“Don’t Blame Sun for Global Warming, Study Says”. National Geographic. September 13, 2006 – “But sunspot-driven changes to the sun’s power are simply too small to account for the climatic changes observed in historical data from the 17th century to the present, research suggests. The difference in brightness between the high point of a sunspot cycle and its low point is less than 0.1 percent of the sun’s total output. “If you run that back in time to the 17th century using sunspot records, you’ll find that this amplitude variance is negligible for climate”‘…according to Solar astronomer Peter Foukal of Heliophysics, Inc.
In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles lead to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.
Foukal, Peter; et al. (2006-09-14). “Variations in solar luminosity and their effect on the Earth’s climate.”. Nature. Retrieved on 2007-04-16.