If this volcano starts to erupt again, then the temperature in Denmark will then drop from + 8-9 degrees Celsius to -2. Everything is explained by the fact that after the eruption comes the so-called volcanic winter.
Most likely, the author suggests, in those distant times after the eruption of the volcano and the onset of the volcanic winter, people lived in the area of Lake Toba. However, we can only speculate how the temperature change affected them. Most likely, when it became warm again, only a few thousand people remained on Earth. That is, it turns out that because of this catastrophe we have become an endangered species.