About 55 Million years ago the earth went through a rapid warming phase, somewhat similar to what is happening right now. It was triggered by massive releases of Carbon dioxide, probably of volcanic origin but research is still continuing.
Life did not die out completely but seas were MUCH higher and palm trees grew in the arctic. This event happened about 10 times more slowly than the current human carbon dioxide injection and there were no people around.
Here’s a link to a recent National Geographic article with more details
Santa Cruz Climate Action Network 