WGI primarily used the term 
scenarios and WGIII mostly used the term modelled emissions and mitigation pathways. The SYR primarily uses scenarios when referring to WGI and modelled emissions and 
mitigation pathways when referring to WGIII. {WGI Box SPM.1; WGIII footnote 44}
103 Around half of all modelled global emissions pathways assume cost-effective approaches that rely on least-cost mitigation/abatement options globally. The other half look 
at existing policies and regionally and sectorally differentiated actions.  The underlying population assumptions range from 8.5 to 9.7 billion in 2050 and 7.4 to 10.9 billion 
in 2100 (5–95th percentile) starting from 7.6 billion in 2019. The underlying assumptions on global GDP growth range from 2.5 to 3.5% per year in the 2019–2050 period 
and 1.3 to 2.1% per year in the 2050–2100 (5–95th percentile). {WGIII Box SPM.1}
104 High mitigation challenges, for example, due to assumptions of slow technological change, high levels of global population growth, and high fragmentation as in the Shared 
Socio-economic Pathway SSP3, may render modelled pathways that limit warming to 2°C (> 67%) or lower infeasible (medium conﬁdence). {WGIII SPM C.1.4; SRCCL Box SPM.1}
105 SSP-based scenarios are referred to as SSPx-y, where ‘SSPx’ refers to the Shared Socio-economic Pathway describing the socioeconomic trends underlying the scenarios, and 
‘y’ refers to the level of radiative forcing (in watts per square metre, or Wm–2) resulting from the scenario in the year 2100. {WGI SPM footnote 22}
106 Very high emission scenarios have become less likely but cannot be ruled out. Temperature levels > 4°C may result from very high emission scenarios, but can also occur from 
lower emission scenarios if climate sensitivity or carbon cycle feedbacks are higher than the best estimate.