Thanos wasn't radical enough.
Average surface temperatures have risen by a few decimal points since the 1880's. If we assume this change is beyond our control, then there's nothing we can do about it.
If we assume this change is primarily man-made, then the level of emission we had in the 1880's was too much, as we already had an increase back then. Thus the "fix" is to reduce emissions to a level we had before 1880. Back then there were around 1.3 Billion people, a fifth of what we have today, so killing 80% is a good start, 50% isn't enough. We also need to do something about consumption, as the remaining 20% are still consuming far too much per person, the solution is that we'll simply have to back to mid 19th century standards of living.
It seems like the only way to actually accomplish that politically is through mass genocide and destruction of of civilization, so all-out nuclear warfare.
