Editor’s Note: There’s a 12 percent chance of something really disruptive hitting Earth in the next decade. How worried should we be?

Very, says astronomer Donald Goldsmith. To put that risk in context, that’s about the same likelihood of another earthquake striking California. But the risk Goldsmith warns about is a lot less publicized: a type of solar flare, known as a coronal mass ejection, could cause economic damages up to $2 trillion, according to the National Academy of Sciences. (NewsHour science reporter Jenny Marder explained how the sun ejects these balls of gas in this 2011 story.)

These potentially destructive solar storms are in the news this summer because of a recently published paper revealing just how close a CME came to hitting Earth two years ago. If this extreme solar storm had hit in July 2012, the University of Colorado’s Daniel Baker said, “We’d still be picking up the pieces.”

Even more troublesome is how close it came. Had the Earth been a week earlier in its rotation, impact would have been made, Baker said.

Weighing in on a different and unrelated risk, environmental economist Marty Weitzman has argued on this page that even if we’re not sure climate change is actually happening, the costs of not believing the warnings and not preparing are extraordinarily high.

Goldsmith makes a similar point in the following column. There may not be an obvious solution to avert the effects of the next CME, but the costs of continued ignorance are too high. It’s time to get the word out.

Fittingly, Goldsmith has made a career in the popularization of astronomy and science education, consulting for Neil Tyson’s “COSMOS” series, writing for Scientific American and the PBS series “NOVA,” and authoring sixteen books, including, most recently, “Origins,” with Tyson.

— Simone Pathe, Making Sen$e Editor

Californians have become accustomed, to the point of deep mental burial, to the fact that sooner or later, the San Andreas, the Hayward, or another of the numerous faults that crisscrosses the state will produce an earthquake at least as great as the 1906 event that led to the destruction of much of San Francisco.

We can easily analogize this now-familiar trope to the solar explosion that will some day devastate, if only temporarily, the complex edifice we call civilization. Our life-giving star has demonstrated a marvelous constancy through thousands of millennia, probably varying by no more than a few percent during the past billion years. In our lifetimes, the sun’s yearly energy output has changed, as part of the 11-year sunspot cycle, by only a fraction of a percent. The sun’s almost unvarying outflow has allowed life on Earth to flourish and to evolve for the past billions of years. But this constancy over longer-time scales masks the short-term phenomena that lurk in our future: solar flares and the resultant coronal mass ejections.

Every second, deep in the sun’s core, nuclear-fusion reactions release the energy equivalent of a hundred thousand billion tons of TNT, mainly in the form of increased kinetic energy of the elementary particles that participate in fusion. Through uncounted (for this report, at any rate) collisions, the newly released energy makes its way to the solar surface, taking about a million years to do so, from where it propagates into space, primarily in the form of visible light, ultraviolet and infrared radiation. By intercepting about 20 parts in a trillion of this continuous outflow, the Earth basks in the temperatures that life finds appropriate — not by accident, of course.

The sun’s constancy arises from the self-regulation of the nuclear-fusion processes in its core, which are highly sensitive to the exact multi-million-degree temperature. If the core happens to grow a bit hotter, the rate of nuclear fusion ramps up, releasing more energy — which expands the core and brings it back to its normal state. Likewise, temporary cooling in the core reduces the rate of nuclear fusion, allowing the core to shrink under the weight of the overlying layers, so that the core heats to resume its familiar temperature and rate of fusion. (Herein lies the secret of safe fusion energy: Create a fiery plasma that initiates nuclear fusion at 10 million degrees, wrap it in a few hundred thousand Earth masses of material, and stand back.)

But the sun has its burps. Every so often, on time scales measured in days or weeks, a solar flare appears in the sun’s outermost layers, called the solar corona — the gauzy envelope that surrounds our star’s visible surface. A significant flare can release an amount of energy equal to a fair fraction of the sun’s total per-second output. Many flares produce coronal mass ejections (CMEs is the term of art) —great flows of charged particles that head outward in whatever direction toward which the outburst happens to point. Although the sun continuously pushes charged particles outward to form the “solar wind,” a CME outburst produces many times the normal particle flux, and contains particles moving at higher speeds than the ordinary solar-wind particles possess.

The energy source for flares and CMEs long remained a mystery, but today, astronomers feel sure that they arise from the release of the energy stored in the magnetic fields that surround the sun, which bend and twist in complicated patterns that can realign themselves in sudden reconfiguration.

