Carbon budget · Glossary

A carbon budget is the amount of carbon dioxide that humanity can still emit if a given temperature target is to be met with a specified probability. It links a climate target, such as limiting global warming to 1.5 degrees or well below 2 degrees Celsius relative to 1850 to 1900, to a quantity of emissions. The Intergovernmental Panel on Climate Change (IPCC) distinguishes between two terms. The total budget is counted from pre-industrial times. The remaining budget is counted from a more recent reference date. A carbon budget is therefore not a target for a single year. It is an accounting of quantities over time.

It is based on an approximately linear relationship: the more CO₂ has been emitted in total, the more the temperature rises. According to the IPCC, global surface temperature increases by about 0.45 degrees for every 1,000 gigatonnes of CO₂ emitted (likely range 0.27 to 0.63 degrees). What matters is the sum of emissions, because a considerable share of CO₂ remains in the atmosphere for a very long time. One can picture a bathtub with an almost blocked drain: even a weaker inflow keeps filling the tub. A country, a sector or a power plant fleet can therefore report falling annual emissions and still use up much of the remaining budget if emissions stay high until then. The carbon budget thus directs attention to the entire emissions pathway and not only to the target year.

Orders of magnitude and current estimates

According to the IPCC, about 2,400 ± 240 gigatonnes of CO₂ were emitted net from 1850 to 2019. That is roughly four fifths of the total budget of about 2,900 gigatonnes with which warming would be limited to 1.5 degrees with a 50 percent probability. For the beginning of 2020, the IPCC's Sixth Assessment Report gave the following remaining budgets as best estimates: 500 gigatonnes of CO₂ for 1.5 degrees at 50 percent probability and 1,150 gigatonnes for 2 degrees at 67 percent probability.

Since then, the remaining budget has shrunk considerably because of continued high emissions and newer methods. Two updates are particularly important at present:

The Indicators of Global Climate Change 2025 (published in June 2026) estimate the remaining budget from the beginning of 2026 at 130 gigatonnes of CO₂ for 1.5 degrees at 50 percent probability. At around 42 gigatonnes of CO₂ per year, that lasts a little over three years. For 2 degrees, the study gives 1,050 gigatonnes at 50 percent, 860 gigatonnes at 67 percent and 690 gigatonnes at 83 percent probability.
The Global Carbon Budget 2025 gives, from January 2026, around 170 gigatonnes of CO₂ for 1.5 degrees, 525 gigatonnes for 1.7 degrees and 1,055 gigatonnes for 2 degrees. At current emission levels, this corresponds to about 4, 12 and 25 years.

The differences between the estimates show that remaining budgets are not fixed constants of nature. They depend on data, models and assumptions and are recalculated regularly. In practice, the core message of both sources is the same: at current emission levels, the 1.5-degree budget will be used up within a few years.

Unit of measurement and frame of reference

Carbon budgets are stated in tonnes, millions of tonnes or billions of tonnes of carbon dioxide. At the global level, the gigatonne of CO₂, i.e. one billion tonnes, is customary. Smaller budgets are derived from it for countries or sectors, for example for Germany, the energy sector or individual branches of industry. Such derivations are never purely scientific. The global remaining budget can be estimated with climate models and probabilities. How it is allocated among countries, generations and sectors, by contrast, is a normative and political decision.

A budget is incomplete without a statement of probability. A budget for a 50 percent chance of meeting a temperature target is larger than one for 67 or 83 percent. If this information is missing, the figure appears more precise than it is. A carbon budget is not an exact account from which emissions are mechanically debited. It is a scientifically based range. It depends, among other things, on how strongly the climate responds, how other greenhouse gases and aerosols develop and which assumptions are made about carbon sinks. According to the IPCC, the uncertainty in non-CO₂ emissions alone amounts to about ± 220 gigatonnes. The 2020 budget for 1.5 degrees could therefore, depending on how these gases develop, have been as low as 300 or as high as 600 gigatonnes.

