Reading Theo Gray's Mad Science I found that silver never tarnished before the Industrial Age. Now that power plants have put more sulfur in the atmosphere, silver will react with that to form black silver sulfate. Back in the pirate days (see previous post), bars of silver would sit out and not tarnish. But now we've changed the chemistry of our atmosphere and silver tarnishes.
This doesn't even necessarily say anything specifically about global warming. But it does say we can change the chemistry of the world, which at the very least is something to consider.