The old saying that "what goes up must come down" doesn't apply to carbon dioxide pollution in the air, which just hit an unnerving milestone.
The chief greenhouse gas was measured Thursday at 400 parts per million in Hawaii, a monitoring site that sets the world's benchmark. It's a symbolic mark that scientists and environmentalists have been anticipating for years.
See how I combined CO2 levels and helium? It's like a chemistry and economics lesson on elements 2, 6 and 8 all in one post. You're welcome.