Solar flares are massive energy emissions from the Sun, usually associated with sunspot activity. When they strike the Earth, they can cause a range of meteorological phenomena. More disturbingly, however, particularly powerful solar flares can also cause disruptions to the electricity grid and to global communications systems.
The Sun is still not fully understood, but current research indicates that a cycle of increasing and decreasing solar flares is also associated with the observable cycle of increasing and decreasing sunspots. (Sunspots are scheduled to peak in the next couple years, which has contributed to popular fears about some sort of cataclysm occurring in 2012.) In particular, the flares are massive amounts of energy emitted when opposing magnetic fields within the sun clash. The resulting flare can vary immensely in strength, in terms of multiple orders of magnitude. The most common scale for measuring solar flare strength has five categories (A, B, C, M, and X) and multiple levels within each category, with each incremental step up equating to a twofold increase in strength, so that, for instance, an M3 solar flare is twice as powerful as an M2 solar flare.
In general, the Earth’s own magnetic field and other defences are enough to protect life on the surface from the increased radiation emitted as part of a solar flare. However, our relatively fragile network of global power and communications systems is more vulnerable, particularly to the strongest solar flares. The same is true of astronauts and spacecraft once they leave the protective layer of the atmosphere.
The stream of radiation which occurs as part of a solar flare could – in historical reality as well as in theory – disrupt electrical systems which rely on carefully regulated streams of electromagnetic radiation. These include both the electrical grid and worldwide communications systems, such as radio, satellite communications, and even telephone wires. Minor problems actually crop up on a relatively regular basis. In 2003, for instance, the strongest solar storms in 2003 ranged from X10 to X17 in strength and resulted in brief power outages and other disruptions, such as a disruption of power to tens of thousands of customers in Sweden.
The worst-case scenario for an extremely powerful solar flare would still not be an actual imminent threat to human and animal life. That said, however, history does provide an important lesson for the fragility of the power and communications system. In 1859, solar storms believed to be several times as strong as those which struck in 2003 occurred during September. Auroras, or northern lights, were reported as far south as the Caribbean. Most significantly, the North American and European telegraph systems suffered large (but not quite complete) catastrophic failures, blow-outs, and even fires. It took considerable time for the system to recover and for damage to be repaired.
In 1859, the telegraph system was still in its infancy, and electronic communications were a helpful innovation rather than an essential component of daily life for all but a few cutting-edge investors and military personnel. If wireless communications or electrical power systems were to suffer widespread failure today, by contrast, the consequences would be far more drastic. The failure of satellites in orbit, and of ground communications and power systems, could cause considerable chaos. That said, claims that a solar flare would destroy human civilization and cause a new stone age are of course exaggerated. It would take some time, and in the meantime disruptions to the normal flow of food and other essential goods could be serious, but it would not be technically difficult to repair the damaged systems.