The Paris pact was pretty toothless: it was a step forward, but heavily aspirational. Indeed, one of the stated complaints of conservatives was that it did nothing to enforce compliance. The Trump Administration could have just as well ignored it without formally reneging on it and forced the next administration to deal with the egg on the face for failing to meet the USA's commitments.
But instead he's seriously undermined people's ability to trust in the United States maintaining its international obligations for anything except the shortest periods of time. To be fair, what he's done already with NATO is far worse. But this underlines the reality that the United States government is so unstable that it's unable to uphold any coherency in policy or meet its stated commitments.
If you're a small country who's been trusting the USA as a solid ally against regional competitors, you're looking at this and going, "well, maybe I should be focusing more on ties with the EU/Russia/China/India."
It's honestly pretty humiliating for the USA, even aside from the fact that we've just fucked the next generation just to give the finger to elitist liberals and scientists.