The article in the link above names Dallas Abbott, a research scientist at the Lamont-Doherty Earth Observatory of Columbia University. Abbott believes that in about 535 CE, a meteorite hit in Australia and tossed up a bunch of schmutz that affected climate all the way to England.
About her current research, Abbott says:
The primary focus of my present research is on submarine impact craters and their contribution to climate change and megatsunamis. This research grew out of my work on the thermal history of the earth when Ann Isley and I discovered that mantle plumes had the same periodicity as impact cratering events (Isley and Abbott, 2002, Journal of Geology, 110, 141-158). We did a compilation of impact cratering events and found that the record was woefully undersampled (Abbott and Isley, 2002. Earth and Planetary Science Letters, 205, 53-62). As a result, I started to look for impact craters on the ocean floor. I found that the Holocene age impact crater candidates could be located using a combination of bathymetry derived from satellite altimetry and the directions to the source of chevron dunes. I am now part of a research group (the Holocene Impact Working Group) that is focusing on two goals: looking at the effect of submarine impacts on climate and determining if chevron dunes are megatsunami deposits. We have located a candidate crater set in the Gulf of Carpentaria, Australia with an inferred age of AD 572±86 (Abbott et al., 2007). The craters have produced impact spherules of magnetite, impact glass, and probable shocked quartz. The date of AD 572±86 is, within error, the same as the age of the climate downturn at AD 536. Ice core work is underway to see if the samples of the GISP2 ice core dating to 536 A.D. contain impact ejecta from the Carpentaria craters. We are also working on samples from chevron dunes in Madagascar to see if they contain impact ejecta. [Emphasis added]However, Dr. Abbott's hypotheses have met some controversy:
Other researchers led by U.S. physicist Mark Boslough have dismissed Dr Abbott's theory.Surely some enterprising writer will develop the Beowulf in Space film: A young warrior, twelve retainers, a kick-ass sword, and one chance to save a hearty people from below-average temperatures.
Dr Boslough said if a large impactor had broken up as it approached Earth's surface, the fragments should have 'essentially behaved as one piece', creating just one crater.
He added that Dr Abbott and others had found 'evidence' of more impact events than astronomer's believe are possible.