National Geographic : 2004 Jul
Solar storms can overload power lines, causing (Continuedfrom page 21) They contained sig nificantly more carbon 14 than trees before and after the period. That meant that higher amounts of cosmic radiation had been reaching Earth during that time. (A magnetically active sun reduces the cosmic radiation we receive.) So, Eddy concluded, there might be a connection after all. Eddy's investigation also drew attention to another sunspot dearth from 1460 to 1550. Put ting that episode next to the Maunder dates, scientists realized that these extended minimums coincided with the core of a famously frigid period in Europe and elsewhere known as the Little Ice Age (1400-1850), during which the Thames River in London and the Lagoon of Ven ice regularly froze. It might seem as if fewer sunspots should mean a brighter sun. But the sun's luminosity is actually greater when there are more sunspots, because their magnetism creates extra-bright areas called faculae (page 16). Sunspot activity has indeed been high over the past century as Earth's temperatures have climbed. But according to a recent NASA report, greater luminosity seems to account for only half of the global temperature increase before 28 NATIONAL GEOGRAPHIC * JULY 2004 1940, and less than that in later years as green house gases have continued to rise. Swings in solar activity are only part of the puzzle. Moreover, our knowledge of those swings is limited. Our best helioseismological studies and high-tech spacecraft observations only cover about 15 years. And as Joel B. Mozer, senior phys icist at the Air Force Research Laboratory at Sac ramento Peak, New Mexico, points out, "Since the beginning of the space age in the 1950s, we've had only four solar cycles. All our understanding is based on that. But there's plenty of evidence that these don't represent the extremes." From computer simulations, scientists have a fair idea of how sunspots might arise and dis sipate. But there are still too few highly detailed observations to compare with theory. "The hope is that helioseismology will even tually give us better magnetic field observations at crucial depths," says Spiro Antiochos of the Naval Research Laboratory in Washington, D.C., who models the physics of solar outbursts. "Now we have to infer from the surface what's going on below. Even the simple question of the struc ture of the magnetic field under a sunspot-we just don't know."