There have been many disasters in human history, such as plague epidemics, mass famine, destructive wars, influenza pandemics and others, which posed a threat to the entire human civilization.
But the worst year is probably AD 536. At least that's what Professor Michael McCormick, an archaeologist and medieval specialist at Harvard University, says.
In 536, all of Europe, the Middle East and part of the countries of Asia plunged into an 18-month twilight, which almost hid the Sun, caused, possibly, by strong smoke from an unknown source.
In China, this immediately caused heavy snowfalls, and then crop failures, hunger, disease, and drought followed. In other countries that fell under dusk, the same was observed. In general, almost the entire northern hemisphere of the Earth was covered.
According to McCormick, the smoke was most likely caused by the eruption of a large volcano in Iceland and the countries of the northern hemisphere recovered from the consequences of this cataclysm only by 640.
The countries seemed to be shrouded in a gloomy fog, which stood for 18 months and people practically did not distinguish between days and nights. The impact on the climate was so severe that the Irish sagas noted that in 536-539 people did not eat bread at all.
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In the summer of 536, temperatures dropped by an average of 1.5-2.7 degrees Celsius, and this seemingly small drop was the cause of the coldest decade in the last 2300 years. Therefore, this period is also sometimes called the "ten-year winter".
Basically, historians call this period the literal Dark Ages (usually this term means the general cultural lag of Europe in the 6-10 centuries from Byzantium, the Muslim world and China). And the reasons for this blackout still remain a mystery. The volcano version is just one hypothesis.
For the first time, archaeologists faced this mystery when studying the annual rings of trees, finding out that in a certain period of the 6th century AD, the temperature suddenly dropped dramatically for several years, and this was observed in many countries.
Professor McCormick of Harvard and glaciologist (ice specialist) Paul Majewski of the University of Maine's Climate Change Institute are confident that they have found the answer to this mystery and it is all about the Icelandic volcano. A study of ice core samples showed that Iceland had two major volcanic eruptions in 540 and 547. This is, of course, later than 536, but very close to it.
When the Eyjafjallajökull volcano woke up again in April 2010 in Iceland, the smoke was enough to severely restrict almost all European flights for the period of late April and early May. But this eruption was relatively weak.