In unit 4 we discuss a strange event known as the Maunder Minimum, ... a 70 year period where the sunspot cycle apparently shut off (1645-1715), ... and at the same time Europe experienced record low temperatures. The question now becomes, ... is there any evidence that the sun (and earth) experienced similar episodes even before the Maunder Minimum? Since we have no telescopic observations prior to 1609 (Galileo), it becomes a matter of indirect evidence. Is there any? Is there any evidence that the opposite effect occurred. That is, the sun was overactive and the earth got hotter as a result?
We have 400 years of data that clearly shows the sun does vary over time. Within the 11 year sunspot cycle are dips (Maunder & Dalton Minimum) and climbs (Modern Maximum). However, is it possible to go back further in time ... before direct observations and say something about the sun and its possible connection with the climate on the earth? The answer is YES.
Credit Wikipedia
The key is to look for changes in solar activity beyond direct observation. By looking at tree rings, scientists can determine the level of Carbon 14 there was in the atmosphere going back as far as 11,400 years ago. They can also find the same patterns from air trapped in annual layers of glacial ice. C14 is a radioactive isotope of carbon 12 and astronomers know that it tends to drop in the atmosphere at times of high solar activity. The reasoning goes like this: C14 is made when a cosmic ray enters our upper atmospheric and hits a nitrogen atom. During periods of high solar activity, the sun has an extended magnetic field that tends to reduce the number of cosmic rays entering our solar system. Short answer: active sun = low C14 in atmosphere. This fits rather well for the 400 years of direct observation. Extrapolating this to the past, atmospheric C14 levels (and presumably, the sun) has, indeed shown long term variations. These variations also match well with known changes in climate. For example, during the Medieval Maximum, Greenland was colonized by the Vikings .... when the climate was much warmer.
Modified from Wikipedia
Note: This correlation is still highly debated and far from conclusive.