There's more methane in the atmosphere now. It probably smells like a fart.

Cannabis. At least most major cities in Europe/North America I find it really common now to openly smell cannabis all hours of the day. Combination of the strains being MUCH stronger and legalization. Even just 20 years back, of course in the Haight in SF or certain parts of NYC you'd smell it, or outside clubs/bars at night. But today I walk through Downtown SF at 830am and smell it every other block. Was in the design district in NYC a few weeks back and same deal.

People from the 40s would recognize the current smell of the world.

I'm hoping car exhaust takes that role.