When we think about global risks, we tend to extrapolate from the headlines. The troubles in the Euro-zone could spiral out of control, the revolution in Syria could flare into a wider war, the H5N1 bird flu virus could mutate and spread from person to person in a global pandemic. Those dangers and many others are clear and present, and society would be well-advised to prepare for them.

But this year the World Economic Forum and Nature teamed up to explore another category of risk: the dangers that could sneak up, largely unnoticed. Many of these risks – which we call X factors – are unexpected consequences of our own scientific and technological quests. To borrow a phrase from the technology writer Edward Tenner, “Things bite back.”

Neuroscientists, for instance, are enthusiastically pursuing drugs and devices that could deliver real cognitive enhancement—not just sharpening our alertness and ability to focus, as drugs can do today, but upping our intellectual firepower. From students to business executives, the demand for such drugs would be huge—and so would the potential for unintended consequences. Few drugs strike just a single target, and the neurotransmitter systems important to cognition play a role in other functions as well, raising the spectre of serious side effects: for example, a drug that boosts memory might also make the user more prone to impulsive behaviour. And then there are the ethical conundrums: should the market decide who gets the benefits of these drugs and who must get along without them? Should they be banned, to level the playing field—or subsidized for the same reason?

With far more mixed feelings, climatologists are studying schemes for geoengineering—deliberately altering the climate system to combat the effects of rising greenhouse gases. In one scenario, high-flying jets or balloons would release a haze of reflective sulphur particles into the stratosphere, dimming the sun’s rays and cooling the planet. Wary that geoengineering could affect the climate system in unexpected ways, researchers have deliberated, studied, and proposed only the smallest of experiments. But an X factor looms: geoengineering is simple and cheap enough that a rogue nation, even a company, could deploy it on a large scale before its risks are well-understood, perhaps triggering a widespread climatic crisis.

X factors are part science and part speculation, and unpredictable by nature. But Nature’s technical editors and journalists pooled their expertise to identify the most plausible of them, and this year’s Global Risks 2013 report features our top five.

Author: Tim Appenzeller is Chief Magazine Editor at Nature magazine, which collaborated with the World Economic Forum on the X Factors section of the Global Risks 2013 report. Nature’s editorial coverage includes Planetary disasters: It could happen one night and Rumour Research Can Douse Digital Wildfires.

Image: A bird stands on a statue as planet Venus transits across the sun REUTERS/Jamal Saidi