According to their study, these factors would lead to ultraviolet-related DNA damage about 1,000 times higher than normal, and general ultraviolet damage to plants about 500 times higher than normal.

Ultraviolet radiation can cause mutations, cancer, and cataracts. It can kill plants or slow their growth, suppressing the photosynthesis which forms the base of the world's food chain.

Smaller asteroid impacts, which have happened far more frequently in Earth's history, theoretically might cause similar or even worse problems with ultraviolet exposure, the researchers say.

The ozone depletion would be less, but there would also be less of a protective dust cloud.

"Part of what we're trying to stress here is that with an asteroid collision, there will be many synergistic effects on the environment that go far beyond the initial impact," said Cockell, a researcher with the British Antarctic Survey who did some of this analysis while formerly working with NASA.

"Effects such as acid rain, fires, the dust clouds, cold temperatures, ozone depletion and ultraviolet radiation could all build upon each other."

During the K-T event, the scientists said, many of the animals may actually have been spared most of the ultraviolet spring they envision. That impact, oddly enough, hit a portion of the Earth's crust that was rich in anhydrite rocks.

This produced a 12-year sulfate haze that blocked much of the ultraviolet radiation. But it was a lucky shot -- that type of rock covers less than 1 percent of the Earth's surface.

So when the next "big one" comes, the scientists said, the ecological repercussions may be more savage than any of those known in Earth's long history. The collision will be devastating, the "impact winter" deadly.

But it will be the ultraviolet spring that helps finish off the survivors.