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File:Sunspot Numbers.png


Changes in carbon-14 concentration in the Earth's atmosphere, which serves as a long term proxy of solar activity.

This figure summarizes sunspot number observations. Since c. 1749, continuous monthly averages of sunspot activity have been available and are shown here as reported by the Solar Influences Data Analysis Centre, World Data Centre for the Sunspot Index, at the Royal Observatory of Belgium. These figures are based on an average of measurements from many different observatories around the world. Prior to 1749, sporadic observations of sunspots are available. These were compiled and placed on consistent monthly framework by Hoyt & Schatten (1998a, 1998b).

The most prominent feature of this graph is the c. 11 year solar magnetic cycle which is associated with the natural waxing and waning of solar activity.

On longer time scales, the sun has shown considerable variability, including the long Maunder Minimum when almost no sunspots were observed, the less severe Dalton Minimum, and increased sunspot activity during the last fifty years, known as the Modern Maximum. The causes for these variations are not well understood, but because sunspots and associated faculae affect the brightness of the sun, solar luminosity is lower during periods of low sunspot activity. It is widely believed that the low solar activity during the Maunder Minimum and earlier periods may be among the principal causes of the Little Ice Age. The Modern Maximum is between 1900 and 1950.


This figure was prepared by Robert A. Rohde and is part of the Global Warming Art project.

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  • Hoyt, D. V., and K. H. Schatten (1998a). "Group sunspot numbers: A new solar activity reconstruction. Part 1.". Solar Physics 179: 189-219.
  • Hoyt, D. V., and K. H. Schatten (1998b). "Group sunspot numbers: A new solar activity reconstruction. Part 2.". Solar Physics 181: 491-512.
  • Stott, Peter A.; Gareth S. Jones and John F. B. Mitchell (15 December 2003). " Do Models Underestimate the Solar Contribution to Recent Climate Change". Journal of Climate 16: 4079-4093.
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