CO₂ · Glossary

CO₂ is the chemical formula for carbon dioxide. This colourless and odourless gas consists of one carbon atom and two oxygen atoms. For the energy system, CO₂ is important above all because it is produced when carbon-containing energy sources are burned and, as a long-lived greenhouse gas, warms the atmosphere. Coal, crude oil and natural gas contain carbon. When this carbon burns with oxygen, carbon dioxide is formed. The energy released in the process is used in power plants, heating systems, engines and industrial installations.

CO₂ is not the same as energy consumption, air pollution or the total climate impact. The term refers to a specific molecule. It matters for climate balances because additional CO₂ in the atmosphere changes the Earth's radiation budget. According to measurements by the US agency NOAA, the mean global CO₂ concentration in the atmosphere in 2025 was around 425.6 ppm (parts per million). Since 1979 it has risen on average by just under 2 ppm per year. Among the long-lived greenhouse gases, CO₂ contributes by far the most to the increase in radiative forcing. According to the European Copernicus Climate Change Service, the concentration in 2025 was the highest in at least two million years.

Methane and nitrous oxide also act as greenhouse gases. However, they remain in the atmosphere for different lengths of time and have a different climate impact per unit of mass. To make them comparable in balances, they are often converted into CO₂ equivalents. CO₂ is the reference gas here, with a factor of 1.

CO₂, CO₂ equivalents and emissions

CO₂ quantities are usually given in kilograms, tonnes or millions of tonnes. A tonne of CO₂ is a mass, not a quantity of energy. The link to energy arises via the fuel and its carbon content. When one kilogram of pure carbon is burned, around 3.67 kilograms of CO₂ are produced. This is due to the ratio of the molar masses: 44 grams per mole for CO₂ compared with 12 grams per mole for carbon. Most of the mass therefore comes from the oxygen in the air. This is why the specific emissions of fossil fuels differ markedly. Per kilowatt hour of electricity generated, lignite generally causes more CO₂ than hard coal, and hard coal more than natural gas. How high the balance turns out exactly depends on the efficiency of the plant, the fuel and the system boundary of the calculation, for example on whether upstream chains such as extraction and transport are counted.

CO₂ equivalents, often written as CO₂e, are a calculation unit. They combine different greenhouse gases according to their climate impact. Methane does not become CO₂ just because it appears as CO₂e in a balance. It is merely converted to a common reference unit, usually over a period of 100 years. This distinction matters because debates about climate balances otherwise mix up three different things: the physical emission of a substance, its effect in the atmosphere and the chosen assessment method.

The term emissions is also broader than CO₂. It can refer to greenhouse gases, air pollutants or other released substances. Nitrogen oxides, sulphur dioxide and particulate matter matter for health and local air quality, but they are not CO₂. Conversely, at usual concentrations CO₂ in outdoor air is not a classic air pollutant that burdens the respiratory tract. In poorly ventilated indoor spaces, however, it serves as an indicator of air quality. For climate policy, what counts is the worldwide accumulation of CO₂ in the atmosphere.

Significance in the electricity system

In the electricity system, CO₂ is produced mainly where fossil power plants generate electricity. A coal-fired or gas-fired power plant emits CO₂ as long as it burns fuel. Wind turbines, photovoltaic systems, hydropower plants and nuclear power plants do not emit CO₂ from combustion during operation. However, indirect emissions also arise with these technologies during manufacturing, construction, maintenance and decommissioning. It is therefore necessary to distinguish between operational emissions and life-cycle emissions.

A well-known indicator is the CO₂ emission factor of the electricity mix. According to calculations by the Umweltbundesamt (German Environment Agency), generating each kilowatt hour of electricity consumed in Germany in 2025 produced on average around 344 grams of CO₂. In 2024 it was 353 grams, and in 2023 still 379 grams. The UBA cites as reasons for the decline the higher share of renewable energies, the reduced electricity consumption during economic stagnation and the surplus of electricity imports. It expressly points out that the falling factor therefore shows only to a limited extent how durably the climate protection measures in the electricity sector are working. Anyone comparing such values should check whether they refer to generated or consumed electricity and whether upstream chains or other greenhouse gases (then in CO₂e) are included.

An average annual value is not sufficient for the climate impact of electricity consumption. In the grid, generation and consumption must match at every moment. If additional consumption in a particular hour is covered mainly by a fossil power plant, it has a different short-term effect than in an hour with plenty of wind and solar power and little fossil generation. Analyses therefore distinguish between average values, marginal values and hourly emission factors. An average emission factor indicates how much CO₂ arises per kilowatt hour in the electricity mix as a whole. A marginal emission factor asks which generation is triggered by an additional kilowatt hour of consumption.

This distinction influences practical decisions. A heat pump, an electric car or an electrolyser can have very different effects on emissions. What matters is how efficient the installation is, when it runs and which electricity generation it displaces or additionally requires in the respective period. This does not make electrification questionable in terms of climate policy. However, the focus shifts from the individual device to the interplay of electricity consumption, generation structure, grids, storage and flexibility.

Common oversimplifications

CO₂ is often equated with "climate damage" without naming the system boundary. A national emissions balance usually covers the emissions arising within a country. A consumption-based balance allocates emissions to the goods and services consumed in a country, even if they were produced abroad. Both perspectives answer different questions. The territorial balance fits many governmental responsibilities and international reporting obligations. The consumption-based balance shows which emissions are co-caused by demand for imported products. The same applies in the electricity sector: if a country imports more electricity, its domestic power plant emissions fall, although the emissions are in part merely shifted abroad.

