Missing land processes shrink carbon budgets in CMIP6 models

Some of the world’s leading climate models may be overestimating how much carbon dioxide humanity can emit while staying within key warming limits, according to new research published in Communications Earth & Environment.

The research centres on the Transient Climate Response to cumulative Emissions, or TCRE, a cornerstone metric in climate science. TCRE describes the near-linear relationship between the increase in global mean temperature and cumulative emissions of CO2, and it underpins estimates of carbon emissions budgets to future warming targets. Because TCRE is derived from sophisticated Earth System Models, any processes those models omit can bias the resulting budget estimates.

The team used multiple configurations of UKESM, a full-complexity Earth System Model developed in the United Kingdom, to estimate how the TCRE of 11 models that participated in the sixth Coupled Model Intercomparison Project (CMIP6) would change if each included six terrestrial carbon cycle related processes that are present in UKESM. Among the processes considered are nitrogen limitation of photosynthesis and fire-vegetation interactions.

The findings are striking: accounting for the processes not included in the models increased their TCRE by an average of 21.4 percent. A higher TCRE means that each tonne of emitted carbon dioxide produces more warming, leaving less room in the budget compatible with a given temperature target. Translated into emissions terms, the authors report a reduction of 16.0 percent, with an uncertainty of plus or minus 0.8 percent, in the carbon budget for the transition from 1.5 degrees to 2 degrees of global warming.

The study highlights a practical challenge for the climate modelling community: several Earth System Models that contribute to international assessments lack processes known to have a large impact on TCRE. By emulating how the missing processes would behave if they were included, the researchers offer a way to quantify the effect without rerunning every model from scratch. The work, published open access on 24 October 2026, suggests that carbon budgets estimated from models lacking these land processes should be interpreted with caution, since incorporating the missing mechanisms points to tighter remaining budgets than current multi-model estimates imply.

Subject of Research: How emulating missing terrestrial carbon cycle processes in CMIP6 Earth System Models reduces estimated carbon budgets for warming targets

Article Title: Emulating absent land processes reduces carbon budgets in CMIP6 models

Article References: Liddicoat, S. K., Jones, C. D., Mercado, L. M., Robertson, E., Sitch, S., & Wiltshire, A. (2026). Emulating absent land processes reduces carbon budgets in CMIP6 models. Communications Earth & Environment. https://doi.org/10.1038/s43247-026-04111-9

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04111-9

Keywords: carbon budget, TCRE, CMIP6, Earth System Models, UKESM, nitrogen limitation, fire-vegetation interactions, carbon cycle, climate modelling, warming targets, Emulating, absent