disposal of sold products |
Scope 1 itself breaks into four source types, each with its own page: combustion (split into stationary and mobile), process and fugitive. The direct versus indirect split is what stops the same tonne being counted twice in one company’s total — though it is counted again, legitimately, as scope 1 in the footprint of whoever owns the source.
One tonne of CO₂ from a power station is scope 1 for the generator, scope 2 for the factory that buys the power and scope 3 for the factory’s customers. The atmosphere receives it once.
Gross, net, negative, avoided, residual
“Emissions” gathers qualifiers quickly, and each changes the meaning of the number in front of it. These are the ones that cause the most confusion in reports and targets.
| Term | What it means | Counts towards the inventory total? |
|---|---|---|
| Gross emissions | Everything released, before any removals or credits | Yes — this is the inventory |
| Net emissions | Gross minus removals (and, in some frameworks, credits) | Reported separately; never replaces gross. See gross vs net |
| Negative emissions | CO₂ taken out of the atmosphere and stored, so the net flow is below zero | No — a removal, reported alongside |
| Avoided emissions | Emissions that would have happened without a product or project (sometimes called “scope 4”) | No — a comparison against a baseline, never subtracted |
| Residual emissions | What is left after all feasible reductions, to be balanced by removals at net zero | Yes — part of the gross total |
The pattern is consistent across the major frameworks: report gross emissions in full, report removals and credits next to them, and never let one quietly cancel the other inside a single figure. A “net” number with no gross number beside it is a warning sign.
Worked micro-example
A delivery depot in the UK records three activities for the year. Each is multiplied by its factor to give CO₂e. Factors are the UK DEFRA 2026 values at the time of writing (diesel per litre, UK grid electricity per kWh) and the IPCC AR6 100-year GWP for fossil methane; the figures are illustrative.
| Activity | Activity data × factor | Emissions | Scope |
|---|---|---|---|
| Diesel in company vans | 1,000 L × 2.58354 kg/L | 2,583.5 kg CO₂e | 1 |
| Grid electricity | 20,000 kWh × 0.131 kg/kWh | 2,620.0 kg CO₂e | 2 |
| Methane leak from an on-site gas line | 50 kg CH₄ × 29.8 | 1,490.0 kg CO₂e | 1 |
| Total | — | 6,693.5 kg CO₂e ≈ 6.7 tCO₂e | — |
By mass the methane leak is 50 kg against more than 2.5 tonnes of CO₂ from the vans, yet it contributes over a fifth of the total once its warming effect is applied. Scope 1 is 4,073.5 kg CO₂e and scope 2 is 2,620.0 kg CO₂e. None of these numbers was measured: they are three bills, three factors and three multiplications.
Common mistakes
- Treating “emissions” and “carbon emissions” as the same. CO₂ is about three-quarters of global greenhouse-gas emissions; the rest is methane, nitrous oxide and F-gases. A CO₂-only figure understates the total.
- Mixing greenhouse gases with air pollutants. A cut in particulates or NOx is an air-quality gain, not necessarily a climate one. Check which family a claim refers to.
- Assuming the number was measured. Most emissions are calculated from activity data and factors. Ask which factor set, which year and which GWP basis.
- Netting removals or credits into the headline. Gross emissions are the inventory; removals, offsets and avoided emissions are reported beside it, not subtracted from it.
- Comparing totals across different boundaries. A global figure with land use and one without differ by several gigatonnes. Corporate totals with and without scope 3 can differ tenfold.
- Counting biomass CO₂ in the scope totals. Biogenic CO₂ from combustion is reported separately; its methane and nitrous oxide still go in scope 1.
Turn an emissions figure into something you can picture, or calculate your own scope 1 fuel emissions.
Frequently asked questions
An emission is something released or sent out from a source. In environmental and climate contexts it means gases or particles discharged into the air — most often greenhouse gases such as carbon dioxide, methane and nitrous oxide from burning fuel, industry, agriculture and waste. The GHG Protocol defines emissions as “the release of GHG into the atmosphere.”
Emissions are what a source releases; pollution is the harmful presence of a substance in the environment. Air pollutants such as particulates, nitrogen oxides and sulphur dioxide cause local health damage and are regulated by concentration. Greenhouse gases are not toxic at normal levels but warm the climate wherever they are released, so they are counted as tonnes of CO₂e per year rather than as concentrations.
Not quite. “Carbon emissions” is often used loosely for all greenhouse gases, but strictly it means carbon dioxide. Fossil CO₂ was 74.5% of global greenhouse-gas emissions in 2024 according to EDGAR; methane, nitrous oxide and F-gases made up the rest. Corporate inventories report all seven Kyoto gases converted to CO₂ equivalent, so a “carbon footprint” normally includes every gas.
Mostly they are calculated rather than measured. Organisations multiply activity data — litres of fuel, kilowatt-hours, tonnes of material — by a published emission factor, convert each gas to CO₂e using its global warming potential, and add the results. Direct measurement through continuous stack monitors or leak surveys is used at some large industrial sites and in methane detection.
They are the three categories the GHG Protocol uses to assign emissions to a company. Scope 1 covers direct emissions from sources the company owns or controls. Scope 2 covers emissions from generating the electricity, heat, steam and cooling it buys. Scope 3 covers all other indirect emissions across its value chain, from purchased goods to the use and disposal of the products it sells.
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