A second oversimplification concerns the terms "CO₂-neutral" and "climate-neutral". CO₂-neutral can mean that an activity, on paper, causes no net CO₂ emissions. Climate-neutral is stricter if all greenhouse gases and their climate effects are included. In practice, such statements depend on accounting rules, offsetting measures, certificates and time periods. If fossil CO₂ is released and later compensated by afforestation, this is physically not the same as an avoided emission. Forests store carbon, but they can burn, die off or be used differently. Fossil carbon was sequestered over geological periods and only enters the active carbon cycle additionally through combustion.

With biomass, too, the balance is not automatically zero. Plants absorb CO₂ as they grow and release it again on combustion or decay. Whether energy use benefits the climate depends on cultivation, land competition, harvest cycles, transport, processing and the alternative displaced. Anyone who treats biomass across the board as CO₂-free overlooks time lags and competing uses. In many balances, biogenic CO₂ at the stack is reported separately and not counted. Physically, it is nevertheless produced.

Rules, prices and responsibilities

In the energy system, CO₂ is not only a scientific term but also a steering variable for markets and regulation. Under the EU Emissions Trading System (EU ETS 1), certain installations in the energy industry and manufacturing need allowances for their greenhouse gas emissions. The quantity of allowances is limited. Their price affects the operating costs of fossil power plants and industrial installations. CO₂ thus does not change how a power plant works technically, but it does change its economic position in the market. A higher CO₂ price makes particularly emission-intensive generation more expensive and can shift the merit order of power plants, for example in favour of gas-fired over coal-fired plants.

For fuels in buildings and road transport, the EU has created a second system with Chapter IVa of the Emissions Trading Directive 2003/87/EC (EU ETS 2). There, it is not the individual heating systems or vehicles that are subject to surrender obligations, but the fuel suppliers. The system was originally to start in 2027. Following an EU legal act amending the decision on the Market Stability Reserve, the start was postponed to 2028 (as of this revision). In Germany, national CO₂ pricing under the Brennstoffemissionshandelsgesetz (Fuel Emissions Trading Act) applies to these sectors until then. Specific prices and transitional rules should be checked in the current legal text.

Alongside emissions trading, other instruments are at work: energy taxes, support rules, reporting obligations, product standards, guarantees of origin and public procurement. These rules lie partly at European level, partly at national, regional or municipal level. Conflicts arise where a technical saving option exists, but investment incentives, grid access, permitting or responsibility do not fit. CO₂ reduction is therefore not purely a question of switching fuels. It concerns market rules, infrastructure planning, industrial value creation and the distribution of costs.

In the electricity market, CO₂ also shows that a low short-term exchange price should not be equated with low overall costs. If climate impacts are not, or only partly, included in the price, fossil energy sources appear cheaper than they would be if external costs were fully taken into account. A CO₂ price attempts to translate this difference, at least partly, into business decisions. However, it does not automatically replace grid expansion, storage, flexible loads and the safeguarding of security of supply.

Distinction from carbon and decarbonisation

CO₂ contains carbon but is not the same as carbon. Carbon occurs in many forms: in coal, natural gas, crude oil, biomass, plastics, soils, limestone and living organisms. Anyone comparing quantities should check whether "tonnes of carbon" or "tonnes of CO₂" is meant. The difference is a factor of 3.67. In the energy system, decarbonisation usually means displacing fossil carbon from energy conversion, heat, transport and industrial processes or permanently preventing its release. Some industrial processes, such as cement production, release CO₂ not only through energy consumption but also through the chemical conversion of the raw material: when limestone is burned, CO₂ is split off. There, renewable electricity alone is not sufficient to avoid all emissions.

CO₂ capture and storage, usually called CCS, is intended to capture carbon dioxide at large point sources and store it permanently in geological formations. If CO₂ is taken directly from the air and stored, this is called direct air capture with storage. These technologies can be important for hard-to-avoid processes and residual emissions. However, they do not undo an emission. Their effect depends on capture rate, energy demand, transport, tightness of the storage sites and liability rules. In fossil electricity generation with CO₂ capture, emissions from the upstream chain, such as methane losses in natural gas extraction, and the portion not captured also remain.

CO₂ is thus both: a precise name for a substance and a central accounting variable. The term does not by itself say which measure is effective, fair or economical. But it forces us to disclose material flows, time periods and system boundaries. In the electricity system, CO₂ describes not only what leaves the stack. It also shows which generation a consumption triggers, which rules put a price on emissions and which residual emissions remain after technical avoidance.

Sources

CO₂-Emissionen pro Kilowattstunde Strom 2025 nur leicht gesunken — Umweltbundesamt; unbekannt (2026). Einleitung/Kurzbeschreibung der Meldung
Entwicklung der spezifischen Treibhausgas-Emissionen des deutschen Strommix in den Jahren 1990 - 2025 (Climate Change 16/2026) — Umweltbundesamt; 2026-03. Publikationsseite und Kurzbeschreibung
Annual Greenhouse Gas Index (AGGI) — NOAA Global Monitoring Laboratory; unbekannt (Daten bis 2025). Abschnitt zu CO₂ und Strahlungsantrieb
Climate Indicators: Greenhouse gas concentrations — Copernicus Climate Change Service (C3S/ECMWF); unbekannt (2026). Abschnitt Carbon dioxide
Update CO2-Bepreisung: Verschiebung des ETS2 — Stiftung Umweltenergierecht; 2025-12-05. Darstellung Art. 1a des Änderungsentwurfs zum MSR-Beschluss

Related glossary entries

Edited by eCMS on 11.11.2026 17:34