We might expect solar variations of 0.2 % are possible since that is twice the present 11 - year solar variation.The 0.2 % variation gives a surface temperature variation of (aps.org)
Proxima b's star - hugging orbit may give it a surface temperature amenable for water, but also subjects it to atmosphere - eroding stellar flares and tidal forces that lock the planet's rotation, leaving the side always facing the star airless and eternally bathed in starlight while the atmosphere freezes out on the world's cold, permanently dark side. (scientificamerican.com)
Applying the adiabatic lapse rate from the effective radiating height to the surface as per the S - B Law then gives a surface temperature which is some 33C higher than it «should» be. (newclimatemodel.com)