The face of the sun is now breaking out in more magnetic storms, known as sunspots, than at any other time in the previous millennium, a team of physicists reports. If true, the unusual solar activity might account for some of the recent warming of Earth's climate. But some researchers say the evidence for increased activity is itself spotty.
Spots are clearly more numerous in recent decades than at any other time since Galileo first trained a telescope on the sun in 1610. To gauge earlier activity, researchers study rare isotopes whose concentration varies with the magnetic activity on the sun. For example, beryllium-10 is produced when cosmic rays crash into Earth's atmosphere, and it accumulates in annual layers of ice near the poles. The sun's magnetic field deflects cosmic rays away from Earth, so fewer spots should mean less beryllium-10 and vice versa.
Detailed models now provide a precise calculation of sunspot history, Ilya Usoskin of the University of Oulu, Finland, Sami Solanki of the Max Planck Institute for Aeronomy in Katlenburg-Lindau, Germany, and colleagues report. They strung together three models--one that relates the number of sunspots to the strength of the sun's magnetic field, another that shows how field strength determines the number of cosmic rays, and a third that accounts for how the rays produce beryllium-10. Plugging in beryllium-10 concentrations from Greenland ice dating back to 1424 and Antarctic ice dating back to 850, the researchers calculated past sunspot activity, as they report in a paper to be published in Physical Review Letters. Since the 1940s, the researchers say, the sun has shown nearly twice as many spots as it did during any equal time period in the preceding millennium.
Some scientists argue that magnetic activity on the sun affects Earth's climate. If so, the results suggest that solar activity may have contributed to rising temperatures during the 20th century. However, Usoskin stresses that such "solar forcing" alone probably cannot account for all the warming. Humanmade pollution likely drives temperatures up, too, he says.
The results tighten the link between beryllium-10 and solar magnetic fluctuations, says Jürg Beer of the Swiss Federal Institute for Environmental Science and Technology in Dübendorf. "Everybody agrees that we seem to be in a very active phase" of the sun, Beer says. Well, not quite everybody. Grant Raisbeck of the Center for Nuclear Spectroscopy and Mass Spectroscopy in Orsay, France, points out that the new work considers data from Antarctic ice formed only before 1900. Since then, he says, Antarctic ice shows no drop in beryllium-10, which contradicts the data from Greenland ice. The discrepancy calls the method into question, Raisbeck says, and undermines the conclusion that the sun is currently cranking out spots at an extraordinary rate.