So, as far as a casual one on one conversation, I don't know. But some sort of formal petition to the public or a person of power, I would look for known geological activities like earthquakes and volcanic eruptions before going back in time. Weather events are too fickle. We can't really affect geological events though.

It looks like Mount Iliamna had a known eruption in 1876. You could spend a few months getting attention before it happens. The same way people who give very specific doomsday predictions do. This way, once the eruption happens everyone will be certain you're telling the truth. Or at least, most people won't think you're crazy. They may still be skeptical, but this will be enough to get people to lend you and ear and take warnings seriously. It is probably too early in the industrial revolution to really get people to slow down their progress if you want to stop climate change. A similar stunt around the time the first suppressed reports of climate change happened would be better at that.

Holocene eruptive activity from Iliamna is little known, but radiocarbon dating seems to indicate at least a few eruptions, all before the European settlement of Alaska. Prehistoric eruptions have been dated to 5050 and 2050 BCE (VEI-4), 450 BCE and 1650. Historically observed eruptions took place in 1867 (VEI-2) and 1876 (VEI-3), with unconfirmed eruptions in 1933, 1947, 1952 and 1953.[7]