WASHINGTON (AP) - 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.
While this week's number has garnered all sorts of attention, it is just a daily reading in the month when the chief greenhouse gas peaks in the Northern Hemisphere. It will be lower the rest of the year.
But not for long - the trend is going up and at faster and faster rates.
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