Global carbon dioxide levels in the atmosphere have surpassed 400 parts per million, and will almost certainly remain there indefinitely, according to new numbers from the Scripps carbon dioxide monitoring program at the Mauna Loa observatory in Hawaii.
The 400-level has long been considered a benchmark of irreversible damage to the environment.
"We're really in uncharted territory," said Ralph Keeling from the Scripps Institution of Oceanography, who directs the program. "It's too bad we're this deep into it already, but that's the fact."
The level has swung above 400 parts per million before, but this is the first time it will have stayed that high for all 12 months of the year.
While the year isn't over yet, the month of September almost always has the lowest levels, because it comes at a time when plentiful summer plants in the Northern Hemisphere slow down their carbon dioxide uptake and begin to die off in the fall.