So, why are these volcanic ridges causing melt now, rather than 300 years ago?

While you're right that there are many variables we haven't considered, and that correlation is not causation, there certainly does seem to be a strong causal link between industrialisation and climate change.

Of course, we could be wrong, and implement all these climate change laws that do nothing to stop ice melt, but if that's the case, what's the downside? We get clean air, more sustainable power sources, etc. There's very little reason not to push for better climate regulations.