"Irradiance variations" refers to changes or fluctuations in the amount of light or radiation that is received or absorbed by a surface or object. Full definition
Krivova, N. A., Solanki, S. K., Fligge, A. & Unruh, Y. C. Reconstruction of solar irradiance variations in cycle 23: is solar surface magnetism the cause? (nature.com)
Data records of sufficient duration are expected to reveal the extent of irradiance variations over time that reflect a combination of cause and effect of climate change. (esrl.noaa.gov)
It misses the point when it comes to the earth's climate's non-linear response to solar irradiance variation for example, though. (cfact.org)