The remarkable cold lingered in Europe for weeks. Lakes, rivers, and the sea froze over, and the soil solidified a meter deep. The cold cracked open trees, crushed the life out of livestock huddling in stables, and made travel a treacherous undertaking. It was the coldest winter in the past 500 years, and one of the coldest moments in a larger global phenomenon known as the Little Ice Age. Likely causes include volcanic activity, oceanic currents, and/or reforestation due to Black-Death-induced population decline. It is nearly certain, however, that it has something to do with the unusually low number of sunspots that appeared at that time, a phenomenon referred to as the Maunder minimum.
We now know that such solar minima correlate quite closely with colder-than-normal temperatures on Earth, but science has yet to ascertain exactly why. Solar maximums, on the other hand, have historically had little noteworthy impact on the Earth apart from extra-splendid auroral displays. But thanks to our modern, electrified, interconnected society these previously innocuous events could cause catastrophic economic and social damage in the coming decades.
Astronomers first began to monitor sunspots in the early 1600s. Although no one knew the nature of the dark blemishes on the surface of the sun, the sort of people who keep painstaking records of things began to keep painstaking records of sunspots. Scientists have since learned that sunscabs arise when the sun's magnetic field lines twist and tangle due to layers of plasma inside the sun rotating at different rates. This tangling produces magnetic loops which can protrude from the photosphere and smother the underlying convection, thereby decreasing the amount of heat reaching the surface. Sunspots only appear dark in contrast to the extremely bright areas around them; viewed in isolation a sunspot would still be quite squint-worthy. Over time the field lines become more and more tangled, resulting in more protruding loops, until the magnetic disarray reaches a tipping point at about 11 years where the magnetic field snaps into a new orientation and the cycle starts anew.
A CME plasma wave is magnetically charged, with positive and negative poles inherited from the magnetic loop that spawned it. Consequently, when such a wave impacts the Earth's magnetic field the nature of the interaction depends upon the relative orientation of the two magnetic fields. The way that f*cking magnets work is that opposite poles attract and like poles repel; so if a powerful wave hits the Earth's magnetic field aligned mostly positive-to-positive, the plasma will be repelled. If it hits positive-to-negative essentially all of the plasma will pour into the atmosphere via the north and south poles, causing a geomagnetic storm.
In late summer 1859, the sun was in a particularly persnickety solar maximum, and a complex cluster of tightly packed sunspots was drawing a lot of attention from astronomers. On 28 August the night sky was alight with brilliant auroras much further from the poles than usual. Operators of the fledgling telegraph system reported some unusual technical glitches the same evening. There were widespread outages, electrical arcs from telegraph receivers, and a few small fires at telegraph stations. Unbeknownst to the telegraphers and astronomers these effects were due to a CME causing extreme magnetic variation in the atmosphere. Faraday's law of induction observes that a magnetic field changing with time will induce a voltage changing with time, and some telegraph wires were long enough to be exposed to a quite a range of magnetic variation. Consequently a high-voltage induced current was created up and down the wires.
This second coronal mass ejection, now known as the Carrington Super Flare, had sent the energy equivalent of a few billion atomic bombs towards the Earth. Moreover, the flare from two days prior had swept aside most of the ambient solar wind plasma, allowing the X-class Carrington flare to arrive with much greater speed and energy than usual. It reached the Earth in a mere 17 hours rather than the typical 3-4 days, and it triggered the most spectacular geomagnetic storm in recorded history.
Since 1859 the quaint cabling of the Victorian electrical and telegraph networks have multiplied into millions of miles of power and telecommunication conductors worldwide, all interconnected via fragile electrical transformers. These days power line voltages are much higher to improve transmission efficiency, which has the side effect of making the lines more sensitive to induced currents. Moreover, many high-voltage transformers are directly connected to the ground to offset lightning strikes and other power surges, but this also provides a back door for strong geomagnetic currents.
If the sun were to fling another such super flare towards the Earth today, solar observatories operated by the US Air Force and the National Oceanic and Atmospheric Administration (NOAA) would detect the telltale X-ray burst of a solar flare just minutes after the eruption, but with insufficient data to assess whether it would be a threat. Space-based instrumentation between the Earth and Sol would record and relay information regarding the trajectory, intensity, and orientation of the plasma wave, but not until the CME was well on its way. An official assessment of the incoming CME risk would become available just a few hours before impact.
Many electrical transformers--particularly the high-voltage variety--would be destroyed due to overheating, resulting in large-scale grid failures within a few seconds. Some sections of power lines could heat sufficiently to melt the wire. Apart from damage caused by power surges, small electronic devices would not be harmed by such a slow, large-scale magnetic event; however they would become essentially useless if there is no electrical grid to recharge their batteries and no communications grid to tickle their antennae. Automobiles and airplanes would likewise be spared, but fuel may become difficult to obtain.
In the aftermath of a sufficiently intense storm, sizable populations could be left without electricity for an extended period of time. For example, a flare about 15% as energetic as Carrington struck in 1989 and left the Canadian province of Quebec without power for nine hours due to severe transformer damage. The most vulnerable Extra High Voltage (EHV) transformers are costly and time-consuming to custom-build, and in this hypothetical geomagnetic apocalypse the factories that construct them could also be without power, exacerbating the outages into months or years. During the widespread blackouts it may become difficult or impossible to obtain clean water, food, fuel, medicine, and emergency services in many areas. A 2008 National Academy of Sciences report estimated that a Carrington-level event would could cause "extensive social and economic disruptions" in the United States, requiring $1-2 trillion and 10 years to fully recover. This is not taking into account the economic losses from lack of transportation, electrification, communication, and refrigeration, nor the attendant health costs. The ozone layer would take about four years to recover to current levels.
As for solar minima such as the Maunder Minimum that caused the Great Frost of 1709, although the correlation between low sunspot activity and low temperatures is strong, the reason for this is poorly understood. Scientists note that lulls in sunspot activity correspond to an uptick in cosmic rays on Earth, which may in turn increase the proportion of reflective clouds and therefore reduce the solar energy absorbed by the atmosphere. Recent observations have also suggested that the sun's UV output may be related to its progress in its solar cycle, which could affect the Earth's weather in as-yet-unexplained ways. Minima aren't necessarily altogether negative phenomena; some scientists cheerfully point out that future severe minima may help to offset global warming for brief periods. Additionally, researchers have hypothesized that the "perfect sound" of Stradivarius's violins may have been due to a higher-density wood that grew during the Maunder Minimum. Still other researchers hypothesize that those researchers are just being silly, pointing to double-blind experiments demonstrating that modern musicians cannot hear a significant difference between a Stradivarius violin and an inexpensive modern one. Debate continues.
In any case, another Maunder-level minimum is inevitable eventually, as is a future encounter with a Carrington-level super flare. Both are normal, healthy functions of a giant, 4.57-billion-year-old, naturally-occurring nuclear fusion furnace that is indifferent to the skim of life that has developed on a puny sphere of rock 93 million miles away. Let us just hope that humanity is ready when the sun sneezes a billion or so tons of charged plasma at our planet sometime in the next 350 orbits. Perhaps we'll luck out and the Yellowstone supervolcano will erupt first, smothering half the planet in warm ash and saving at least some of us from experiencing the horror of an Internetless Earth.