A cold snap in AD 535is called the start of the Dark Ages in a May, 2000 PBS documentary episode of Secrets of the Dead. Since the Medieval Warm Period is put at AD 950 to 1250 that leaves over four centuries of despair, that began with two to five years of brutal crop failure, reduced sunlight, unpredictable rains and floods and cool temperatures.
Events that triggered the “Extreme weather events of 535-536” and the weather of the Dark Ages.
While scientists study the period we will never know precisely what happened, and it may be a confluence of factors. Two probabilities most likely are a volcanic explosion and a comet strike.
Wikipedia mentions but explains poorly the work of Dallas Abbott with Greenland ice cores. Ms. Abbott is a scientist interested in comet strikes. We have heard her postulate that tropical organisms in these ice cores could only have been caused by extra terrestrial strikes in the ocean or very near the ocean and in the tropical belt of the planet. Such an impact would have launched huge amounts of sea water and organisms into the atmosphere and carried them to Greenland to be snowed onto the ice cap.
Many others credit a volcanic explosion, though exactly which volcano is still a source of uncertainty. Some studies point to North American activity, which could mean Mexico, Alaska, Washington State, maybe east central California, or even Yellowstone, and of course possibly Alaska or British Columbia. Other Canadian volcanoes have not erupted in millions of years.
It could even be Mount St. Helens. When that volcano blows, it makes it difficult for geologists to date prior eruptions, because everything got melted and scattered and generally disrupted.
Yellowstone is unlikely. As a super caldera it has not blown for millions of years. A BC region that is studied is the Tseax Cone, thought to have last erupted sometime 17th or 18th century. Wikipedia ranks the event as the 15th deadliest of ALL TIME! Wikipedia authors made this ranking by using known deaths, and the 2,000 deaths they assign to event, around 600 miles north of Vancouver, seems to be just a wag.
But there are more volcanoes than comet strikes, for sure. Still, it is conjecture that a volcano could be big enough to cause such a big disruption in weather as the 535 weather anomaly. But recall that in 1985 a volcano in Columbia South America killed 23,000 people, more than died in the 2011 Japan Tsunami. Less than 100 miles from Bogota, Columbia, Nevado del Ruiz was not really that big, but its eruption and lahar buried the village of Armero.