In the meantime, many smaller clouds could hit our area at any time, with densities of hydrogen 100 times greater than we're encountering now. These clouds, called Local Fluff, are much more common than the heavy stuff that's still 50,000 years away. But they're hard to see, and almost impossible to analyze. Zank's work, at this point, is mostly theoretical, but he believes the historical record may support his conclusions. Ice cores from the South Pole contain higher levels of a form of beryllium, a rare metal, which is created when cosmic rays strike Earth.
Nobody can explain why.
He sees one possible explanation. If a cosmic cloud passed by Earth then, it could have weakened Earth's cocoon enough to allow much higher levels of cosmic rays to reach the ground, producing the layers of beryllium. Guesses range from a greenhouse effect to an ice age. No one can do anything about it. The people who worry about asteroids think if they have enough warning they can alter the course of an asteroid that would otherwise crash into Earth.