The problem, in Hills' view, is that impacts by asteroids or comet fragments big enough to cause years of environmental disaster are relatively rare on a human time scale. But to have four in that time - I would be rather skeptical. Baillie finds evidence in tree-ring analyses for three to five periods of ecological upheaval in the past 4,500 years. The dust kicked up by the blast produced spectacular sunsets and unusually bright nights over much of Europe and middle Asia for several weeks, but it caused no lingering environmental problems. Hills says a Tunguska-size object is likely to crash into land at least once every 300 years.
Comets are more complicated.
The danger, he says, is real, if quite unpredictable, and solutions are possible. Yet it has never hit the consciousness of the people. They just can't make that psychological leap. That amazes me.