The last time below 400?
In the late 1950s, professor Charles Keeling of UC San Diego/Scripps Institute established a technique for measuring the abundance of carbon dioxide in the Earth’s atmosphere. To begin understanding how CO2 varied across the world, they took measurements in several locations and selected Mauna Loa, a volcano in Hawaii with a peak that reaches high into the atmosphere and a substantial distance between it and developed continents as a location where they could measure a well-mixed, average sample of the atmosphere.
They began measuring CO2 abundances on Mauna Loa in 1958; at the time they measured about 310 parts per million of CO2 at that site. Within only a few years, that measurement demonstrated the main changes that happen in atmospheric CO2; a yearly variation of about 6 ppm on top of an annual increase of about 2 ppm.
The yearly variation reflects the growth of vegetation; the northern hemisphere has much more land area than the southern hemisphere so during northern hemisphere summer when trees are blooming and organisms are reproducing, CO2 drops, but all of that CO2 is re-released during northern hemisphere winter. The annual increase on top of it was comparable to the amount of CO2 released by humans through burning fossil fuels for energy and changing land use patterns.
This measurement has continued multiple times per day on Mauna Loa for nearly 50 years and during that time CO2 has marched relentlessly upwards due to human activities. In 2013, CO2 levels on Mauna Loa crossed 400 ppm for the first time; an increase of nearly 1/3 since Dr. Keeling’s first measurements and an even larger increase compared to the atmosphere prior to the industrial revolution and wholesale burning of fossil fuels.
The plot of these CO2 measurements over time is now known as the Keeling Curve in honor of its creator. In fact, today, Dr. Keeling’s work continues to be run out of Scripps Institute and the current head of that program is Dr. Ralph Keeling: Charles Keeling’s son.
Last week, Dr. Ralph Keeling proposed that we’re about to see another fascinating milestone on this plot; the last time it goes below 400 ppm.
CO2 is going up by about 2.2 ppm per year right now. It first reached 400 ppm during late winter in 2013 (http://on.fb.me/1LVRS9l), meaning by 2016, CO2 levels will have increased by over 6 ppm since that point. The yearly variation in CO2 levels is about 6 ppm, so that would put it right on schedule.
Furthermore, 2015 is showing the formation of a strong El Niño. The last strong El Niño, in 1998, was associated with a larger-than-normal jump in CO2 contents during the subsequent summer due to drought conditions in the tropics that reduced growth of plants during the summer. If that pattern, which is expected as the typical El Niño weather pattern, holds in 2016, CO2 will not drop as far as normal next summer and the daily averages may never cross below 400 again.
There is no official global significance to the number 400 ppm. The planet only recognizes that there is 2 more ppm CO2 in the atmosphere than there was last year and that CO2 acts to absorb more of the heat radiating away from the surface. 400 ppm is only a milestone number because of how we count numbers. That said, it’s disconcerting to realize there is now a good chance we have permanently crossed 400 ppm CO2.
Over the long term, geologic processes will work to push CO2 back down. When there is an atmospheric CO2 spike, the planet warms up and as a consequence the rate of weathering of rocks increases (chemical reactions happen faster when the temperature is higher). Weathering of rocks releases components like calcium and other nutrients for life, allowing life and ocean chemistry to gradually push CO2 contents back down. This process, however, takes tens of thousands to perhaps hundreds of thousands of years to happen.
The world will eventually get rid of the CO2 pulse released by humanity, but unless we either find a way to actively remove CO2 from the atmosphere, CO2 contents will never drop below 400 ppm in the lifetime of anyone reading this post.
CO2 tends to go up by about 1.5 ppm per month during northern hemisphere fall and as I’m writing this post CO2 contents are at just over 398 ppm. CO2 contents should cross 400 ppm in either late November or early December, and it is possible that will be the last time any human alive ever sees atmospheric CO2 below the 400 ppm mark. I’ll watch next summer to see if it drops below 400 again; if it does, then next summer will be all but guaranteed to be the final time below 400 due to the never-ending burning of fossil fuels.
-JBB
Image credit: Scripps https://scripps.ucsd.edu/programs/keelingcurve/
References: http://bit.ly/1WgyQ20 http://news.bbc.co.uk/2/hi/science/nature/7120770.stm