Global Carbon Budget: Advances and Remaining Challenges

The Global Carbon Project published its 18^th edition of the annual update of the Global Carbon Budget in December 2023. The goals of this effort are (1) to provide the most up-to-date trend analyses and flux data (to the current year of publication) of all major natural and anthropogenic fluxes contributing to the global carbon budget, and, therefore, to the net atmospheric carbon balance; (2) to bring together the broader research community to encourage and guide the further deployment of observations and modeling development to address the biggest uncertainties of the budget and its drivers of change; and (3) to liaise and reach out to national (e.g., governments) and international (e.g, UNFCCC, SBSTA, WMO, NGOs) policy bodies to provide data, trends, and scientific advice.

The presentation will cover:

1. Highlights of the latest carbon budget, 2023, with preliminary 2024 data.
2. Recent improvements in the science and modelling contributing to the latest carbon budgets, including
   1. New insights from oxygen on land/ocean partitioning.
   2. Earth system models now provide current year prediction of land and ocean sinks
   3. Extended discussion on fire emissions
   4. Further partition of gross fluxes of the net land use change flux, including estimates of deforestation and re/afforestation.
   5. Comparison between the GCP GHG budgets and the National Greenhouse Gas Inventories.
   6. CO₂ and climate attribution of the evolving CO₂ land and ocean sinks.
3. Major remaining uncertainties that require attention, including
   1. LUC emissions continue to have large uncertainty, with a highly uncertain long-term decline now offsetting the increase in fossil emissions
   2. Increasing divergence among different ocean estimates (observation-based estimates suggest higher sink than of Global Ocean Biogeochemical Models)
   3. Limited agreement on the land sink distribution between DGVMs and atmospheric inversions.
   4. Oxygen data suggest stronger ocean sink, weaker land sink.