What destroyed the earth in the Netflix movie The Midnight Sky?

The film is now number 1 among the German trends on Netflix and it is also relatively good. But somehow I didn't realize how they managed to destroy the earth within a couple of days. Can someone tell me? Atomic bombs or something?

Unfortunately, what happened is not explicitly explained, neither in the film nor in the book. But the film provides a few clues. For example, that people are to blame or have caused it. In addition, one hears of extreme levels of radioactive radiation in the air, which spread from large cities across the entire planet. The people who are still left are sick and spend their last weeks in shelters. This suggests that the "event" was due to either an environmental disaster or a nuclear war.

Great thank you