Check out the 1.5C special report. Go to section 2.2.1.2, last paragraph says
> The reduced complexity climate models employed in this assessment do not take into account permafrost or non-CO2 Earth system feedbacks, although the MAGICC model has a permafrost module that can be enabled. Taking the current climate and Earth system feedbacks understanding together, there is a possibility that these models would underestimate the longer-term future temperature response to stringent emission pathways
https://www.ipcc.ch/sr15/chapter/chapter-2/#:~:text=Geophysi...