We know how the number of sunspots has varied since before the industrial revolution. Satellites have measured the total solar irradiance at the top of the atmosphere since the late 1970s. There is a strong positive correlation between the number of sunspots and the solar irradiance. The number of sunspots is therefore a good proxy when estimating the solar irradiance before the satellite era.
NOAA summarizes the connection between solar irradiance and global temperature on their web site: 'If the Sun were to intensify its energy output then, yes, it would warm our world. Indeed, sunspot data indicate there was a small increase in the amount of incoming sunlight between the late 1800s and the mid-1900s that experts estimate contributed to at most up to 0.1°C of the 1.0°C (1.8°F) of warming observed since the pre-industrial era. However, there has been no significant net change in the Sun’s energy output from the late 1970s to the present, which is when we have observed the most rapid global warming.'
The three figures in this blog post are based on monthly values that are updated up to and including December 2020.
Solar intensity and sunspots
The solar intensity varies by approximately 0.1 per cent over a solar cycle. Both the variations and the average value of the intensity differ a little from one cycle to the next. A solar cycle lasts on average for just over 11 years.
Figure 1 shows the solar irradiance and the number of sunspots since the mid-1880s. It covers a little more than 12 solar cycles. Appendixes A.1 and A.2 explain from where the Total Solar Radiance and the Sunspot numbers are downloaded.