Global Carbon Budget

The Global Carbon Budget is a key concept in climate science that refers to the balance between the sources and sinks of carbon dioxide (CO₂) on Earth, as well as the remaining amount of CO₂ that can be emitted while limiting global warming to specific temperature targets. It provides both a scientific framework for understanding the global carbon cycle and a policy tool for setting emission limits to achieve climate goals such as those under the Paris Agreement.

Concept and Definition

The term “Global Carbon Budget” has two primary meanings:

1. It describes the accounting of carbon fluxes—how much CO₂ is emitted, how much is absorbed by the land and oceans, and how much accumulates in the atmosphere.
2. It represents the remaining allowable CO₂ emissions that humanity can emit to keep global temperature rise below a certain threshold, such as 1.5°C or 2°C above pre-industrial levels.

Both meanings are essential for assessing human influence on the climate and for shaping global mitigation strategies.

Components of the Carbon Budget

The Global Carbon Budget provides a detailed breakdown of the sources, sinks, and atmospheric changes of carbon dioxide, following a principle of mass balance:
1. Carbon Sources

• Fossil Fuels and Industry: Emissions from burning coal, oil, and natural gas, as well as cement production, are the largest contributors to atmospheric CO₂.
• Land-Use Change: Deforestation, land degradation, and biomass burning release stored carbon from vegetation and soils into the atmosphere.

2. Carbon Sinks

• Terrestrial Sink: Forests, soils, and vegetation absorb CO₂ through photosynthesis and soil storage.
• Ocean Sink: The world’s oceans absorb a significant portion of emitted CO₂, where it dissolves and forms carbonic acid, affecting ocean chemistry.

3. Atmospheric GrowthThe remaining CO₂ that is not absorbed by land or ocean sinks accumulates in the atmosphere, increasing the global concentration of greenhouse gases and driving climate change.
This relationship is summarised by the equation:

Emissions (fossil fuels + land-use change) = Atmospheric increase + Land uptake + Ocean uptake

The Remaining Carbon Budget

In climate policy, the “remaining carbon budget” refers to the total amount of CO₂ that can still be emitted while keeping global temperature rise below a given limit with a certain probability.
Key Points:

• The relationship between cumulative CO₂ emissions and global temperature increase is approximately linear.
• Staying within a defined temperature target therefore requires keeping total emissions below a calculable threshold.
• For a 50% chance of limiting warming to 1.5°C, the remaining budget is estimated to be roughly 400–500 gigatonnes (Gt) of CO₂ from 2022 onwards.
• Given that annual global CO₂ emissions are approximately 37 Gt, this budget could be exhausted within about a decade if emissions continue at current levels.

Policy and Climate Implications

The Global Carbon Budget translates scientific data into actionable climate policy by:

• Setting emission ceilings: Governments and international bodies can use the budget to plan emissions pathways compatible with climate targets.
• Highlighting urgency: Each year of continued high emissions reduces the remaining budget, tightening future options for mitigation.
• Defining net-zero goals: To stabilise global temperatures, CO₂ emissions must fall to net zero by the time the remaining budget is depleted.
• Supporting fairness and equity discussions: Allocation of the global carbon budget among nations raises questions about responsibility, capacity, and historical emissions.
• Guiding adaptation and removal technologies: If emissions exceed the budget, carbon dioxide removal (CDR) methods will be required to restore balance.

Scientific and Practical Challenges

While the carbon budget is a powerful framework, several uncertainties affect its precision:

• Non-CO₂ Forcings: Gases such as methane and nitrous oxide influence warming, making exact carbon limits difficult to determine.
• Earth System Feedbacks: Warming-induced feedbacks like permafrost thawing and forest dieback may release additional carbon, reducing the effective budget.
• Climate Sensitivity: Uncertainty in how strongly the climate responds to CO₂ changes can alter budget estimates.
• Temporal and Probability Assumptions: Budgets depend on the likelihood of meeting targets (e.g., 50%, 66%) and on time horizons used in models.
• Implementation Equity: Translating a global limit into national targets remains politically complex.

Current Global Trends

• Rising Emissions: Global fossil fuel emissions remain high and have largely returned to pre-pandemic levels.
• Diminishing Budget: At the current rate of emissions, the carbon budget for limiting warming to 1.5°C may be fully consumed within the next decade.
• Regional Contributions: Emissions are concentrated in industrialised and rapidly developing economies, though carbon sink capacities vary by region.
• Policy Momentum: The concept of the carbon budget underpins global net-zero commitments and informs climate negotiations under the UN Framework Convention on Climate Change (UNFCCC).

Importance of the Global Carbon Budget

The Global Carbon Budget serves both as a diagnostic tool for understanding the flow of carbon through Earth’s systems and as a prescriptive instrument guiding international climate action. Its importance lies in:

• Clarifying the link between emissions and temperature rise.
• Enabling transparent tracking of global progress toward emission reductions.
• Emphasising that climate stability depends on cumulative emissions, not just annual rates.
• Encouraging rapid transition to low-carbon energy systems.