Same sun. Different views.
Long considered a constant, the sun is under new scrutiny as scientists discover that small changes in solar output may lead to significant changes in Earth's climate patterns.
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.Skip to next paragraph
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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.