Earth's clouds are shrinking, boosting global warming

A naive question: why is global warming bad for the earth, especially for the environmentalists? I mean I get it that it will be bad for human, but the biosphere thrived in much warmer pre-historical ages, right? Or rain forests still have the highest biodiversity nowadays, right? For people who hate human activities to preserve a thriving earth, wouldn't they welcome global warming?

On an ecological scale, it isn’t the heat that is a problem, it is the speed it is increasing.

In the past, temperature changes have been slow enough for evolution and ecological systems to adapt, but now it is happening fast enough that these systems can’t adapt fast enough.

Speed. Adaptation to change takes time, fast changes lead to mass extinctions.

And we are not in a stable situation, we don’t know wha will be the new normal, nor when it will be reached, and finding more positive feedback loops like this one put everything in the extreme side of things.