What forces, human or otherwise, are now warming the Earth? Debate over this highly politicized question continues to simmer. And a publication in Nature last October by solar physicist Sami K. Solanki of the Max-Planck-Institut für Sonnensystemforschung and four of his colleagues is bound to intensify the arguments. Solanki and coworkers attempted to estimate "sunspot numbers," a general barometer of solar activity, for times long before the beginning of the observational record, which starts four centuries ago. Their main result is expressed in the title of their paper: "Unusual activity of the Sun during recent decades compared to the previous 11,000 years."

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They came to this conclusion after comparing historical observations of sunspots with the amount of carbon-14 and beryllium-10 stored, respectively, in tree rings and ice cores. Carbon-14 and beryllium-10 are "cosmogenic nuclides," created when cosmic rays (energetic particles arriving from outside the solar system) strike Earth's atmosphere. The solar magnetic field partially shields the Earth from cosmic rays—more so when the Sun is highly active; less so during quiet periods. Thus, after correction for various confounding factors (including the additional shielding that the Earth's magnetic field provides and the ongoing radioactive decay of carbon-14), the tree-ring and ice-core archives of these cosmogenic nuclides can serve to document solar activity.

Solanki's recent work suggests that sunspot numbers (averaged over the 11-year cycle in solar activity) have mostly remained below 50, often far below. In the 15th century, for example, average sunspot numbers were typically well under 20. But as the observational record clearly shows, average sunspot numbers have been increasing in fits and starts since, rising sharply over the 20th century to the modern level of about 75.

Judith Lean, a solar-terrestrial physicist at the Naval Research Laboratory, says that "this is startling news—I'm just not sure that it's right." Solanki fully accepts that his 11,000-year record might yet require refinement: "I'm too much of a scientist to make the claim, 'Yup, this is it.'"

Why are sunspot numbers such a hot topic? Intuition suggests that lots of dark spots on the face of the Sun should diminish solar irradiance, but as Solanki explains, "the Sun is actually a little bit brighter when there are many dark spots." The reason is that bright areas called "faculae" tend to accompany sunspots. Although not easily discerned in telescopic views of the Sun, these features are brilliant enough that the overall effect of the combined spots and faculae is to boost irradiance levels. The question is by how much.

Direct satellite measurements of changes in the Sun's output (which have only been available since 1978) show that solar irradiance indeed moves in step with sunspot numbers, but the effect is quite small—about a tenth of a percent. "If you plug that into a climate model, you get zip," explains Peter Foukal, an investigator who runs Heliophysics, a research company in Nahant, Massachusetts. Yet it appears that changes in the Sun have influenced climate. Gerard Bond, a geologist at Columbia University's Lamont-Doherty Earth Observatory, and nine coworkers demonstrated this long-debated link particularly well in 2001 when they compared dramatic swings in the climatological state of the North Atlantic over the past 12 thousand years with matching swings in the activity of the Sun. Presumably, these ancient changes in solar irradiance were on par with the 0.1-percent variation observed over the last two of the Sun's 11-year cycles. How such a tiny change can affect climate is puzzling. "One needs an amplifier," says Solanki's coauthor Ilya G. Usoskin, a physicist at a the University of Oulu in Finland.

Two amplifying mechanisms are being actively considered. The first invokes changes in the stratosphere. These changes include variation in the high-level winds that circle the Arctic in winter, which are observed to speed up and slow down in pace with the 11-year solar cycle. Solar ultraviolet radiation is thought to be the culprit. Although UV constitutes only a small fraction of the Sun's total energy, this portion of the solar spectrum varies much more than does total irradiance. So there is a significance change in the heating that goes on in the stratosphere when high-level ozone absorbs the Sun's UV light. Also, increases in the UV output of the Sun tend to create more stratospheric ozone than normal, further amplifying the warming that goes on there. The curious thing is that certain climate models indicate that this rather subtle high-altitude effect might be felt near the surface, by influencing the way the underlying atmosphere distributes its own heat. Drew Shindell, of NASA's Goddard Institute for Space Studies, Lean and three other colleagues demonstrated this connection with the surface through modeling studies in 1999.

The second possible amplifier involves cosmic rays. The argument goes like this: When cosmic rays hit the atmosphere, they not only form the cosmogenic nuclides, they also form ions, which give rise (through complicated and as yet poorly known ways) to cloud-condensation nuclei. That is, cosmic rays may spur the growth of clouds. An active Sun partially shields the Earth from the normal barrage of cosmic rays, leading, presumably, to fewer clouds. Clear skies in turn let in more light and heat the surface more intensely than normal.

Although the physics and chemistry of the cosmic ray-cloud link are murky, there is evidence that cloudiness varies in synch with the 11-year solar cycle, at least in some places. Usoskin, for example, along with four colleagues presented evidence last August in the journal Geophysical Research Letters that variations in cosmic-ray induced ionization affect cloudiness in the lower atmosphere at middle latitudes. Michael E. Schlesinger, a climate modeler at the University of Illinois at Urbana-Champaign, says he's "inclined to think that this influence on clouds is chimerical," but he admits "that doesn't mean it's not possible." Solanki isn't so dismissive, pointing out that "the mechanism is just too beautiful to ignore."

Further work may help determine whether cosmic rays indeed affect cloudiness and, if not, how it is that tiny changes in the Sun can manage to alter Earth's climate. But perhaps more pressing than the question of how is the question of how much. Solanki and colleague Natalie A. Krivova attempted to answer that question in 2003, when they published a paper in the Journal of Geophysical Research titled "Can solar variability explain global warming since 1970?" Their reckoning revealed that the Sun is responsible for less than 30 percent of the rise in surface temperature experienced since that time. Although other investigators have made similar estimates, the jury is still out, and gauging the role of the Sun in global climate change remains a key area of research.

One reason to want to know exactly what the Sun contributes is obvious from Solanki's long history of sunspot numbers: If the Sun continues to follow the pattern seen in that record, it should move out of its high-activity state within the next decade or two. This change might thus help to combat some of the forces  that have lately been pushing climate toward a hotter world.

Investigations of the Sun's influence on climate are relevant for other reasons as well. For one, this  work should help to sort out what portion of recent climate change is natural. In addition, it will improve the ability of climate modelers to assess Earth's sensitivity to increasing levels of carbon dioxide. (Right now it's anyone's guess whether a doubling of atmospheric carbon dioxide could reasonably be expected to heat the Earth by 2 degrees or by 5.) As Schlesinger aptly notes, "It's important to sort this out."