The sun's energy output varies slightly as sunspots wax and wane on the star's surface.
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," Foukal said.
The researchers obtained accurate daily sunspot measurements dating as far back as 1874 from institutions such as the Mount Wilson Observatory near Pasadena, California, and the Royal Observatory in Greenwich, England.
Older records exist all the way back to when the telescope was invented in the 17th century, though the data become increasingly patchy with age (see a related graphic of how the Hubble Space Telescope works).
The team also derived the sun's historic strength by looking at the presence or absence of isotopes, such as beryllium 10, in ice samples from Greenland and Antarctic that reflect the past contents of Earth's atmosphere.
(Related news: "Climate-Change Forecast? Ask the Antarctic Ice" [November 2004].)
Such isotopes are formed when cosmic radiation penetrates the atmosphere.
In periods of high activity, a brighter sun emits more magnetic and plasmatic particles that shield Earth from the galaxy's rays, resulting in fewer isotopes.
Measuring the historical record of such isotopes from ice yields useful, though debatable, estimates of the sun's past power on Earth.
"If you see that these isotopes were low for 50 or 100 years, it's a darn good bet that the sun was more active then," Foukal said.
Sun Not Off the Hook for Warming
The authors and other experts are quick to point out that more complicated solar mechanisms could possibly be driving climate change in ways we don't yet understand.
Climate change carries such high stakes that even more unlikely possibilities may capture scientific attention.
"There are numerous studies that find a correlation [between solar variation and Earth climate]," said Sami Solanki of the Max Planck Institute for Solar System Research in Lindau, Germany.
"These authors have looked at the simplest mechanism, and they find that this mechanism does not produce the same level of change that has been observed," he continued.
"This could be suggesting that there are other mechanisms acting for the way that the sun influences climate."
Solar ultraviolet (UV) rays are one possibility, though that theory creates its own challenges.
"UV is only a small fraction of total solar output, so you'd need a strong amplification mechanism in the Earth's atmosphere," study co-author Spruit said.
Magnetized plasma flares known as solar wind could also impact Earth's climate. Solar wind influences galactic rays and may in turn affect atmospheric phenomena on Earth, such as cloud cover.
Such complex interactions are poorly understood but could be crucial to unlocking Earth's climatic puzzle.
"I think the main question," the Max Planck Institute's Solanki said, "is, How does the sun [in general] act on climate? What are the processes that are going on in the Earth's atmosphere?"
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