Variable Solar Irradiance

Daily Tech

A study from NASA’s Goddard Space Flight Center in Greenbelt, Maryland looking at climate data over the past century has concluded that solar variation has made a significant impact on the Earth's climate. The report concludes that evidence for climate changes based on solar radiation can be traced back as far as the Industrial Revolution.

Science Daily
Total solar irradiance is variable over many different timescales, ranging from seconds to centuries due to changes in solar activity.
The solar irradiance measurement is much higher during solar maximum, when sunspot cycle and solar activity is high, versus solar minimum, when the sun is quiet and there are usually no sunspots.