September 20, 2015


By Michael J. McGee


This story is told by numbers.  Earth's CO2 crossover to 400 ppm CO2 started in 2012 and it's not yet complete.  The crossover from the 300s started in the high northern latitudes where the greatest seasonal swings show up in the atmospheric CO2 readings.  And it started with daily averages that are more variable than monthly and annual averages.

The table below is a kind of story teller.  It has 4 years, seven numbers, seven time periods, two locations and two scientific institutions.  Some of these elements are more important than others.  This small table tells a short story that provides a chance to gain some interesting and useful insights into the way the earth system works.  Atmospheric readings for CO2 at one or two locations are globally and regionally signficant.  With atmospheric CO2, you don't need a million data points to get an earth systems story.


Selected Milestones from Earth's
Crossover to 400 ppm CO2

ppm = parts per milion

Year Date Milestone for Atmospheric CO2 Data
2012 April 1st monthly average >400 ppm anywhere* NOAA-Alaska: 400.01 ppm
2013 May 9 1st daily average >400 ppm at MLO

NOAA Media: 400.03 ppm

Scripps Media: 400.08


May 26-Jun 1

May 25-31

1st weekly average >400 ppm at MLO

NOAA: 400.01 ppm

Scripps: 400.13 ppm

2014 April 1st monthly average >400 ppm at MLO

NOAA: 401.29 ppm

Scripps: 401.33 ppm

2015? Full Year 1st annual average >400 ppm at MLO

To be confirmed (or
not) in January 2016

MLO = Mauna Loa Observatory
* Data from other northern latitudes has not been checked (e.g. Denmark and Finland). 
It is posible that the first monthly average may have been recorded at a monitoring site other than Barrow, Alaska.

We're still waiting to see what happens when Scripps and NOAA post the annual average for 2015.  That could happen in the first couple of days or weeks of 2016.  But even if the 2015 average surpasses 400 ppm, Earth's 400 ppm CO2 crossover is not over.  The 300s have started to dissapear from all the Mauna Loa records.  But for a few years still, they will continue to show up in some of the records at the more northern latitudes.

In time, the 400 ppm CO2 crossover may happen in the other direction.  It is not known whether or when that will happen.  If it does, crossing back to levels below 400 ppm CO2 will take much longer.  This is because the natural rate of decline is less than the current rate of human-caused increases.