Carbon Dioxide Bidding Farewell to 400-ppm Benchmark

On an otherwise unremarkable day last week--November 11, 2015 (noted mainly for being Veterans Day in the U.S.)--a crucial milestone in global climate was quietly transcended.  The daily average concentration of carbon dioxide in the air that day at Hawaii’s Mauna Loa Observatory was 399.68 parts per million.  On November 12 it rose to 401.64 ppm, and it’s quite possible that we’ll never see another day in our lives with the daily Mauna Loa CO2 reading below 400 ppm.  Greenhouse gases have been building in our atmosphere for more than a century, so this news doesn’t come as a shock so much as a reminder of what our continued use of fossil fuels is doing to the atmosphere.  The data also serve as a prelude to the upcoming United Nations climate talks in Paris, which face some unexpected obstacles ... as a result of the city’s terrorist attacks of November 13.

If it seems like you heard the news about the atmosphere reaching 400 ppm quite a while ago, you can attribute your deja vu to the seasonal cycle.  Figure 2 (below) shows what’s been going on.  The 400-ppm mark was first reached in May 2013--but only for a few days, during the annual peak of atmospheric CO2.  Along with the year-on-year rise due to fossil-fuel use, CO2 ebbs and flows in the atmosphere each year as vegetation grows and dies back in the Northern Hemisphere (where the majority of the world’s plant life is located).  In 2014, the daily Mauna Loa readings stayed above 400 ppm for more than three months.  This year they rose above 400 ppm even longer, again dipping below 400 ppm in August before climbing back above the benchmark this month.

Ordinarily, we might expect one more northern summer with CO2 values below 400 ppm, but El Niño could prevent that.  A strong El Niño event, like the one now under way, tends to produce drought in some of the world’s most heavily forested areas, such as Indonesia. Averaged across the globe, this temporarily reduces the total amount of CO2 soaked up by Earth’s vegetation. In addition, the large fires common in drought-stricken areas pour even more CO2 into the air.  Based on this prospect, Ralph Keeling, who directs the CO2 measurement program at Mauna Loa for the Scripps Institute of Oceanography, made a fairly bold prediction on October 21:  “By sometime in the next month or two, CO2 will again rise above 400 ppm.  Will daily values at Mauna Loa ever fall below 400 ppm again in our lifetimes?  I’m prepared to project that they won’t, making the current values the last time the Mauna Loa record will produce numbers in the 300s.”

Read more at Carbon Dioxide Bidding Farewell to 400-ppm Benchmark