Free Death from the Skies! by Ph. D. Philip Plait Page B

1800s, the scientist E. W. Maunder summarized Spörer’s findings and published them. We now call this period of sunspot deficit the Maunder Minimum.
    All of this would be somewhat academic if not for one rather critical point: the years 1645 to 1715 were also a time of much lower than average temperatures across Western Europe and North America. It was so cold that the Thames River froze over (which it generally does not do, even in winter), glaciers in the Alps advanced, destroying whole villages, and the Dutch fleet was frozen solid in its harbor. This period was called the Little Ice Age.
    It’s awfully tempting to directly connect the Maunder Minimum with the Little Ice Age, but we have to be very careful. In nature, it’s rare for a single effect to have a single cause, especially when the effect is as dramatic as a prolonged climate change. Usually, there are a number of events that have to occur to manufacture such a big change.
    It turns out the Little Ice Age may have started long before the Maunder Minimum, even as early as the mid-thirteenth century. Caspar Ammann, a solar physicist who has extensively studied the connection between the Sun’s output and the Earth’s climate, notes that the Little Ice Age was not one continuous event, but instead consisted of “several pulses of cooling episodes . . . the first one started in the 1250s through 1300, after a medieval warming period.” Clearly, there were other causes of the temperature drop.
    The biggest culprit is probably volcanic activity. There are clear signals of eruptions during the Little Ice Age, mostly seen in ice cores: atmospheric gases trapped in polar ice can be studied to determine what was happening in the Earth’s air during certain times in history. Interestingly, in the 1690s, the Little Ice Age got very severe, especially in Western Europe—there are stories of birds literally freezing to death sitting in branches. At this very time, there is a large spike in the amount of atmospheric sulfur found in ice cores, indicating large levels of volcanic activity. Volcanoes launch sunlight-reflecting dust and gases into the air, reducing the amount of visible light reaching the Earth’s surface. This cools the planet by lowering the amount of heat the surface can absorb.
    By itself, this could not cause the severest parts of the Little Ice Age. But together with the Maunder Minimum, when the global temperatures would have dropped, it could have lowered the Earth’s average temperature even more.
    Still, if this were a global effect, why was Western Europe hit so much harder than everywhere else?
    It turns out there is a third player in this game. This gets a little complicated, so strap yourself in.
    During a sunspot minimum, there is less solar activity in general. Besides there being less visible light, there is a drop in the amount of sunlight across the spectrum, including ultraviolet light. This turns out to be important: UV light is what helps create the Earth’s ozone layer; it turns normal atmospheric oxygen (O 2 ) into ozone (O 3 ). If there is less UV, there is less ozone. Ozone is actually quite important in the temperature balance of the upper part of the atmosphere, called the stratosphere. When there is lots of ozone the stratosphere is warm (because it absorbs UV light), and when there is less ozone the stratosphere is cooler.
    Most, but not all, of the ozone creation happens in the tropics, at low latitudes near the equator. That’s because that’s the part of the Earth getting the most sunlight, and therefore the most UV. In the summer, ozone can be created both at the equator and at the pole, because that whole hemisphere is in sunlight. In that case, the difference in temperature in the stratosphere from pole to equator is minimal.
    But in the winter, the pole is in darkness. No UV reaches the stratosphere, so no ozone is created there. That in turn means there is a big temperature difference in the ozone layer between