We are now approaching the annual low point in the Mauna Loa CO 2 curve, which typically happens around the last week of September but varies slightly from year to year. Recent daily and weekly values have remained above 400 parts per million. From this it’s already clear that the monthly value for September will be above 400 ppm, probably around 401 ppm. September is typically but not always the lowest month of the year.

The low point reflects the transition between summer and fall, when the uptake of CO 2 by vegetation weakens and is overtaken by the release of CO 2 from soils.

Is it possible that October 2016 will yield a lower monthly value than September and dip below 400 ppm? Almost impossible. Over the past two decades, there were four years (2002, 2008, 2009, and 2012) in which the monthly value for October was LOWER than September. But in those years, the decrease from September to October was at most 0.45 ppm – which would not seem to be enough to push October values below 400 ppm this year. The monthly value for October will therefore almost certainly also stay above 400 ppm and probably will be higher than 401 ppm. By November, we will be marching up the rising half of the cycle, pushing towards new highs and perhaps even breaking the 410 ppm barrier.

Concentrations will probably hover around 401 ppm over the next month as we sit near the annual low point. Brief excursions towards 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.