Climate change: remaining carbon budget to meet targets of the Paris Agreement is smaller than previously thought

The remaining carbon budget represents the net quantity of CO₂ that can be emitted without overshooting a particular warming threshold. The most used estimates relate to a budget that gives a 1 in 2 chance of staying below that global temperature threshold. This is one of the key parameters for planning decarbonization pathways under the Paris Agreement, which sets the goal of limiting warming to below 2°C, aiming to limit warming to 1.5°C and minimize the worst impacts of climate change. However, despite its importance, calculation of the remaining carbon budget is associated with large levels of uncertainty, owing to the influence of various factors, including the impact of non-CO₂ gases and the ongoing effects of emissions that are not factored into existing models. 

Led by Robin Lamboll, a researcher at the Imperial College London, the study, published in Nature Climate Change, includes contributions from several IIASA researchers in the Energy, Climate, and Environment Program. Using an updated dataset and methodology, the team estimated the remaining carbon budget and characterized some of the existing uncertainty. Their calculations suggest that the remaining carbon budget, for a 50% chance of constraining warming to 1.5 °C, is 250 gigatons of CO₂ , as of January 2023. This is smaller than the number reported in the IPCC Sixth Assessment Report, not only because of the emissions that have happened since that publication, but also due to a change in the estimates of the effects of aerosols on global temperatures. The researchers suggest that at current emissions levels, the remaining carbon budget to limit warming to 1.5 °C may be exhausted within 6 years. The authors also estimate that the remaining carbon budget, for a 50% chance of keeping warming below 2 °C,  is 1,200 gigatons of CO₂, which is very close to existing estimates. They suggest that if CO₂ emissions continued at 2022 levels (around 40 gigatons of CO₂ per year), this is roughly equivalent to a remaining carbon budget of 23 years and 12 years for a 66% or 90% chance, respectively, of limiting warming to 2 °C.

Other factors, including aerosols and other greenhouse gas emissions, such as methane and nitrous oxide, contribute to the comparatively large uncertainty surrounding the remaining carbon budget for keeping temperatures below 1.5°C. This uncertainty also underscores the impact of political decisions made at national and global levels.  

“Acknowledging that decisions should be considerate of uncertainty is important. If we want to have a higher than 1 in 2 chance of limiting warming to 1.5°C or 2°C, we have even less time to get to net-zero. Science cannot provide 100% precision on the remaining carbon budget, not least because it depends in part on the policies to reduce emissions other than CO₂,” emphasizes Jarmo Kikstra, study coauthor and a researcher in the IIASA Energy, Climate, and Environment Program.

At the end of November 2023, governments will meet in Dubai for COP28 and conduct the first global stock take to monitor progress and outline further action to stay on track with regard to the targets set in the Paris Agreement.

“What is certain is that more action is needed. Even for countries with net-zero targets in the 2050s, the ambition still appears to be as fast as necessary but as slow as possible. These revised estimates raise the alarm on the urgent need for immediate, rapid, and sustained emissions reductions towards net-zero within this decade,” concludes Edward Byers, study coauthor and senior researcher in the IIASA Energy, Climate, and Environment Program.

Reference

Lamboll, R., Nicholls, Z., Smith, C., Kikstra, J., Byers, E., and Rogelj, J. (2023) Assessing the size and uncertainty of remaining carbon budgets Nature Climate Change DOI: 10.1038/s41558-023-01848-5