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Section 3
Section 1
Section 3
3.3 Mitigation Pathways
Limiting human-caused global warming requires net zero anthropogenic CO2 emissions. Pathways consistent 
with 1.5°C and 2°C carbon budgets imply rapid, deep, and in most cases immediate GHG emission reductions in 
all sectors (high conﬁdence). Exceeding a warming level and returning (i.e. overshoot) implies increased risks 
and potential irreversible impacts; achieving and sustaining global net negative CO2 emissions would reduce 
warming (high conﬁdence).
3.3.1 Remaining Carbon Budgets
Limiting global temperature increase to a speciﬁc level requires 
limiting cumulative net CO2 emissions to within a ﬁnite carbon 
budget126, along with strong reductions in other GHGs. For every 
1000 GtCO2 emitted by human activity, global mean temperature rises 
by likely 0.27°C to 0.63°C (best estimate of 0.45°C). This relationship 
implies that there is a ﬁnite carbon budget that cannot be exceeded in 
order to limit warming to any given level. {WGI SPM D.1, WGI SPM D.1.1; 
SR1.5 SPM C.1.3} (Figure 3.5)
The best estimates of the remaining carbon budget (RCB) from 
the beginning of 2020 for limiting warming to 1.5°C with a 50% 
likelihood127 is estimated to be 500 GtCO2; for 2°C (67% likelihood) 
this is 1150 GtCO2.128 Remaining carbon budgets have been quantiﬁed 
based on the assessed value of TCRE and its uncertainty, estimates of 
historical warming, climate system feedbacks such as emissions from 
thawing permafrost, and the global surface temperature change after 
global anthropogenic CO2 emissions reach net zero, as well as variations 
in projected warming from non-CO2 emissions due in part to mitigation 
action. 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.