sunspots. Over the last 267 years, 23 of these have been observed with an average length of 11 years. A period of time when there are few sunspots is called a solar minimum. The longest one was the Maunder Minimum, and it corresponded with the Little Ice Age (1650 to 1850).
Sunspot cycles are important because of their link to climate change on the earth. When there are few sunspots, the earth tends to get cooler, and vice versa. This may be the main driver of global warming and cooling.
- ...there is some evidence that a lack of sunspots can cause moderate cooling because of the way the sun interacts with Earth's outer atmosphere, said Alexei Pevtsov, staff astronomer at the National Solar Observatory in Sunspot, N.M.
- "Lower-than normal temperatures tend to occur in years when the sunspot cycle is longest." 
- The cause-and-effect relationship of solar cycle length and the Northern Hemisphere air surface temperature - Richard Reichel, Peter Thejll and Knud Lassen 
- "Our analyses show statistically significant relationships between the double sunspot cycle and the “January thaw” phenomenon along the East Coast and between the double sunspot cycle and “drought” (June temperature and precipitation) in the Midwest." [Journal of the Atmospheric Sciences (1978)]
- "The number of sunspots reaches a maximum about every 11 years ..." Davison E. Soper, Institute of Theoretical Science, University of Oregon
- "That period also correlates with the Little Ice Age, which started in 1650 and ended in the mid-1800s." Low sunspot cycle fascinates scientists, who say there's no reason to panic - Santa Fe New Mexican
- Are sunspots to blame for climate change? - Paul MacRae, May 31, 2008