The concept of a carbon budget translates abstract temperature targets into a concrete, finite constraint.
Because global temperature rise is closely linked to cumulative carbon dioxide emissions, there is only a limited amount of CO₂ humanity can emit while retaining a given probability of staying below a temperature threshold such as 2°C. Once this budget is exhausted, additional emissions commit the planet to higher long-term warming.
According to the Global Carbon Budget, the remaining carbon budget consistent with staying below 2°C is approximately 1,100 billion tonnes of CO₂.
This figure is not exact. It depends on climate sensitivity, non-CO₂ greenhouse gases, and modelling assumptions. But its order of magnitude is robust—and its implications are stark.
Source: https://betterenergy.org/blog/carbon-budget-101-what-it-is-and-why-it-matters/
Global CO₂ emissions currently exceed 35–40 billion tonnes per year. At this rate, the remaining budget will be exhausted in less than 30 years, even before accounting for uncertainty or potential feedback loops.
This timeline creates two critical realities:
Every year of continued high emissions reduces future options and increases reliance on unproven or risky interventions.
Precision matters more over time
As the remaining budget shrinks, errors of a few billion tonnes become increasingly consequential.
In early decades, uncertainty could be absorbed. In the coming decades, it cannot.
Carbon budgets are expressed in probabilities, not certainties. Staying within a 1,100-billion-tonne budget does not guarantee staying below 2°C; it offers a reasonable chance of doing so.
Exceeding the budget does the opposite: it shifts the odds toward outcomes that include overshoot, tipping points, and long-duration damage.
This probabilistic nature means that systematic bias (consistent over- or under-counting of emissions or removals) can quietly erode climate safety margins without being immediately visible.
Because emissions cannot fall to zero instantly, carbon removal is unavoidable. The carbon budget therefore has two sides:
In theory, removals extend the budget. In practice, they only do so if they are real, additional, durable, and accurately measured.
If removals are overstated due to uncertainty or methodological weakness, the budget is consumed faster than reported, and warming continues or even accelerates. This represents a fundamental structural risk in climate change mitigation.
At a remaining budget of 1,100 billion tonnes, a 20% uncertainty in major carbon sinks represents hundreds of billions of tonnes of potential mis-accounting over time.
As the budget declines, uncertainty itself becomes a driver of overshoot:
In this context, improving measurement is not a matter of optimisation. It’s a matter of necessary risk management.
Understanding the carbon budget makes one conclusion unavoidable:
if forests are to play a meaningful role in extending the remaining budget, then their contribution must be measured with far greater confidence than is currently possible.
That brings us to forest carbon and why it is both essential and uniquely challenging.