I saw that the other day, Jcan. It's a bit difficult to conclusively come up with one year being a completely bad for humanity.
The article itself opens with the claim:
"Ask medieval historian Michael McCormick what year was the worst to be alive, and he's got an answer: "536." ..... In Europe, "It was the beginning of one of the worst periods to be alive, if not the worst year," says McCormick, a historian and archaeologist who chairs the Harvard University Initiative for the Science of the Human Past."
Which itself is a bit confusing, as his main point seems to be 536 was the beginning of bad times within Europe and not outside of it. He mentions some crop failures in China, but doesn't really elaborate on it being some widespread climate change effect, and fails to mention anything about the Americas, South Asia, Australia or subsaharan Africa.