>represented an analogy for the threat of global nuclear war; however, since 2007 it has also reflected climate change,[4] and new developments in the life sciences and technology that could inflict irrevocable harm to humanity.[5]

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

It has been this way for nearly a decade. It's also plausible that they are linked - climate change destroying natural resources (land, food, etc.) certainly has the potential to inflict political instability and conflict.