Most scientists have long thought that the sun shone too steadily to affect conditions on Earth. If climate lurched willy-nilly from one extreme to another over the millennia, they reckoned, sunshine was not to blame. Now, a new study suggests that cycles in climate actually move in synch with the waxing and waning of the sun.
The new sun-climate correlation rests on a combination of highly detailed records of both changing climate and solar activity. Paleoceanographer Gerard Bond of the Lamont-Doherty Earth Observatory in Palisades, New York, and his colleagues found that rocky debris deposited in the North Atlantic by melting ice had jumped in abundance roughly every 1500 years as the ice surged farther out into a temporarily colder Atlantic. To trace ancient solar activity, Bond and colleagues consulted measurements of carbon-14 in tree rings and beryllium-10 in cores of Greenland ice. Both isotopes are the products of cosmic rays striking the upper atmosphere; when the sun is more active, it magnetically fends off cosmic rays and decreases isotope production.
The test for a sun-climate connection came when the scientists compared the two types of records. As the researchers report online this week in Science, they found a close match between the peaks and troughs of the climate record and those of the solar record. The climate of the northern North Atlantic has warmed and cooled nine times in the past 12,000 years in step with the waxing and waning of the sun. Their conclusion: The sun is driving millennial climate change.
"The Bond et al. data are sufficiently convincing that [solar variability] is now the leading hypothesis" to explain the oscillation of climate seen since the last ice age, including the Little Ice Age of the 17th century, says glaciologist Richard Alley of Pennsylvania State University, University Park. The sun could also add to the greenhouse warming of the next few centuries. Still, he cautions that much is left to sort out. "It remains a little hard to figure out exactly how the sun has mattered to [recent] climate," Alley says, "and why it has mattered so much."