We have passed a grim CO2 threshold — Possibly for good

Posted Sep 28, 2016 by Karen Graham
Despite the fact that September is usually the month with the lowest levels of carbon dioxide in the atmosphere, this year may go down in the history books as being the first time September failed to register CO2 levels below the 400 ppm threshold.
NOAA s Mauna Loa Observatory in Hawaii.
NOAA's Mauna Loa Observatory in Hawaii.
In 2013, scientists at the Mauna Loa Observatory in Hawaii announced that atmospheric concentrations of carbon dioxide had reached a daily average above 400 parts per million for the first time since records had been kept, reports the Huffington Post.
"CO2 concentrations haven’t been this high in millions of years,” said scientist Erika Podest at the time. “This milestone is a wake-up call.” That wake-up call has now arrived, and climate scientists are saying we have reached that magical 400 ppm threshold.
The dashed red line with diamond symbols represents the monthly mean values  centered on the middle ...
The dashed red line with diamond symbols represents the monthly mean values, centered on the middle of each month. The black line with the square symbols represents the same, after correction for the average seasonal cycle. The latter is determined as a moving average of SEVEN adjacent seasonal cycles centered on the month to be corrected, except for the first and last THREE and one-half years of the record, where the seasonal cycle has been averaged over the first and last SEVEN years, respecti
Typically, September is the month when CO2 levels in the atmosphere are at their lowest because all summer long, plants have been sucking the gas up while growing, here in the Northern Hemisphere. But with autumn, plants begin losing their leaves which in turn, decompose back into the atmosphere.
But climate scientists at Mauna Loa say that while the annual process has begun, the atmospheric CO2 levels have remained above 400 ppm. Engadget writes that Ralph Keeling, who monitors C02 levels at the Scripps Institute for Oceanography, states that while there may be the occasional dip in the levels, it is also true that we won't be seeing any monthly levels below 400 ppm this year.
The really disturbing part about all this is that carbon dioxide stays in the atmosphere, sometimes for hundreds or even thousands of years, so we won't see any lower CO2 levels for years to come, and certainly not in our lifetime.
Gavin Schmidt, NASA’s chief climate scientist, told Climate Central that if somehow, our CO2 emissions were to plummet to zero tomorrow, we would not see any appreciable difference in atmospheric CO2 levels for at least a decade. He added, “In my opinion, we won’t ever see a month below 400 ppm.”
What does the 400 ppm threshold mean?
You could say it doesn't mean a whole heck of a lot. Climate scientists say there isn't a great deal of difference in the climate impact of a reading of 395 ppm and 405 ppm. In other words, it is more of a "symbolic" number. But the psychological effects of the 400 ppm number is significant.
Keeling told the Huffington Post in an earlier interview that when the focus is on moving through a threshold, people come to appreciate the meaning more. He said, "I hope people remember this moment so that when they hear the carbon dioxide levels are 420 ppm in a matter of years, they’ll say, ‘I remember when it was 400.’”