Our Sun is a variable star. Its most notable variables are brightness, or irradiance; eruptive activity, including solar flares and geomagnetic storms; and cosmic rays. These variables manifest in cycles of 11, 22, 88, 106, 213, 429, and more years.
At a cycle's low point, or minimum, both solar irradiance and eruptive activity tend to decrease, leading to a decrease in solar warming. Changes in visible light brightness are small but measurable (about 0.1 percent during an 11-year cycle), while decreases in ultraviolet, or UV, radiation are greater (6 to 8 percent during the same period). We know that when UV rays combine with oxygen to produce ozone, heat is generated. Fewer UV rays means less ozone—and thus less heat—is produced.
When eruptive activity on the Sun decreases, solar winds weaken; fewer solar flares occur; and fewer geomagnetic storms reach Earth's atmosphere. A decrease in solar activity leads to an increase in cosmic rays reaching Earth's lower atmosphere.
Cosmic rays increase low cloudiness, which reflects solar radiation. Thus, an increase in cosmic rays leads to cooling. (Conversely, fewer cosmic rays result in fewer low clouds, more sunshine, and warmer temperatures.)
The direct changes in solar brightness, combined with the indirect cycles of UV and cosmic rays, help to bring about temperature oscillations, or cycles, on Earth. Studies have shown that long-term cycles of solar activity correlate with weather station-based annual mean temperatures very well (see graph).