In the debate over global warming, at least one thing seems constant: the sun.

In the debate over global warming, at least one thing seems constant: the sun.

But satellite measurements show that our local star dims and brightens slightly in concert with sunspot cycles, which range from nine to 14 years. Could such tiny fluctuations be responsible for changes in climate?

Centuries of scientific study failed to find a link between the cycles, weather and climate.

But in 1991, Danish scientists reported a statistical correlation between the length of sunspot cycles and Northern Hemisphere temperatures over the past 130 years. Coupled with the fact that sunspot activity had climbed steeply between 1900 and 1960, the results led the Exxon-backed George C. Marshall Institute to argue that the sun might be to blame for global warming.

The problem is, no one has been able to figure out how minuscule changes in sunlight could raise temperatures significantly. Of the 1.2-degree increase in average global temperatures over the past century, less than 15 percent can be blamed on changes in the sun, the newest estimates say.

Some scientists have proposed elaborate mechanisms that would amplify weak solar fluctuations, but so far there’s little evidence for any of the ideas.

An intriguing possibility was raised in the early 1990s by Harvard astrophysicist Sallie Baliunas, a prominent global-warming skeptic who has received funding from the American Petroleum Institute. She found that stars similar to the sun wax and wane in cycles that last centuries. Perhaps the sun has similar ups and downs?

But later studies found Baliunas’ stars didn’t really resemble the sun after all, and other researchers couldn’t reproduce her results with larger numbers of stars.

All the efforts to blame the sun for global warming founder on one simple observation that most scientists accept as true: For the past three decades — when warming has intensified and accelerated — solar activity hasn’t increased.