There's a lot of bad things that happen at 3C.

The first is that 3C represents a lot more energy in the atmosphere. That translates to more water evaporating from the oceans creating bigger more violent storms (think more frequent flash floods).

It changes the ocean currents which can be really bad. Right now Europe is warm for it's latitude because of a weakening current from the equator to the UK brings a lot of heat. If that completely collapses, Europe can enter an ICE age.

The rising temperature also ends up weakening the vortex of the north pole which mostly keeps the arctic temperatures sealed up north. As that vortex weakens, spills of crazy blizzards can hit unusual places pretty hard. The winter storm in 2021 is an example of that happening.

Then of course there's the potential melting of the ice caps which will release a lot of methane into the atmosphere (speeding up warming). That will ultimately cause sea levels to rise which won't be great for the state of Florida.

Mass migration, crop instability, more frequent and more extreme weather. It's just a combo of bad things that all come together at once.