Photosynthesizers (plants, algae, etc.) have chloroplasts that selectively fix lighter isotopes of carbon. Therefore, the act of burning their remnants (logs or in this case fossil fuels) releases CO2 with a mass that is a neutron or two lighter than the CO2 coming from other sources, including volcanoes.
The industrial revolution really got going around 1850. This correlates both with the beginning of our CO2 climb and with the dilution of our Carbon-13 percentage in the atmosphere as Carbon-12 percentage has risen.
So we have a so-called diminishing "C13/C12 ratio" for atmospheric CO2. That's new to this period. How can we know that? Because even though we never bothered to detect atmospheric CO2 masses before the last few decades, nature had all the while been producing fixed and dated records, waiting for us to become keen to notice.
Specifically, this can be well dated by the carbon make-up of tree rings and corals. These are organisms that build durable structures which can be read, almost like consecutive pages of a historical record, where instead of alphabets and syntax we see the chemical make-up of the environment as it existed when those structures/layers were built.
The record for trees that are aged to hundreds of years or older show a very stable C13/C12 ratio prior to ~1850. And then a sudden, consistent shift with C13/C12 lowering ever since. Perhaps there are other explanations than mankind, but volcanoes at least don't fit the isotopic facts.
Meeeeeeeanwhile: even if a person rigidly denounces anthropogenic climate change despite the evidence, being good to the planet "just because" also happens to be responsible. It also fits all kinds of identities, whether you're a left-leaning übercrunchy hippie believing it because your healing crystals said so or a right-leaning evangelical aiming to be a good steward of god's creation. It's also a philosophy that keeps our oceans low in plastic, our cities low in smog, and our drinking water low in toxins. Even if you disagree on the cause, caring (even caring A LOT) isn't that controversial.