# Table 1 Key choices or uncertainties of terms affecting estimates of the remaining carbon budget

Term Symbol Key choices or uncertainties Type Level of understanding
Temperature limit Tlim Choice of temperature metrics used to express global warming, choice of preindustrial reference period, and consistency with global climate goals Choice Medium to high
Historical human-induced warming Thist Choice of temperature metrics used to express global warming, choice of preindustrial reference period, and consistency with global climate goals Choice Medium to high
Historical human-induced warming Thist Incomplete coverage in observational data sets, and methods to estimate human-induced component Uncertainty Medium to high
Non-CO2 contribution to future global warming $${T}_{{{\rm{nonCO}}}_{{\rm{2}}}}$$ The level of different non-CO2 emissions that are consistent with global net-zero CO2 emissions, which depends on policy choices but also on uncertain success of their implementation Choice and uncertainty Medium
Non-CO2 contribution to future global warming $${T}_{{{\rm{nonCO}}}_{{\rm{2}}}}$$ Climate response to non-CO2 forcers, particularly in the level of aerosol recovery and temperature reduction from lower methane emissions Uncertainty Low to medium
Zero-emissions commitment TZEC Sign and magnitude of zero-emission commitment at decadal time scales for current and near-zero annual CO2 emissions Uncertainty Low
Transient climate response to cumulative emissions of CO2 TCRE Distribution of TCRE uncertainty, linearity of TCRE for increasing and stabilizing cumulative CO2 emissions, and impact of temperature metrics on TCRE estimate Uncertainty Low to medium
Transient climate response to cumulative emissions of CO2 TCRE When extended beyond peak warming (Supplementary Text 1), uncertainty about linearity, value and distribution of TCRE for decreasing cumulative CO2 emissions Uncertainty Low
Unrepresented Earth system feedback mechanisms EEsfb Timescale and magnitude of permafrost thawing and methane release from wetlands and their representation in Earth system models, as well as other potential types of feedback Uncertainty Very low
1. Each of the terms in equation (1) is listed. ‘Level of understanding’ indicates our assessment of the current level of understanding of the various uncertainty components.