[Alas Babylon!]

However, some natural environments, like volcanoes and hydrothermal vents, also release potential toxins.

See, it's not just us. We're actually one supervolcano away from a year without Summer--or double the CO2 parts per million, depending on what gases get belched out.

[Reily]

See 536 AD the worst year ever!

https://en.wikipedia.org/wiki/Volcanic_winter_of_536

The earth it does things!