There are three main sources of uncertainty in climate projections: emission scenarios, regional climate responses and internal climate variability . Emission scenario uncertainty is captured in Representative Concentration Pathways  for greenhouse gases and aerosols. RCP2.6 represents low emissions, RCP4.5 medium emissions and RCP8.5 high emissions. Regional climate response uncertainty and internal climate variability uncertainty are captured in climate model simulations driven by the RCPs.
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report