Unlike light from the sun, which leaps the 150 million kilometers to Earth in only eight and a third minutes, the charged particles in CMEs — mainly electrons and protons that travel at a few thousandths of the speed of light — take two or three days to span the Earth-sun distance. When they do arrive, the Earth’s magnetic field guides them toward the magnetic poles. Upon encountering atoms high in the atmosphere, the particles excite the atoms to produce stunning auroral displays. For most flare activity and the resultant CMEs, the aurora provides most of the excitement. However, every ham radio operator (are there any left?) knows that when solar activity grows more intense, the increased outflow of fast-moving charged particles also disrupts the ionosphere, the atmospheric layer of charged particles that reflects relatively long-wavelength radio waves and thus allows AM radio stations to be heard despite interference from the Earth’s curvature.

A sufficiently large CME will amplify this effect to the point that chaos reigns. What sort of chaos? Consider the greatest CME recorded on Earth: the “Carrington event” that ended August 1859. Intense streams of charged particles from the sun then induced auroral displays as far from the poles as the Caribbean and Queensland. According to Sten Odenwald and James Green, the astronomers who wrote about this event for Scientific American in 2008, many people thought that cities were on fire, and Rocky Mountain miners awoke during the night believing it was time to prepare breakfast. On the technology front, the incoming particles shook the Earth’s magnetic field, inducing electrical currents that put telegraph systems on the fritz around the world.

Today, of course, we no longer rely on the telegraph. Instead, we routinely depend on transformers — unsung heroes of our modern world — to regulate the electrical currents that power pretty much everything. A sufficiently powerful CME could burn out these transformers and deaden the electrical grid that moves electricity from its source to its users; it might also induce short circuits in anything that uses electrical circuits, such as home appliances, office equipment, water pumping stations, and any vehicle made since about 1920.

Why, then, have we never heard of CMEs as potential dangers? To be sure, 1859 was a while ago. On the other hand, just 130 years later, on March 3, 1989, a CME much less powerful than the Carrington event produced a power failure through Quebec, damaged transformers in New Jersey and the United Kingdom, and induced several hundred power anomalies across the United States. Before long, of course, power was restored and the event forgotten.

CMEs have reappeared as a news item, thanks to a recently published analysis by Daniel Baker at the University of Colorado of a vast outburst just two years ago that had no effect whatsoever on Earth. This solar eruption passed through the Earth’s orbit on July 23, 2012, happily for us, at a time when our planet was about one-fiftieth of the way around our orbit from the point at which the dangerous outflow intersected the orbit. According to Baker, had the Earth been in the direct line of fire, as it was a week earlier, the outcome would have been catastrophic.

The National Academy of Sciences estimates that the economic damage could reach $2 trillion; a moment’s thought reveals that we must count not just the cost of replacing, for example, all the burnt-out transformers, but the unknown losses arising from the length of time needed to get back to normal. One may, in fact, conclude that in addition to the immense economic impact, the greatest damage could lie in the loss of our belief that we have created a system that will satisfy our needs for water, power, transportation, heat and light, with only temporary and localized interruptions.

When can we expect the big one to arrive? Fittingly, the odds are roughly the same as those for the next great earthquake in California. Earlier this year, the physicist Peter Riley, who works at Predictive Science, Inc. (good to know it’s a science), published his estimate in Space Weather . Analyzing the records of CMEs for the past half-century, Riley estimates that the chance of a CME packing the punch of a Carrington-like event during the next decade at 12 percent, which he called “a sobering figure.” Only 12 percent! Obviously we have good odds of passing the next decade or two with only our familiar woes. But when the big one comes, and it seems sure that it will, all hell will break lose, semi-metaphorically. When we regroup (who is this “we,” anyhow?), we may decide that the safe approach will be to bury any part of the electrical grid that we can, and to develop some sort of shielding (not favored ecologically) for anything that stands or rides above ground, and no longer to rely on using radio waves to transmit messages and television broadcasts. Could other civilizations, on similar planets orbiting sunlike stars, have already taken this route? Could this explain why attempts to “eavesdrop” on their (supposed? imagined?) radio communications have so far proven unsuccessful?

Afterword

For those who rate blissful ignorance our best approach to dealing with disasters that remain impossible to predict, we may note that Riley’s estimate of a 12 percent probability for a devastating CME during the next decade was published in February 2012. We have already sped through one-quarter of the succeeding decade, with only the near miss of July 23 capable of impinging (barely!) upon our powers of denial.