Did concentrations of carbon dioxide (CO2) exceed 400 parts per million at Mauna Loa?
Milestones have a way of focusing our attention. They are symbolic. For example, racing a mile in under four minutes is somehow more meaningful than racing a mile in 4:00:29.
And so the announcement last week that CO2 concentrations at the Mauna Loa Observatory in Hawaii had surpassed the 400 parts per million (ppm) mark over a 24-hour period that ended last Thursday at 8 p.m. EDT proved to be quite newsworthy. Headlines splashing across the front door of major news outlets sounded a similar theme:
- “Heat-Trapping Gas Passes Milestone, Raising Fears“
- “Atmospheric carbon dioxide reaches 400 parts per million concentration milestone“
- “What’s in a Number? New Carbon Dioxide Level Unseen in Human History“
- “CO2 levels hit new peak at key observatory.”
(The Wall Street Journal took a different tack with an opinion page piece just days before entitled "Harrison H. Schmitt and William Happer: In Defense of Carbon Dioxide.”)
The media hubbub was rightly so, I suppose. The Mauna Loa Observatory is the site of the oldest near-continuous record of remote CO2 observations we have. That record dates back to 1958 when Charles Keeling began monitoring atmospheric CO2 concentrations and first documented that they were indeed on the rise.
So I guess this can be viewed as a real wake-up call about climate change. Well, yes and no.
The CO2 Daily Mean: A Number (Somewhat) in Flux
After that surge in news coverage last week, yesterday we learn that the CO2 at Mauna Loa might not have actually surpassed 400 ppm last week. It turns out that there are two independent programs stationed at Mauna Loa monitoring CO2. One is operated by the National Oceanic and Atmospheric Administration (NOAA) and the other by the Scripps Institute of Oceanography, Keeling's scholarly home.
On Monday NOAA noted in its data records that it had revised its measured concentration for Thursday downward to only 399.89 ppm. Such revision is routine for CO2 data; as explained on the NOAA website, ”These data are still preliminary, pending recalibrations of reference gases and other quality control checks.” (Note it did not amend or otherwise revise its press release of May 10.)
So, phew, close, yes, but we avoided passing the dreaded 400 ppm mark, right? Saved by a mere 0.11 ppm. Guess we can cancel last week's wakeup call, right?
Of course not; that's taking this whole milestone thing a little too seriously. We shouldn't need a 400 ppm measurement at Mauna Loa to wake us up to climate change.
News of CO2 Above 400 ppm Is a Year Old
Sure, data from Mauna Loa holds special symbolic meaning since that's where Keeling first set up shop and it’s the oldest instrumental record we have, but it's not the only place where we measure CO2. Another site is Barrow, Alaska. Way back on May 31, 2012, not quite a year ago, NOAA announced that:
"[t]he concentration of carbon dioxide in the atmosphere of Barrow, Alaska, reached 400 parts per million (ppm) this spring, according to NOAA measurements, the first time a monthly average measurement for the greenhouse gas attained the 400 ppm mark in a remote location."
And those Alaskan measurements, it turns out, were not alone -- the 400 ppm threshold was crossed in Greenland, Norway, Iceland and Mongolia.
In other words last week’s announcement that CO2 exceeded 400 ppm at Mauna Loa is pretty much old news.
And Then There Are CO2 Equivalents
As I bet you know, CO2 is not the only greenhouse gas we put in the atmosphere. In addition to CO2, for example, the Kyoto Protocol recognizes methane, nitrous oxide and hydrofluorocarbons, and uses the term CO2 equivalents to assess the global warming impact of all of the gases combined.
And when it comes to CO2 equivalents, crossing the 400 ppm mark is old hat -- actually, a very old hat. That milestone was crossed about 20 years ago. Today in CO2 equivalents, the milestone we are now flirting with is not 400 ppm but 450 ppm to 475 ppm. (For more on these marks as well as a discussion of dueling uses of the term CO2 equivalents, see this RealClimate post.)
We Have Air Pollution to Thank?
The one saving grace, if you want to call it that, is that aerosols and particulate matter, the stuff we normally associate with air pollution, reflects sunlight and provides enough cooling to cancel out much of the warming caused by the non-CO2 greenhouse gases bringing us back, when all is said and done, to the 400 ppm mark.
Now there's a dubious milestone to celebrate, thanks to air pollution, we can keep the focus on 400 ppm instead of 450 ppm ... for the time being.
Time history of atmospheric carbon dioxide from 800,000 years ago until January, 2012.