Pope's work is based on new mathematical models and a review of scientific literature regarding a thin, global layer of dust determined to be 65 million years old. The dust was generated when a roughly 6-mile-wide (10-km) asteroid slammed into Mexico's Yucatan Peninsula, where a crater was found more than a decade ago.

The regional effects would have been devastating -- no one argues that point.

But most researchers have assumed that the dust blotted out the Sun for months on end, creating a "cosmic winter" that prevented photosynthesis in plants -- the basic chemical transaction that supports most life on Earth. Widespread extinctions resulted, including that of most dinosaurs.

Not everyone subscribes to the dust hypothesis, which is about 20 years old.

"I have been skeptical of the 'darkness at noon' scenario for years," says Jay Melosh, a University of Arizona researcher who models asteroid impacts.

Melosh did not comment on Pope's study, but he said some researchers, himself included, have shown that impacts might not generate enough fine dust to have blocked the Sun enough to cut off photosynthesis. Only tiny dust, which remains aloft longer than larger dust grains, could have such an effect.

That's exactly what Pope's study concludes.

"The [previous] studies were in error," Pope says of work that supports the cosmic winter hypothesis. "They really overestimated the effects of dust."

Pope speculates on how his results might apply to impacts of smaller rocks, which pose a more real threat because they are more likely.

While no immediate threat to Earth exists based on the present census of asteroids, rocks are out there that will eventually hit the planet, Pope and other researchers agree.

Asteroids larger than a half-mile (1 kilometer) are thought to hit Earth once every 100,000 to 300,000 years. Most asteroid experts -- including top NASA researchers -- have said that such an impact could cause global devastation and might threaten human civilization, in part as a result of climate effects on crop production.

"My new impact dust estimates indicate that death by an asteroid is far less likely and that such medium-sized asteroid impacts would not have catastrophic global effects," Pope said.

"This seems extremely over-optimistic and goes against a widely held scientific consensus," said Benny Peiser, an expert on asteroid risk at Liverpool John Moores University in the UK. "Crucially, [Pope] ignores the social and economic knock-on effects of such a global disaster. While we as a species would not become extinct as a result of such an impact, it is almost certain that the world as a whole would suffer to the extent of civilization collapse and Dark Age conditions."

Peiser said, however, that the new study is likely to stimulate vigorous debate and trigger new research designed to further pin down the risks.

Kevin Pope doesn't entirely disagree with Peiser's assessment.

"There's a whole other set of processes that could be set in motion" by an asteroid impact, he said, referring to potential economic consequences, especially if multiple large cities were wiped out.

But as for a dust-induced cosmic winter, Pope said previous assumptions are flawed. The data supporting such a worldwide catastrophe is only partly based on dust from impacts, because such dust is hard to find. Researchers have therefore used what's known about atomic bomb blasts to help support the claim.

But it would take many atomic bombs to equal the effects of one large asteroid. So researchers have to dramatically scale up the bomb data, and Pope said this scaling creates errors.

Pope did not do any new field research. But his review of recent studies of real impact dust, which he says were more revealing than previous studies, shows that much of the dust in the layer is larger than had been thought -- too large to stay aloft long enough to generate the hypothesized climate catastrophe.

Pope's case is not airtight. And several researchers told SPACE.com they are concerned the study may be flawed.

Pope examined patterns of coarse dust bits to create a model that showed how the small dust particles were dispersed. To truly understand the influence of impact dust, he says, scientists need to find a way to directly measure the amount of small dust particles.

The work was made available to reporters in a press release put out Jan. 23 by the Geological Society of America. But scientists said that from what they could see, the study did not appear promising, yet they needed more time to fully digest Pope's full scientific paper, which they had not seen.

These researchers were dismayed that such a strong challenge to a well-established idea was made available via a press release before they had had time to read the actual scientific paper and thus be prepared to comment on its validity.

"If you wanted to make a press release of this nature, you really ought to notify some colleagues," said Alan Harris, an asteroid researcher at NASA's Jet Propulsion Laboratory. After having discussed the press release and an abstract of Pope's work with several colleagues, Harris said the jury was still out on its merits.

Brian Toon, a University of Colorado atmospheric scientist, helped work out some of the widely accepted notions that Pope's work refutes. Toon was also part of a group that peer-reviewed, on behalf of Geology, an early version of Pope's paper.

Toon recommended rejecting the paper for publication because, he said, small dust particles "have no simple relation to large ones." He said it was akin to saying "I saw a panther today, lions must be going extinct."

Toon said Geology did not let him see the final paper or Pope's responses to his criticisms.

Amid all this uncertainty is one solid fact: The extinctions that occurred 65 million years ago are well documented by paleontologists. The fossils exist in the same layer of soil where the dust is found, the so-called K-T boundary that marks a division between Cretaceous and Tertiary periods of geologic history.

Noted University of Chicago paleontologist David Raup has been embroiled in these issues for more than two decades and now views it all from the comfortable distance of retirement from active research.

"The strong implication [of Pope's study] is that the impact explanation of the K-T extinction will fall if the dust cloud hypothesis falls," Raup told SPACE.com.

"Given the paucity of hard data, who knows which one to choose?" Raup wonders.

Indeed, if Pope's work is to be verified or refuted, there is likely some more field work to do.