Here's a story from earlier this month in New Scientist:
MULTIPLE comet impacts around 1500 years ago triggered a "dry fog" that plunged half the world into famine.
Historical records tell us that from the beginning of March 536 AD, a fog of dust blanketed the atmosphere for 18 months. During this time, "the sun gave no more light than the moon", global temperatures plummeted and crops failed, says Dallas Abbott of Columbia University's Lamont-Doherty Earth Observatory in New York
It seems to me that the ability to find possible impacts has really increased, with finer-resolution cores and ways of finding the microscopic ejecta. Regardless of the scale of effects of any given instance, it will be nice when we can get some idea of the rate of such events through the past several tens of thousands of years.
