September will be remembered as the month when atmospheric carbon dioxide levels permanently exceeded the symbolic limit of 400 parts per million, “never to return below it in our lifetimes,” Climate Central reported earlier this week.
“Is it possible that October 2016 will yield a lower monthly value than September and dip below 400 ppm? Almost impossible,” wrote Ralph Keeling of the Scripps Institute for Oceanography’s CO2 monitoring program. “Brief excursions toward lower values are still possible, but it already seems safe to conclude that we won’t be seeing a monthly value below 400 ppm this year—or ever again for the indefinite future.”
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September is usually the month when CO2 levels hit their annual low, “after a summer of plants growing and sucking it up in the northern hemisphere,” Climate Central notes. “As fall wears on, those plants lose their leaves, which in turn decompose, releasing the stored carbon dioxide back into the atmosphere.” So the trend for the months ahead should be upward, not down.
2016 is also almost certainly to be the hottest year on record: “The planet has edged right up against the 1.5°C/2.7°F warming threshold, a key metric in last year’s Paris climate agreement,” notes correspondent Brian Kahn.
Even if emissions ceased tomorrow, the carbon we’ve already loaded into the atmosphere would keep concentrations in the 400 ppm range for decades.
“At best (in that scenario), one might expect a balance in the near term, and so CO2 levels probably wouldn’t change much—but would start to fall off in a decade or so,” NASA Chief Climate Scientist Gavin Schmidt told Kahn. “In my opinion, we won’t ever see a month below 400 ppm.”