Younger Dryas cooling event said to be comet related

SH –
You say, “Asteroids with a one km diameter strike earth every 500,000 years on average. Larger collisions, of five-km objects, happen approximately once in twenty million years. The last known impact by an object of 10 km or more in diameter was the K-T (or P) extinction event 66 million years ago.”
I can put up numbers, too… A bit on the evolution of the estimates of frequencies of impacts on Earth:
Gerrit L. Verschuur, in “Impact!: The Threat of Comets and Asteroids (p. 157)” wrote:
“…Specific odds were offered by Sir Richard Gregory in 1893: “about once in about twenty million years.” In 1897 David Todd offered an estimate of an impact every 15 million years, but even if the earth should pass through the head of a comet, it might bring universal death to nearly all forms of animal life. The famous textbook of Russell, Dugan, and Stewart in 1926 stated flatly that comet collisions would happen once in 80 million years.” [Note that no size was given. These were blanket statements about ALL comets.]
“…Representing the Lone Ranger point of view, Clark Chapman and David Morrison published their odds in Nature in January 1994. In “Impacts on the Earth by Asteroids and Comets: Assessing the Hazard,” they concluded that the chance that a large (2-kilometer diameter) object will slam into the planet and terminate civilization during the next century is I in 10,000. (pg 159)”
Note that this is the same David Morrison who is the director of NASA, and this was the same year that Shoemaker-Levy 9 broke into 20 large fragments which slammed into Jupiter over the course of several days in July of that year.
Note also that until the Shoemakers and Levy found SL-9, NO astronomers or anyone at NASA thought that they would ever in their lives see a comet hit a planet. And, to add insult to injury, TWO other comets have since 1994 been seen to hit Jupiter.
So much for the estimations of frequencies of comet strikes on planets? Maybe, but we raren’t done yet.
Continuing. . .
“In July 1994 an interesting article appeared in Scientific American on what was learned from the Apollo moon landings. The author, G. Jeffrey Taylor, a geophysicist physicist at the University of Hawaii in Honolulu, pointed our that for every impact crater you see on the moon there would be 20 expected on the earth, because of the earth’s larger size. He added that Frederick Horz of the Johnson Space Center estimates that there are 5000 craters on the moon larger than 5 kilometers across which were produced in the past 600 million years. Putting these numbers together implies that half-kilometer objects, which are potentially civilization-destroying, are expected on average every 6,000 years. This rate is consistent with the claims made by the Tollmanns (chapter 8). If earth is hit by a 0.5-kilometer object every 6,000 years, roughly four out of five would be expected to land in the oceans, which means it is very likely that a major flood event would have occurred at least once in the past 10,000 years. (p. 162).”
(Don’t tell the Creationists!)
Continuing……..
“While researching old books and technical articles for this chapter, I noticed something fascinating that illustrates how our conception of what the future holds depends so intimately on our knowledge base. In two centuries, the typical estimated time between comet impacts (from old books) to impacts capable of producing global catastrophe (from new research) has decreased from once every 281 million years (which held for most of the 19th century) to about once ever 5 to 10,000 years in the past year.…
“…Compared to estimates made in the past century, one thing has changed in recent years. The NEAs have entered the picture and therefore the odds of impact have shortened dramatically. Around the end of the nineteenth century the odds had shortened to once every 10 million years or so, which held until the early 1980s when 100,000 years between civilization-destroying impacts began to surface. That change happened because of the sudden increase in information about NEAs, crater statistics, and past mass-extinction events. A spate of at least nine estimates appeared in early 1995, four of which independently set the interval between such collisions at close to 5,000 years. (p. 164)”
On another tack ——– From http://www.pbs.org/wnet/savageplanet/03deadlyskies/03tracking/indexmid.html we have this:
“…Dr. Brian Marsden, director of the Minor Planet Center in Cambridge, Massachusetts, which catalogues all newly discovered near-Earth objects and tries to accurately determine their orbits…
“… In 1998, the center listed 10,000 objects. Two years later, the number has risen to 15,000. Most are objects traveling harmlessly in the asteroid belts between Mars and Jupiter, and more are being discovered everyday…
“…A smaller subgroup, called Potentially Hazardous Asteroids, includes 249 objects that are one-tenth of one mile or larger, and pass within five million miles of the Earth. Last year, about 100 new objects were added to this list.
“Most of these objects are not a danger,” says Dr. Marsden. “But in the last two years, there have been five objects with a significant impact possibility during the next half-century.” Intense efforts are underway to determine the orbits of these objects. All but one has been ruled safe. This one object, which appeared in 1998, but vanished before an orbit could be determined, is listed as a potential threat to the Earth.”
So, the number of known threats becomes higher. IN one year it went up about 40% – and that ws before WISE mission began looking seriously.
At about the end of 1980 9,425 asteroids were known. By 1985 that had grown to 12,214. In 1990 it wsa 26,748. BY 2000 it had grown to 123,114. Then in 2005 the count went to 343,671. In 2010 it was 548,914. In 2013 it had grown to 588,992. Those are all now known, numbered and tagged. The number went up by 62-fold.
These absolute numbers don’t mean that the danger to Earth has increased or decreased. We are not 62.5 times more much in danger than 35 years ago. We just have more information.
But we DO need to be aware that old estimates, based on old numbers, cannot fly. Estimates need to be based on the numbers we DO know about – and WHAT we know about them. If we see that 588,991 of those will never be a threat, the ONE that is left is not much of a threat.
But with the NEAs/NEOs numbers up several-fold since 1980, we DO now know the threat more than ever, and the odds are not once in 500,000 years anymore.
SH, it looks like your numbers are based on information from probably 40-50 years ago.