[Conan the Librarian]

Could be this too?

Extreme Weather Events of 535 AD.
https://en.wikipedia.org/wiki/Extreme_weather_events_of_535%E2%80%93536

[Fred Nerks]

from your link:

David Keys' book speculates that the climate changes may have contributed to various developments, such as the emergence of the Plague of Justinian, the decline of the Avars, the migration of Mongolian tribes towards the West, the end of the Sassanid Empire, the collapse of the Gupta Empire, the rise of Islam, the expansion of Turkic tribes, and the fall of Teotihuacán.[11] In 2000, a 3BM Television production (for WNET and Channel Four) capitalized upon Keys' book. This documentary, under the name Catastrophe! How the World Changed, was broadcast in the US as part of PBS's Secrets of the Dead series...

[21twelve]

I wonder what a series of huge (serious of hugh) methane ice burps would do? Large changes in weather, release of organics, etc.