Strictly speaking, the term refers only to carbon dioxide. In climate policy, however, methane or nitrous oxide are often also included and converted into CO₂ equivalents. This is practical for climate accounting but does not describe the same quantity. Per tonne, methane has a considerably stronger effect than CO₂, but it is broken down within a few decades. A greenhouse gas budget in CO₂ equivalents is therefore a different accounting quantity from a carbon budget in the narrower sense. In the IPCC's global budgets, the other gases are taken into account through assumptions about their future reduction: the more they fall, the larger the remaining CO₂ budget.

Distinction from climate neutrality and annual targets

A carbon budget is not the same as climate neutrality or net zero. Net zero describes a point in time from which human-caused emissions and removals are equal in balance. It must be specified whether only CO₂ or all greenhouse gases are meant. The carbon budget, by contrast, describes how much is emitted in total up to that point. Physically, both are necessary: to limit warming to a given level, cumulative CO₂ emissions must be limited and at least net-zero CO₂ must be reached. If more CO₂ is removed than emitted afterwards, the cumulative total falls again. This can partly offset a temporary overshoot.

This distinction has practical consequences. Two countries can aim for the same net-zero year and still use up very different amounts of the budget. Whoever reduces early emits less CO₂ overall than someone who stays at a high level for a long time and cuts sharply only shortly before the target year. From the budget's perspective, a late, steep decline is therefore not equivalent to an early, steady decline.

Annual reduction targets also show only a snapshot. They specify how high emissions are to be in a given year. They do not automatically say whether the path to that point is compatible with a remaining budget. An annual target can be met formally even though the cumulative budget had already been largely used up beforehand.

Significance for the electricity system

In the electricity system the carbon budget matters because fossil power plants cause large quantities of CO₂ through their operating hours and their technical lifetime. A new gas-fired power plant, a coal unit operated for longer or an expansion of renewable generation that is too slow therefore cannot be assessed solely by the emissions of a target year. What matters is how much CO₂ arises over all years of operation. The IPCC has made a clear assessment in this regard: even the existing fossil infrastructure would, without additional mitigation, emit more CO₂ than the remaining budget for 1.5 degrees at 50 percent probability allows. Existing and planned facilities together would, under past operating patterns, roughly exhaust the budget for 2 degrees at 83 percent probability.

This affects investment decisions. Power plants, grids, storage, industrial facilities and heating infrastructure stay in operation for years to decades. Anyone who builds facilities today that permanently require fossil fuels risks a lock-in effect: either the facility runs for a long time and consumes additional budget, or it is decommissioned prematurely and causes economic losses. The carbon budget thus links lifetime, capital costs and climate target.

For electricity generation, the budget concept shifts the focus from average emission factors to actual operating behaviour. A gas-fired power plant that runs only a few hours a year to safeguard security of supply burdens the budget differently from a fossil power plant in regular base-load or mid-load operation. Likewise, it is not enough to count the installed capacity of renewable energy. What matters is how much fossil generation wind and solar power, together with grids, storage, load shifting and flexibility, actually replace.

The carbon budget also explains why rising electricity consumption due to electrification does not automatically harm the climate. Heat pumps, electric vehicles and electric industrial processes increase electricity consumption. But they can displace oil, gas and coal in buildings, transport and industry. What counts for the budget is the balance: avoided combustion on one side, additional emissions from electricity generation on the other. The faster the electricity mix is decarbonised, the more electrification spares the budget.

Typical misinterpretations

A common misunderstanding is to treat the carbon budget as a politically freely selectable number. Politics decides how a budget is allocated and implemented and which probability is regarded as sufficient. It cannot suspend the physical relationship between cumulative CO₂ and warming. If a budget is exceeded, the probability of meeting the temperature target falls. This does not change if one holds on to a later target year.

A second oversimplification concerns negative emissions. Many scenarios assume that CO₂ is later removed from the atmosphere again, for example through afforestation, bioenergy with carbon capture or direct air capture. Such removals can offset residual emissions from hard-to-abate areas. According to the IPCC, they are even necessary for net zero in all greenhouse gases. But they do not enlarge the budget that can be used today at will. Land requirements, permanence of storage, energy demand, costs and institutional oversight limit their use. Anyone who offsets high emissions today against later removals shifts technical and political risks into the future.

