What are you supposed to say to someone who believes climate change is real but won't accept that human activity contributes to it?

Just show them that one xkcd about how abrupt the current warming is and how nicely it lines up with industrialization. The comparison between the natural slow shifts in global climate vs. what’s been happening the last century or so is the clearest counter to “but the climate has always been changing”.

The other times that things have changed in comparable timescales, it’s been caused by supervolcanoes or other such calamities.

Edit: Oh yeah one caveat: The xkcd is ten years old and we’re now actually already around +1.5°C.

You’re not going to convince anyone with a shitty drawing of the “Earth Temperature Timeline” and then start at 20000BC.