Tell a beach-loving tourist baking under a July sun that old Sol affects climate, and you'll probably elicit a hearty "no duh" and an invitation to describe your stunning discovery to someone else.
Yet the sun's influence on changes in climate - particularly over the past century - has been a topic of hot debate among physicists, astronomers, and more Earth-oriented climate researchers. It cuts to a key question in the policy battles over global warming: How much of the change is due to human influence and how much to natural variations in Earth's climate system?
Climate researchers have reached a consensus - based on measurements and modeling studies - that an increase in human- generated "greenhouse gas" in the atmosphere since the start of the Industrial Age has contributed to the overall warming trend during the 20th century.
Several leading researchers reaffirmed that point in the July 8 issue of Eos, a publication of the American Geophysical Union.
"There is a compelling basis for concern over future climate changes, including increases in global-mean surface temperatures, due to increased concentrations of greenhouse gases, primarily from fossil-fuel burning," they write.
But as the means have improved to measure changes in the
sun's output and space-borne features such as cosmic rays, many have come to acknowledge that these changes may play a more important role than previously thought.
Now, armed with a new sun-watching satellite, an earthbound particle accelerator, and improvements in the way computer simulations take account of the sun's effects on the atmosphere, researchers hope to test ideas about how small changes in the solar output may lead to important changes in climate patterns on Earth.
Over the years, many scientists who study stars, including the sun, have tended "to assume that the sun is dominant and the greenhouse warming is probably fictional," says Robert Cahalan, a senior research scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. On the other side, he adds, are climate scientists, many of whom have tended to treat the sun's output as a constant, deeming its minor fluctuations "irrelevant."
But, he continues, evidence is mounting that "the pro-sun people have a point" when they posit that tiny short-term changes in solar output may imprint themselves on Earth's climate in ways that are indirect but that can have a significant impact.
Getting to this point has been tough, several researchers say. Since the early 1800s, scientists and economists posited links between changes in sunspots and shifts in climate and other earthbound conditions (including the business cycle, in 1884). But on closer scrutiny, the conclusions, the math, or both, proved faulty.
Particularly in the field of sun-climate interactions, flaky results largely discredited the approach, notes Drew Shindell, a research physicist at the Goddard Institute for Space Studies in New York. Most supposed correlations "turned out to be nonsense," he says.
But as statistical techniques became more sophisticated and were more rigorously applied, "some pretty convincing bits of evidence emerged," he says.
In 1976, for example, solar physicist John Eddy examined historical records documenting a series of unusually cold winters in Europe and the virtual disappearance of sunspots between 1645 and 1715 - part of the period dubbed the Little Ice Age. For solar scientists, it's become known as the Maunder sunspot minimum. Dr. Eddy suggested a connection between sunspots and climate that appears to be gaining more credence.
"He really established the modern era" of studies of the sun-climate connection, says Judith Lean, a solar physicist with the US Naval Research Laboratory in Washington, D.C.
Researchers explain that while sunspots themselves reduce solar radiation into space, they are surrounded by bright areas whose output more than compensates for the sunspots' coolness. So the sun's output increases. Satellite measurements during the past 20 years have traced a 0.1 percent swing in the sun's total output during the course of an 11-year sunspot cycle.
But that raises the question of how those changes might tie themselves into the climate. The 0.1 percent change in visible radiation, researchers calculate, is too small to have a significant effect on global average temperatures in the lower atmosphere. But over the length of the sunspot cycle, ultraviolet radiation can change by a more respectable 10 percent. And this appears to have a profound effect on the stratosphere.
In 1999, Dr. Shindell and colleagues used modeling studies and showed that the relatively large swings in UV radiation can heat and cool the stratosphere sufficiently to change its circulation patterns.
Two years ago, they used their computer model as a time machine to test the potential connection between climate and sunspots during the Maunder minimum. They found that the decrease in solar irradiance brought global average temperatures down by 0.3 to 0.4 degrees Celsius. A big effect, however, was felt regionally, as the drop in irradiance imposed changes in circulation over the North Atlantic and Arctic. The model's temperatures for European winters fell 1 to 1.5 degrees C. below normal - which dovetails with the historical record.
Thus, during a sunspot cycle, the total amount of solar energy available to drive climate directly "doesn't change much," Shindell says. You just change how it's spread out" across the planet.
Sunspots, however, are one manifestation of broader phenomena - shifts in the sun's magnetic fields and in the streams of charged particles the sun emits, known as the solar wind - that also are attracting the attention of some researchers, who invoke low-energy cosmic rays as a factor that could affect cloud formation and hence global temperatures.
To some climate scientists, the notion that cosmic rays can play a significant role in cloud formation carries an air of desperation at trying to explain away human influences on climate. But the idea has attracted the serious attention of an international team of researchers who want to use a particle accelerator at the European Center for Nuclear Research (CERN) in Geneva to test the idea.
In 1997, Danish scientists noted a strong correlation between the sunspot cycle, the intensity of galactic cosmic-ray bombardments, and global cloudiness.
Other researchers found that the sun appears to display variations in its magnetic field and solar wind that span longer time scales. According to some researchers, during the past century, the strengths of the solar wind and the sun's magnetic field have doubled.
The cosmic-ray proposition holds that when the sun's magnetic field and the field generated by the solar wind increase, Earth is increasingly shielded from cosmic rays. These charged particles are thought to have the ability to seed cloud formation by triggering processes at the micro level that generate the nuclei around which water vapor condenses. Thus, if Earth's "shield" had been strengthening over the past century, it should have led to lower average cloud cover and warmer temperatures.
Various theories have been offered on how the mechanisms would work in detail, but they haven't been verified experimentally. Thus, the researchers have proposed the CLOUD experiment that would use a particle beam as its source of "cosmic rays" and test the idea in a cloud chamber, ordinarily used to track the path of subatomic particles.
Even as researchers try to test some of the more exotic notions related to the sun and Earth's climate, they also are beginning to draw heavily on data from a satellite launched in January that, for the first time, measures solar irradiance over a wide span of wavelengths.
Dubbed SORCE, the satellite will give researchers more detailed data on the intensity of the sun's radiation at wavelengths that are important within different layers of the atmosphere. The data will be more precise than previous satellites could provide, and, just as important, they will be collected over a long period of time.
One issue the satellite could help resolve, NASA's Dr. Cahalan notes, is whether the sun's output has been increasing since the Maunder minimum. So far satellite measurements haven't been conducted long enough to yield reliable results. Reputable scientists using the same data disagree on whether there has been a change.
Existing sets of satellite data, "just aren't good enough," says Cahalan, SORCE project scientist. "Hopefully, when we have 10 years of SORCE data, we'll be able to answer whether there's a trend or not."