If you’re ever despairing about the state of the world, you can — at least, according to some scholars — be thankful it’s not the year 536 CE. To be fair, it’s medieval scholars, not 21st-century ones, who called 536 CE the worst year to be alive. But hear them out, because it sounds pretty bad. That year, a massive volcano erupted, historians believe, filling the air with volcanic ash. Of course, the majority of people affected by the disaster had no idea what was happening — they just knew it was very suddenly very dark for a very long...