What Climate Models Miss About the Land Makes the Carbon Budget Look Bigger

A Met Office and University of Exeter team has estimated what 11 of the climate models behind global carbon budgets would do if they included six land carbon processes those models leave out. In the emulation, published Oct. 24, 2026, in Communications Earth & Environment, each unit of emitted CO2 produces 21.4% more warming in those models, on average, which the authors say implies a carbon budget 16.0% ± 0.8% smaller for the step from 1.5 °C to 2 °C.

The recalculated quantity is the Transient Climate Response to cumulative Emissions, or TCRE: the near-linear link between total CO2 emitted and global average temperature. The paper calls TCRE the cornerstone of estimates of how much carbon can still be emitted before a warming target is passed, and notes that some of the Earth system models it comes from leave out processes known to have a large effect on it.

The team ran several configurations of UKESM, a full-complexity Earth system model that already carries the six processes, and used those runs to estimate what each of those models, all of them from the sixth Coupled Model Intercomparison Project, would have given with the processes added. Among the six, the authors name the limit nitrogen supply places on photosynthesis and the two-way interaction between fire and vegetation. No model was rebuilt and nothing was newly measured, so the smaller budget is what the emulation implies rather than a revision drawn from observations.

Spencer K. Liddicoat of the Met Office and the University of Exeter is the paper's first author and Andy Wiltshire of the same two institutions its last, with colleagues at the University of Bristol and the UK Centre for Ecology and Hydrology. The journal accepted the work Sept. 19, 2026, and released this accepted version early with a permanent DOI, open access under a CC BY 4.0 license, with a transparent peer review file alongside.

The work was funded by the European Union's Horizon 2020 program, the Met Office Hadley Centre Climate Programme and the UK Natural Environment Research Council.