It is wild to me that in 2025 the question of how much does solar output change during a solar cycle still comes with a ~20% measurement uncertainty.
Jan 22, 2026 10:31I understand why it is hard.
Total solar irradiance is ~1361 W/m², so measuring a ~1 W/m² change is less than a 0.1% change.
And it can only be done with satellites, which often degrade and get replaced, requiring the synthesis of many (sometimes inconsistent) measurements.
Now, forcings due to additional greenhouse gases are clearly larger than the observed solar changes.
But total solar irradiance remains a pretty important boundary condition for all climate and climate change work.
One would think that we would have solar variation nailed down better by now.
