The remaining carbon budget (RCB) is the net amount of CO2 that humanity can still emit while keeping global warming below a specified threshold. This is crucial when considering the pace of decarbonization needed to meet the goals of the Paris Agreement, particularly limiting warming to 1.5°C above pre-industrial levels and keeping it well below 2°C. RCB is used to assess global progress toward these objectives.
Most approaches to estimating the RCB involve allocating it among countries based on various equity principles. However, the latest estimates carried uncertainties. A new RCB analysis by researchers at Imperial College London is the most precise to date.
A Narrowing Window of Safety
In this new study, published in the journal Nature Climate Change, RCB calculations are fully updated and calibrated to match the latest IPCC report. The assessment includes improved climate modeling and better estimates of the cooling effects of aerosols on a global scale.
The calculations reveal that there are less than 250 gigatonnes of RCB left for a 50% chance of limiting warming to 1.5°C before 2030. This means that if carbon emissions continue at their current rate (around 40 gigatonnes per year), this RCB will be depleted by 2029, pushing the planet towards temperatures exceeding 1.5°C above pre-industrial levels. In other words, we have only 6 years to change course and accelerate carbon emission reductions to avoid crossing this critical threshold.
This budget is lower than previously estimated and has halved since 2020 due to the continual rise in CO2 emissions, primarily from fossil fuels. Despite decarbonization efforts, fossil fuels remain the world’s primary energy source. According to Robin Lamboll, the study’s lead author, “Our findings confirm what we already know: we are not doing enough to keep warming below 1.5°C.”
Moreover, for a 50% chance of limiting warming to 2°C, there are only 1200 gigatonnes of RCB remaining. In other words, if current emission levels are not reduced, this budget will be exhausted by 2046. Lamboll believes that the lack of progress in emissions reduction means that the window to maintain safe warming levels is closing rapidly. These results align with the latest UN climate report, suggesting a 1 in 3 chance that the RCB is as constrained as indicated by this study.
A Degree of Uncertainty
However, experts emphasize that this newly estimated RCB is so limited that minor changes in our understanding of climate processes could lead to significant revisions in these estimates. Furthermore, there is still some degree of uncertainty in these estimates, primarily related to the contributions of other greenhouse gases (GHGs) to warming. The dependence of these GHGs on socio-economic and geophysical factors hampers the accuracy of their assessment.
Some previous research has suggested that temperature increases depend not only on specific carbon emissions but also on their accumulation over time. RCB estimates must also account for different CO2 emission scenarios and various GHGs.
In recent IPCC assessment reports, RCB values have been broken down into CO2 and non-CO2 emissions. The CO2 portion is assessed analytically, incorporating data from various sources, while the non-CO2 part is evaluated through climate modeling. In light of these factors, the RCB for the 1.5°C threshold is 100 gigatonnes lower than in previous IPCC assessments.
Furthermore, it is uncertain how other components of the climate system may react to temperature increases. Increased vegetation growth in some regions, for example, could sequester large amounts of CO2 and mitigate warming, while changes in ocean currents and the melting of ice sheets could accelerate it. This underscores the importance of not only considering central estimates but also accounting for the surrounding uncertainty.