CO2 levels in the northern hemisphere climb during the winter and begin to taper off during the spring as plant life becomes active. Then, it will begin to drop during the summer before rising again in the fall as the plant life goes dormant.
Following this pattern, CO2 levels, as measured at Mauna Loa, have reached their maximum for this year and are beginning to fall. Take a look at this plot for the last two years (plots with other times frames can be found here).
|Source: Scripps Institute|
There are several things to note, beginning with the monthly average for May (the last open circle on the right). Reading from the graph, the May average was about 403.8 ppm. It was only May of 2013 that we passed the 400 ppm level at all, and that was just daily readings (small dots). Check out the daily readings now and you can see the maximum daily readings were just under 405 ppm.
Interpolating the data, we can project that June and July will have monthly averages above 400 ppm. I think August will get close but stay below that level. April of last year was the first month to average over 400 ppm and there were three months total last year with an average surpassing that benchmark. This year will have at least seven.
Seven? You might be thinking, "Wait a minute. I only count six."
Don't forget to count December. The level will start rising again in the September/October time frame. The rate of increase is about 2 ppm per year. November 2014 was right at 397 ppm, so it will probably stay below 400. December, though, was at about 398.5 and will surpass 400 ppm this year.
Seven months with averages over 400 ppm this year.
Projecting into 2016 we can see, in addition to the months that exceeded that level this year, January; August; and November will exceed that limit. September and October will be just a little below 400 ppm next year but they will surely cross the line in 2017.
What that means is 2016 is probably the last year we living today will ever see CO2 levels below 400 ppm ever again.