(6 of 11)
It is this vital sunlight that provides the energy for photosynthesis, the process plants use to produce carbohydrates for sustaining their growth. And it is solar energy -- stored in ancient plants that have become today's fossil fuels -- that powers factories and runs automobiles. Sunlight also drives the earth's weather system, supplying the heat that causes atmospheric circulation and evaporates seawater to form clouds and rain. It bombards oxygen in the atmosphere, converting it into the ozone that, paradoxically, screens out much of the sun's lethal ultraviolet radiation. The ultraviolet that does reach the earth gives sun worshipers their tans and, taken in excess, their skin cancers.
Sunlight, so far, has been most reliable. For more than 3.5 billion years it has kept global temperatures within the narrow range necessary to sustain life. Even small changes in solar-energy output could have a profound influence on the planet. A long-term variation in global temperatures of only a few degrees could melt the ice caps, inundating coastal cities -- or bringing on a new ice age.
But is the solar constant a misnomer? Is the sun's energy output really constant, or does it vary with the ebb and flow of sunspots? And can such short-term changes have any significant effect on terrestrial existence? "This has always been an area where cranks and charlatans thrive," says Harry van Loon, at the National Center for Atmospheric Research (NCAR) in Boulder. "Ever since the sunspot cycle has been known, it's been correlated with every darn thing: the length of women's dresses, the number of polar bears, plague in India, even, for a time, the number of Republicans in the U.S. Senate."
Still, some of the coincidences cannot be easily dismissed. One of the most striking involves the Maunder minimum, named for the British astronomer who, after a search of old records, concluded that only a handful of sunspots had been observed during the 70-year period between 1645 and 1715. That long interval of apparently minimum solar activity happened to coincide with one of the coldest stretches of the "little ice age," the period from the mid-15th to the mid-19th centuries. During that chilly interval, many glaciers advanced farther south than they had in 10,000 years, conquistadores walked their horses over the frozen Rio Grande, and the English Channel became the site of winter skating festivals.
In a 20th century follow-up on Maunder's work, John Eddy, at the National Center for Atmospheric Research, also scoured historical records and discovered two other signs of a slackening of solar activity during that 70- year period: auroras were extremely rare, and the corona, the radiant white halo that ordinarily becomes visible during total solar eclipses, could be seen only as a dull, reddish ring of light.
To confirm evidence of the sun's strange hiatus, Eddy turned to one of nature's most reliable forms of record keeping -- tree rings, each of which represents a year of growth. In the rings is a record of cosmic rays, high- speed particles from outer space that constantly collide with molecules in the atmosphere, creating a radioactive isotope called carbon 14. When trees assimilate carbon dioxide through photosynthesis, some carbon 14 is also incorporated.