They claim it's completely human caused, and that's clearly false, because humans didn't even exist during these events, and haven't produced even 1% of the CO2 one big volcanic event did in all of human history.
Here you are breaking the fundamental laws of logic. Just because something was caused by event A in the past, it does not mean it cannot be caused by a completely different event B in the future.
That the current GW is rather mild in comparison to earlier GWs is true, but back then, no human civilization existed. It's the cilivization that is running into big problems with adapting, not the planet itself.
That the current GW is caused by anthropogenic carbon emissions (it's not just CO2, but also methane and others) is as-good-as-proven (>99% probability).
What is not proven, and this is what alarmists get wrong all the time, is how the warming will continue. This we simply don't know for sure. Prognosis and extrapolations are very error-prone.
However, as the events are potentially catastrophic, the precautionary principle has to be applied nevertheless.
And the question which measures are adequate and which are not is as also very difficult, as also @lauch3d explained.