It's worth considering whether it's better to have a warmer planet than a cooler one. As a species, we seem to gravitate to warmer climates and do better there - a few scattered people can survive in a frozen tundra but mostly it's a struggle. Same for other species as well - all the life (both volume and variety) is where the warmth is.
As the population of our planet grows, we need fertile productive land to produce food. If the large land masses where this grows is colder and frozen, there will be mass starvation.
Warmer and more carbon = more greenery, more trees, more food, more life.
So hotter or cooler? Which is really the devastating one?