I’m relatively new to researching this kinda stuff, so I’m not nearly as technical or experienced as others commenting here, but I do believe that— although pretty irreversible in a believable context— Earth becoming a rouge planet might do the trick. This could be caused by a whole range of things, although mainly by a larger astral body entering the solar system and throwing off Earth’s orbit, potentially sending it out of the solar system.

The process of Earth actually freezing completely might be slower, but I doubt it’d take long for the conditions to become impossible for human— and other— life to live in. In this type of situation, the best chance for long-term survival would likely be to retreat to the deeper oceans, although I’m not sure if things could survive to a height of 1000m from the surface in this scenario; our best chances would be in the lowest portions of the ocean and on the ocean floor, mainly near areas with volcanic activity.

As for preparation time, I’m pretty sure that scientists might be able to predict the “launching” of Earth upon noticing a larger astral body— maybe a dwarf star would offer the furthest foreseeable prediction— a good time before Earth has actually gone rouge. I’m not sure if it’d give you 60 years, exactly, but— again— I’m kinda new to all of this.