Sunspot Variation
Because the sun is Earth's greatest source of energy and is the driving force behind its atmospheric circulation, any variation in solar output will influence the weather. Scientists have observed that the number of sunspots on the surface of the sun has been determined to correspond to solar output variability. More sunspots correspond to a higher solar energy output while fewer sunspots correspond to a lower solar output. A record of sunspot numbers has been recorded through time by various indicators including naked eye observations, auroral reports, and C14 isotope concentrations in tree rings (Schaefer, 1977.) Fig. 8 shows that during the MWP there was a high number of sunspots referred to as the Medieval Maximum, while during the LIA there were two periods of very low sunspot numbers called the Spörer Minimum and Maunder Minimum. Although a direct link has not yet been established between sunspot variability and climate change, the data is highly suggestive.
Volcanic Eruptions
Ash and other small particulate matter injected into the stratosphere can effectively reduce incoming solar radiation received at the earth's surface. Sulfur compounds from eruptions condense into very tiny sulfuric acid droplets that form clouds which may stay suspended in the stratosphere for years, further reducing incoming sunlight (Pollock et al., 1976.) Fig. 9 illustrates the process.
Large eruptions at low latitudes can cause the greatest global climate change. Weaker eruptions only send their eruptive materials into the troposphere where weather processes quickly remove them and high latitude eruptions only send their materials into one hemisphere. The explosion of Mt. Tambora in 1815 led to the year 1816 being called "the year without a summer" across much of Europe. The eruption of Mt. Pinatubo in 1991 provided a good example of how a large low-latitude eruption can quickly influence global climate.
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