Wolfgang adds: “Total solar irradiance varies along with the 11-year cycle of solar activity, and one of the most obvious ways to look for long-term energy drift is to compare total solar irradiance between consecutive solar minimia.
“This requires a long time-series of data, ideally coming from multiple instruments because single radiometers will undergo degradation in sensitivity from the hard ultraviolet in the Sun’s rays they are continuously exposed to. That said, any degradation is very gradual: the radiometer aboard the ESA-NASA SOHO solar observatory for instance, which was launched in back 1995, is still working satisfactorily.”