Our sun is a small star with remarkably stable energy output. While many stars pulse dramatically in size and brightness, our sun’s luminosity varies only about 0.1% over the course of its 11-year solar cycle (Figure 1). However, even this small change in incoming radiation exceeds all the other energy sources (such as radioactivity from the earth’s core) so it is important to consider. 
Figure 1. Strip chart showing Total Solar Irradiance (TSI) (top) from several sources, and the number of sunspots (lower) since 1975. (Source: University of Colorado)
The changes in extreme ultraviolet (EUV) radiation can be particularly important, varying by a factor of 10 or more.  EUV can affect the Earth’s upper atmosphere during solar maxima by creating more nitrogen oxides (NOx) which in turn can reduce the ozone levels by a few percent, resulting in more UV radiation reaching the surface. The effect of the UV increase on our weather and climate is complicated, but appears to have an effect on certain regions, such as the Pacific Ocean Basin, more than others. In the Pacific, a La Nina-like pattern seems to occur at the solar maxima, with cooler SSTs in the East Pacific. Solar cycle forcing seems to affect the general circulation more than having a direct temperature signal, with an increase in precipitation the most likely outcome.
One major connection however, was the so-called “Maunder Minimum” of the late 1600’s and early 1700’s (Figure 2), which was the coldest part of the “Little Ice Age.”  This period was characterized by bitterly cold winters in Europe and North America, although there wasn’t actually any glaciation. The solar cycles during this period were extremely weak based on the numbers of sunspots observed (Figure 2). The number of sunspots has risen steadily since the late 1600’s which many scientists believe has contributed to as much as half of the 0.6°C temperature increase since 1900.  However, there is still much uncertainty in these estimates.
Figure 2. Variation in sunspot frequency since the year 1600. The “Maunder Minimum” was accompanied by very cold winters in the Northern Hemisphere. (Source: NASA Marshall Space Flight Center)
It is possible that we are now entering a mini-version of the Maunder Minimum, as solar activity is the weakest it has been in more than 50 years, and is predicted to reach a pronounced minimum by 2020 (Figure 3). Other models suggest a minimum somewhat later, in the 2030’s. Research also seems to suggest that there is an inverse correlation between the length of sunspot cycles and the magnitude of the Earth‘s warming or cooling during the next sunspot cycle (i.e., longer cycles = cooler average temperatures) (Figure 4).  Based on this research, a drop in average temperature during current solar cycle number 24 (ending 2020 or later) of about 1˚ C is possible for parts of the globe. Obviously, this has not happened yet, so time will tell if this prediction will come true.
Figure 3. Recent frequency of sunspots since 1995, along with NASA prediction out to 2020. (NASA MSFC)
Figure 4. Length of sunspot cycle (years) versus global temperature change (C) since 1860. The length axis at right is inverted.
In summary, although the Sun’s energy output has been reliably steady, there is historical evidence for relatively cold periods caused by reduced solar luminosity. The frequency of sunspots and the lengths of sunspot cycles seem to have some relationship to global temperatures. An increase in the number of sunspots may have contributed to a portion of the observed global temperature increase over the past 150 years. Finally, forecasts of solar activity suggest that some cooling may occur in the next two decades as a result.
- Philips, T. 2013: Solar Variability and Terrestrial Climate. NASA Science News, Jan. 8, 2013, available at: http://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate/
- Hathaway, D. 2016: The Sunspot Cycle, available at: http://solarscience.msfc.nasa.gov/SunspotCycle.shtml
- Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report Climate Change, 2007: Chapter 6. Radiative Forcing of Climate Change
- Solheim, J-E, K. Stordahl, and O. Humlum, 2012: The long sunspot cycle 23 predicts a significant temperature decrease in cycle 24. Journal of Atmospheric and Solar-Terrestrial Physics. Vol. 80, pages 267-284.