|MadSci Network: Earth Sciences
This is one of the oldest questions in climate science... the ancient Greek Meton was the first to suspect a connection between climate and sunspots, in 400 b.c.!
The connection between sunspots and climate seems obvious at first: if there are black spots on the sun's surface, it won't be as bright, so less sunlight will hit the Earth's surface, so the Earth will get colder, right?
It's not that simple. First off, when the sun has spots, it's more active, with lots of plumes and coronal ejections; in addition to the black spots which are easy to see, there are bright regions called "faculae" which surround the sunspots. Stellar physicists believe these faculae make the sun actually brighter during the "solar maximum", when sunspot activity is highest, and satellite observations of the sun confirm this.
Also, the Earth's climate very complicated, and events like volcanoes and a wide variety of processes called "internal variability" mean that climate would change a lot without the help of sunspots, and it's nearly impossible to observe sunspot influences through all that "noise". It's like trying to hear the doorbell while listening to your stereo with the volume cranked up to 11.
The simplest connection between sunspots and climate is with Earth's global mean temperature. If the sun is brighter, the temperature should increase. But studies done in the last century directly contradict modern studies. The studies done through the ages have shown:
The record of sunspots over time is a very tricky one: if you play with the mathematics, you can show that everything from birth rates to the stock market are "caused" by sunspot cycles. It's very easy to succumb to "cyclomania", and claim that anything you don't understand is caused by sunspots or other cycles. Most climate scientists, myself included, think that the effect of the 11-year solar cycle on Earth's climate is tiny compared to the climate's internal variability.
The lesson here is that "correspondence does not imply causation" -- that is, you can't say that A causes B because A and B happen at the same time. You might as well say that leap years cause presidential elections!
One last thing: there might be a connection between sunspots and climate on much longer time-scales. For example, from 1645 to 1715, the sunspot cycle turned off, and not one sunspot was visible during these years. During about the same time, the "Little Ice Age" happened, in which temperatures in Europe fell several degrees C. Again, correspondence doesn't imply causation, but it's something to think about.
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