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The Carbon Budget: How Much CO2 Can We Still Emit?
Key Idea
The carbon budget is the most direct quantitative link between human emissions and future warming. It tells us the total amount of CO2 that can be released into the atmosphere while keeping warming below a given temperature threshold. Understanding the carbon budget transforms climate change from an abstract concern into a concrete accounting problem with a finite limit.
What Is a Carbon Budget?
A carbon budget is defined as the maximum cumulative net global anthropogenic CO2 emissions that would result in limiting global warming to a given temperature level with a specified probability. The budget is stated as a total amount, typically in gigatonnes of CO2 (GtCO2), not a per-year rate.
The concept rests on a well-established physical relationship: total warming is approximately proportional to cumulative CO2 emissions. Double the cumulative emissions, and you roughly double the warming from CO2. This proportionality, known as the Transient Climate Response to Cumulative Carbon Emissions (TCRE), is one of the most robust findings in climate science.
Analogy: A Carbon Credit Card with a Hard Limit
Imagine a credit card with a fixed spending limit. Every tonne of CO2 we emit charges the card. Once the limit is reached, further spending pushes the climate into dangerous territory. Unlike a credit card, there is no paying off the balance later: the debt (warming) persists for centuries. Crucially, the limit was set by physics, not by negotiators, and cannot be renegotiated.
The Numbers: What Does the Budget Tell Us?
The IPCC AR6 WGI (Table SPM.2) provides the following remaining carbon budget estimates from January 1, 2020 onwards:
| Temperature Limit | Probability of Staying Below Limit | Remaining Budget from 2020 (GtCO2) | Years at 2019 Emission Rate |
|---|---|---|---|
| 1.5°C | 50% | 500 GtCO2 | ~12 years |
| 1.5°C | 67% | 400 GtCO2 | ~10 years |
| 2°C | 50% | 1350 GtCO2 | ~34 years |
| 2°C | 67% | 1150 GtCO2 | ~29 years |
These numbers are arresting. At 2019 emission rates of roughly 40 GtCO2 per year from fossil fuels and industry, the remaining budget for a 50% chance of staying below 1.5°C would be exhausted in approximately 12 years from 2020, meaning before 2032. Emissions have continued since 2020, shrinking the remaining budget further each year.
Historical Cumulative Emissions: A Key Context
Historical cumulative net CO2 emissions from 1850 to 2019 were approximately 2,400 GtCO2. Of this total, about 58% occurred before 1990 and 42% between 1990 and 2019. Strikingly, about 17% of all historical emissions since 1850 occurred in just the decade from 2010 to 2019.
The IPCC AR6 WGIII notes that, based on central estimates, historical emissions between 1850 and 2019 amount to about four-fifths of the total carbon budget for a 50% probability of limiting warming to 1.5°C. We have already used the vast majority of the safe emissions space that the atmosphere can accommodate.
What 400 Gigatonnes Actually Means
400 GtCO2 sounds abstract. For perspective: a single billion tonnes (1 GtCO2) is approximately what would be released if you burned all the forests in Germany. The entire global economy currently emits roughly 40 GtCO2 per year from fossil fuels alone. The 400 Gt budget for a 67% chance of 1.5°C is therefore roughly equivalent to 10 years of current fossil fuel emissions with nothing else changing. Every year of delay at current levels reduces the remaining budget and narrows the options available.
Uncertainties and Non-CO2 Gases
The carbon budget concept focuses on CO2, but non-CO2 greenhouse gases also contribute to warming. The IPCC accounts for this by adjusting budgets based on the projected path of non-CO2 emissions. The remaining budgets cited above assume a moderate level of non-CO2 mitigation. If non-CO2 gases (methane, N2O, F-gases) are reduced aggressively, the CO2 budget effectively expands by roughly ±220 GtCO2; if they are not reduced, the CO2 budget shrinks accordingly.
Other significant uncertainties include the future behaviour of Earth system feedbacks (such as permafrost carbon release), the exact value of Equilibrium Climate Sensitivity, and potential changes in carbon sink efficiency. These uncertainties are reflected in the probability ranges rather than the central estimates.
What the Budget Demands in Practice
The SR1.5 (IPCC Special Report on Global Warming of 1.5°C) states that to limit warming to 1.5°C with no or limited overshoot, global CO2 emissions must decline by approximately 45% from 2010 levels by 2030 and reach net zero around 2050. For 2°C, the requirement is approximately 25% reduction by 2030 and net zero CO2 by approximately 2070.
These are not arbitrary policy targets. They follow directly from the arithmetic of the carbon budget and the physics of TCRE. Meeting either target requires emissions trajectories that are fundamentally different from current trends, demanding deep transformation across energy, industry, land use, and consumption patterns.
Distributing the remaining carbon budget among countries and peoples raises profound questions of equity. Industrialised nations consumed the largest share of the historical budget. Should they therefore take deeper cuts to leave more space for developing countries to grow?
Different allocation principles yield radically different country-level budgets. Equal per capita allocation, for example, would give India a much larger remaining budget than the United States, since India's per capita emissions are far lower. Historical responsibility-based approaches would require steep cuts by industrialised nations. The IPCC does not adjudicate these questions, but it provides the scientific basis that makes the equity debate concrete and urgent.
Key Takeaways
- 1The carbon budget is the total amount of CO2 that can be emitted while limiting warming to a given temperature, derived from the proportional relationship between cumulative CO2 emissions and warming
- 2From January 2020, the remaining budget for a 50% chance of staying below 1.5°C is approximately 500 GtCO2, equivalent to roughly 12 years at 2019 emission rates
- 3Historical cumulative CO2 emissions from 1850-2019 totalled approximately 2,400 GtCO2, with about 80% of the 1.5°C total budget already consumed
- 4Limiting warming to 1.5°C requires roughly 45% reduction in global CO2 emissions by 2030 and net zero CO2 by approximately 2050
- 5Non-CO2 gas reductions can expand the available CO2 budget by up to approximately 220 GtCO2, making multi-gas mitigation strategies crucial