This figure summarizes sunspot number observations. Since 1749, continuous monthly averages of sunspot activity have been available and are shown here as reported by the Solar Influences Data Analysis Center, World Data Center 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.
The most prominent feature of this graph is the 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. Pictured below, the Frozen Thames, by Abraham Hondius (1677).
Throughout the Little Ice Age, the world experienced heightened volcanic activity. When a volcano erupts, its ash reaches high into the atmosphere and can spread to cover the whole earth. This ash cloud blocks out some of the incoming solar radiation, leading to worldwide cooling that can last up to two years after an eruption. Also emitted by eruptions is sulfur in the form of sulfur dioxide gas. When this gas reaches the stratosphere, it turns into sulfuric acid particles, which reflect the sun’s rays, further reducing the amount of radiation reaching Earth’s surface. The 1815 eruption of Tambora in Indonesia blanketed the atmosphere with ash; the following year, 1816, came to be known as the Year Without a Summer, when frost and snow were reported in June and July in both New England and Northern Europe. Pictured below, the mechanism for volcanic atmospheric cooling.
The Modern Maximum is between 1900 and 1950. (Credits: The 400 Year of Sunspot figure was prepared by Robert A. Rohde and is part of the Global Warming Art project)
The Master of Disaster