Role of Earth system processes in carbon emissions budgets

Estimates of carbon emissions budgets to limit global warming to 1.5°C or 2°C rely on the near-linear relationship between global temperature change and total CO ₂ emitted, the Transient Climate Response to cumulative Emissions (TCRE). Here we use UKESM, a global Earth System Model (ESM), to quantify the impact on TCRE of six processes in isolation: fire-vegetation interactions (TCRE increased by 14.6%); nitrogen limitation of vegetation (+9.7%); diffuse radiation effects on vegetation (+8.5%); changes in vegetation distribution (-1.5%); climate impacts from wetland methane emissions (+5.1%) and from biogenic volatile organic compounds (-1.4%). Emulating their influence on TCRE of 11 ESMs increased the average TCRE by 23.7%, reducing by 18.4% the CO ₂ that can be emitted, starting from 1.5°C, if 2°C is not to be breached. As these processes become more prevalent in models, estimates of remaining carbon budgets will likely reduce: we may be closer to 1.5°C or 2°C than previous model-based analyses suggest.