A third imprecision arises from the choice of accounting boundary. National budgets usually refer to territorial emissions, that is, emissions within a state's territory. Consumption-based approaches, by contrast, attribute emissions to the goods and services consumed in a country, even if they were produced elsewhere. For electricity policy, territorial accounting is important because power plants and grids are regulated nationally or at European level. For industry, imports and supply chains, it is not sufficient on its own.

The term remaining budget is also misleading if understood as a credit that may be spent evenly up to a target year. With a linear allocation, too much would often be emitted in the first years, because later reductions are technically, politically or socially harder to implement. In many sectors lead times are long: permits, grid expansion, skilled workers, industrial conversions and investment cycles limit the pace. A budget that exists on paper is therefore not automatically a margin that can be used in practice. This applies particularly to 1.5 degrees, since the remaining budget there amounts to only a few years of current emissions.

Institutional and economic significance

A carbon budget makes visible what waiting costs: every tonne emitted today reduces the room for manoeuvre in later years. This gives rise to distributional conflicts between sectors. If the electricity sector decarbonises more slowly, industry, buildings, transport or agriculture must reduce faster or carry out additional removals. Conversely, a low-emission electricity system makes the transition easier for other sectors, because electrification then actually reduces emissions.

In emissions trading, this quantity logic appears as the volume of allowances. An emissions trading system sets a cap on certain emissions and makes emission allowances tradable. This is not, however, the same as the total carbon budget. Emissions trading covers only certain sectors and gases, follows its own rules on allowance volumes, market stability and price formation, and is embedded in political target frameworks. The carbon budget provides the climate-policy benchmark against which such instruments can be measured.

For investors and operators in the electricity system, the budget works through expectations. If a country takes a scarce remaining budget seriously, the realistic operating life of emission-intensive facilities falls. At the same time, the requirements for grid expansion, renewable generation, storage, controllable loads and low-emission firm capacity rise. Budget-oriented policy must therefore not only limit fossil generation but also make the alternatives ready for use in time. Otherwise a gap arises between climate target, market incentive and technical feasibility.

The carbon budget is thus an accounting quantity with far-reaching consequences. It does not determine which facility is built, which power plant is decommissioned first or which support instrument is chosen. But it shows which quantity of emissions remains compatible with a temperature target at a given probability, and how closely time, infrastructure and cumulative effect are connected. Whoever uses the term precisely understands climate action in the electricity system not as a point landing in a target year but as a continuous limitation of CO₂ quantities along the entire transformation pathway.

Sources

AR6 Synthesis Report: Climate Change 2023 – Summary for Policymakers — Intergovernmental Panel on Climate Change (IPCC); 2023-03-20. Abschnitte A.1.3, B.5.1–B.5.4 einschließlich Fußnote 41
Indicators of Global Climate Change 2025 – Tracking the human fingerprint (Abschnitt Carbon Budget) — Copernicus Climate Change Service / IIASA (auf Grundlage von Forster et al. 2026, Earth System Science Data); 2026 (Jahresupdate Vol. 4; genaues Datum unbekannt). Abschnitt 05 Carbon Budget, Tabelle nach Temperaturziel und Wahrscheinlichkeit
Global warming reached 1.37°C in 2025 — Copernicus Climate Change Service (ECMWF); 2026-06-11. Pressemitteilung, Kernaussagen
The world's carbon emissions continue to rise. But 35 countries show progress in cutting carbon (Global Carbon Budget 2025) — CSIRO (Mitglied des Global Carbon Project); November 2025 (genauer Tag unbekannt). Abschnitt Tracking global progress
Climate Change 2022: Mitigation of Climate Change – Summary for Policymakers (WG III) — Intergovernmental Panel on Climate Change (IPCC); 2022-04-04. SPM, Fußnoten 10–12 und Begriffserläuterung zu total und remaining carbon budget

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Edited by eCMS on 11.11.2026 17:40