Death from the Skies! by Ph. D. Philip Plait

the equator and the pole.
    The problem is that the jet stream is sensitive to these temperature differences. In the winter, the temperature change across latitudes is large. This drives a strong jet stream, which circulates very firmly around the globe. But in the summer, when the gradient is smaller, the jet stream weakens. Instead of making a tight circle, it meanders, flopping down loosely to lower latitudes. When it does this, it can bring cold air from the Arctic to southern locations, and warm air from the south up to higher latitudes. 18
    As it happens, the jet stream tends to dip down more at certain locations on the Earth than others. Western Europe is one such place.
    This then is the most likely scenario for the very bitter winter cold snap in the 1690s in Europe: volcanic activity dropped the global temperatures, as did the Maunder Minimum. Together they made things cold, but not brutal. But the drop in solar activity dropped the Sun’s ultraviolet output, which lowered ozone production on Earth. This changed the direction of the winter jet stream, bringing the unusually cold Arctic air down to Western Europe.
    And then people could ice skate on the Thames.
    It should be noted that in Western Europe, “the summers were not all that unusual,” according to Ammann. This indicates that whatever caused this intense pulse of cold weather was restricted to winter, which is consistent with the above series of events.
    Like I said, this is complicated. But that’s the whole point. If it were simple, we’d understand it better, and no one would be arguing over how the Sun affects the climate. In fact, these events are all fairly well established in general, but the problem is the magnitude of each one. How much less ultraviolet light was emitted by the Sun during the Maunder Minimum? How much less ozone was created? How far did the jet stream dip south? How much sulfur was spewed into the air by volcanoes? Changing any one of these inputs makes the results different, so knowing how much each one affects the climate is very difficult to figure out.
    The important thing to remember here is that while the Sun affects our climate, changes in its total output over the eleven-year magnetic/ sunspot cycle are small. There is a definite effect on the Earth, but it’s more like a priming charge than the explosion itself. It requires other catastrophic effects—volcanoes, asteroid impacts, man-made emission of CO 2 and methane—to take advantage of the Earth’s climatic sensitivity and cause a disaster. 19 And even then, at least in this particular case, the problems tend to be regional. The global environment of the Earth doesn’t change that much.
    That’s cold comfort to people who are affected, of course. And if the particular region is very sensitive—or that region has global impact itself—then the results can be much worse. A decades-long series of brutal winters in the United States, for example, or China, could cause famine and economic depression. Wars start over such things, and modern wars can wreak far more damage than a simple solar minimum. When it comes to potential extraterrestrial sources of destruction, the last thing we need to do is add our own capabilities to them.
    A more pertinent thought is: could another such minimum occur again? Yes, it could. Worse, it doesn’t look as if such events are entirely predictable. Scientists studying the occurrence of long minima in sunspot numbers show that they don’t appear at regular intervals, meaning they are not an inherently predictable phenomenon in the long term, although it’s marginally possible to make predictions about the very next sequence in the solar cycle. So we might be headed into another minimum a few cycles from now, or it might not happen for a thousand or ten thousand years. But it seems very likely indeed that it will happen again.

HOT PLANETS AND HOT AIR
    So if the Sun can indeed affect Earth’s climate, what about global warming?