Solar variations and climate change

The Sun's energy drives the weather system, so many scientists naturally wondered whether they might connect climate changes with solar variations. But the Sun seemed to be stable over the timescale of human civilization. Attempts to discover cyclic variations in weather and connect them with the 11-year sunspot cycle, or other possible solar cycles ranging up to a few centuries long, gave results that were ambiguous at best. These attempts got a well-deserved bad reputation. Jack Eddy overcame this with a 1976 study that demonstrated that irregular variations in solar surface activity, a few centuries long, were connected with major climate shifts. The mechanism was uncertain, but plausible candidates began to emerge. The next crucial question was whether a rise in the Sun's activity could explain the global warming seen in the 20th century? By the 1990s, there was a tentative answer: minor solar variations could indeed have been partly responsible for some past fluctuations... but future warming from the rise in greenhouse gases far outweighed any solar effects.

The Sun dominates

The Sun so greatly dominates the skies that the first scientific speculations about different climates asked  how sunlight falls on the Earth in different places. The word climate (from Greek klimat, inclination or latitude) originally stood for a simple band of latitude. When scientists began to ponder the possibility of climate change, their thoughts naturally turned to the Sun. Early modern scientists found it plausible that the Sun could not burn forever, and speculated about a slow deterioration of the Earth's climate as the fuel ran out. In 1801 the great astronomer William Herschel introduced the idea of more transient climate connections. It was a well-known fact that some stars varied in brightness. Our Sun is itself a star, so it was natural to ask whether the Sun's brightness might vary, bringing cooler or warmer periods on Earth? As evidence of a connection between Sun and weather, Herschel pointed to periods in the 17th century, ranging from two decades to a few years, when hardly any sunspots had been observed. During those periods, he remarked, the price of wheat had been high, presumably reflecting spells of drought.

Chasing Sunspot Cycles    
     
Scientific speculation increased in the mid-19th century following the discovery that the number of spots seen on the Sun rose and fell in a regular 11-year cycle. It appeared that the sunspots reflected some kind of storminess on the Sun's surface — a violent activity that strongly affected the Earth's magnetic field. Astronomers also found that some stars, which otherwise seemed quite similar to the Sun, went through very large variations. By the end of the century, a small community of scientists was pursuing the question of how solar variability might relate to short-term weather cycles, as well as long-term climate changes. Attempts to correlate weather patterns with the sunspot cycle were stymied, however, by inaccurate and unstandardized weather data, and by a lack of good statistical techniques for analyzing the data. Besides, it was hard to say just which of many aspects of weather were worth looking into.    

Confusion persisted in the early decades of the 20th century as researchers continued to gather evidence for solar variation and climate cycles. For example, Ellsworth Huntington, drawing on work by a number of others, concluded that high sunspot numbers meant storminess and rain in some parts of the world, resulting in a cooler planet. The "present variations of climate are connected with solar changes much more closely than has hitherto been supposed," he maintained. He went on to speculate that if solar disturbances had been magnified in the past, that might explain the ice ages.
     
Meanwhile, an Arizona astronomer, Andrew Ellicott Douglass, announced a variety of remarkable correlations between the sunspot cycle and rings in trees. Douglass tracked this into past centuries by studying beams from old buildings as well as Sequoias and other long-lived trees. Noting that tree rings were thinner in dry years, he reported climate effects from solar variations, particularly in connection with the 17th-century dearth of sunspots that Herschel and others had noticed. Other scientists, however, found a good reason to doubt that tree rings could reveal anything beyond random regional variations. The value of tree rings for climate study was not solidly established until the 1960s.

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