The stronger the reductions in non-CO2 emissions the lower the 
resulting temperatures are for a given RCB or the larger RCB for the 
same level of temperature change. For instance, the RCB for limiting 
warming to 1.5°C with a 50% likelihood could vary between 300 to 
600 GtCO2 depending on non-CO2 warming129. Limiting warming to 2°C 
with a 67% (or 83%) likelihood would imply a RCB of 1150 (900) GtCO2 
from the beginning of 2020. To stay below 2°C with a 50% likelihood, 
the RCB is higher, i.e., 1350 GtCO2130. {WGI SPM D.1.2, WGI Table SPM.2; 
WGIII Box SPM.1, WGIII Box 3.4; SR1.5 SPM C.1.3}
126 See Annex I: Glossary. 
127 This likelihood is based on the uncertainty in transient climate response to cumulative net CO2 emissions and additional Earth system feedbacks and provides the probability that 
global warming will not exceed the temperature levels speciﬁed. {WGI Table SPM.1}
128 Global databases make different choices about which emissions and removals occurring on land are considered anthropogenic. Most countries report their anthropogenic 
land CO2 ﬂuxes including ﬂuxes due to human-caused environmental change (e.g., CO2 fertilisation) on ‘managed’ land in their National GHG inventories. Using emissions 
estimates based on these inventories, the remaining carbon budgets must be correspondingly reduced. {WGIII SPM Footnote 9, WGIII TS.3, WGIII Cross-Chapter Box 6}
129 The central case RCB assumes future non-CO2 warming (the net additional contribution of aerosols and non-CO2 GHG) of around 0.1°C above 2010–2019 in line with stringent 
mitigation scenarios. If additional non-CO2 warming is higher, the RCB for limiting warming to 1.5°C with a 50% likelihood shrinks to around 300 GtCO2.