[bryanlarsen]

You're asking the wrong question. There is no baseline, no natural temperature. The "baseline" temperature has and will change over millions of years.

The right question is about the rate of change. If the temperature changes slowly, ecosystems adapt and migrate. If it changes quickly, they cannot.

Changing a degree or two over ~10,000 years appears to have happened relatively often. This causes some species loss, but in general the ecosystem thrives.

Changing a few degrees over hundreds of years is a leading theory of the cause of the Permian-Triassic extinction event, the largest extinction event since the oxygenation of the atmosphere.

Changing a few degrees over decades? We're going to find out.