Climate change and solar variations

The sun's energy drives our weather systems, so many scientists naturally wondered whether they might connect climate changes with solar variations. But the sun seemed to be relatively 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 confusing at best. These attempts got a well-deserved bad reputation. Jack Eddy overcame this with a 1976 study that showed that irregular variations in solar surface activity, a few centuries long, were associated with major climate changes. The mechanism was uncertain, but likely 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 response: minor solar variations could indeed have been somewhat responsible for some past fluctuations... but future warming from the rise in greenhouse gases far outweigh any solar effects.

The Sun will always dominate

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 climate, inclination or latitude) initially stood for a simple band of latitude. When scientists began to ponder the possibility of climate change, their investigations naturally focused on the Sun. Early 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 was depleted. In 1801 the great astronomer William Herschel introduced the notion 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 very few sunspots had been observed. During those periods, the price of wheat had been high, presumably reflecting periods of drought.

Pursuing 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 period. It appeared that the sunspots reflected some kind of storminess on the sun's surface — the violent activity that strongly affected the earth's magnetic field. Astronomers also found that some stars, which seemed quite similar to the sun, went through very large changes. 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 patterns, 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 accurate statistical techniques for analyzing the data. Besides, it was hard to say just which of many aspects of the weather were worth investigating. Confusion persisted in the early decades of the 20th century as scientists 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 volatile weather 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 could explain the ice ages.

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