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 (see below) as a result of the city’s terrorist attacks of November 13.


Figure 1. Hourly and daily averages of atmospheric carbon dioxide as measured at Mauna Loa Observatory for the week of November 12-18, 2015. Image credit: Scripps/The Keeling Curve.

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.”


Figure 2. CO2 measurements from Mauna Loa Observatory for (top to bottom) the past six months, the past two years, and since the observatory was established in 1958. The final panel shows the Mauna Loa record juxtaposed with CO2 readings deduced from air trapped in ice cores. The ice-core evidence shows that carbon dioxide waxed and waned with a number of ice ages, but the current values near 400 ppm are far greater than any peaks observed in at least the last 800,000 years--and probably much further back than that. Image credit: Scripps/The Keeling Curve.


What next?
Figure 1 shows that the hourly readings at Mauna Loa can vary quite a bit. It’s possible we’ll see more days this month with hourly readings dipping below 400 ppm, as they did on November 18. However, even these hourly readings should remain firmly above 400 ppm within a few weeks. If Keeling’s prediction is accurate, daily readings may stay above 400 ppm in 2015 and for many years thereafter. It’s also still possible that a few hours or even several days might manage to dip just below 400 ppm in mid-2016.

There are other CO2 measurement facilities around the world, although the Mauna Loa record is the one most commonly cited, with its high quality, pristine location, and longevity (Keeling’s father, Charles David Keeling, began regular measurements there in 1958). Different measuring sites will see the 400-ppm mark in their rear-view mirrors at slightly different times, due to local atmospheric variations, but there is no doubt where the global atmosphere is headed. I asked Ralph Keeling on Wednesday for his latest thoughts.

“It's too early to be 100% certain, but I agree that it's starting to look like we are already over 400 ppm for this year, with the last daily and weekly values below 400 ppm occurring earlier this month,” Keeling said. “It also looks like the November monthly average will also be above 400 ppm.”

Climate-change deniers and contrarians typically look beyond CO2 measurements when crafting their talking points, but a few misunderstandings about the role of carbon dioxide still crop up--many of them skillfully rebutted by the website skepticalscience.com. “A possible misconception about the Mauna Loa CO2 record is that the overall increase is influenced by emissions from the volcano,” Keeling told me. “In fact, the volcanic effects are very small and are easily filtered out, like static on a radio signal. Dozens of stations around the world show essentially identical long-term trends, including a record from the South Pole also going back to the 1950s.  The CO2 at these stations might be a little higher or lower than Mauna Loa in a particular season or averaged over the calendar year. But the upward trends are all pretty similar. The rise is therefore clearly a global phenomenon.”

What it means for the Paris climate talks
The 400-ppm news comes just as Earth is experiencing a heat wave fueled by the long-term rise in greenhouse gases and goosed by El Niño. NOAA announced on Wednesday that global temperatures in October 2015 showed the largest departure from the long-term average for any month going back to 1880. The UK Met Office now predicts that global temperature in 2015 will likely end up at least 1°C warmer than the preindustrial average. This would put our planet halfway to the 2°C warming that’s long been viewed by many scientists and policy experts as a level that significantly raises the odds of major climate disruption (although there is nothing magic about 2°C; a smaller rise could still have serious consequences). A brief video from the journal Nature, released on Thursday, serves as a quick guide to the origin and significance of the 2°C goal.

All of these happenings underscore the importance of the two-week-long meeting in Paris that begins on November 30. This is COP21, the 21st annual Conference of the Parties to the United Nations Framework Convention on Climate Change, which was signed in 1992 and ratified by the United States and all other UN members. There has been an unprecedented level of cooperation among the world’s leading carbon-emitting nations in the lead-up to COP21, which raises the odds that a workable agreement for carbon reduction just might be achievable. In contrast to the Kyoto Protocol, which failed to gain support from the world’s two largest carbon emitters--the United States and China--it’s expected that any deal arising from Paris will hinge on voluntary commitments (dubbed “intended nationally determined contributions”, or INDCs) that have already been submitted by more than 160 nations representing more than 90% of global emissions. In an upcoming post, we’ll take a closer look at what to expect and what to watch for as COP21 unfolds. WU climate blogger Dr. Ricky Rood has already filed a series of posts that lay out important context ahead of the Paris meeting. I attended the ill-fated Copenhagen meeting in 2009 (COP15) and came away profoundly discouraged at the lack of progress there. This time around, I am cautiously optimistic that a truly global deal will be struck, although I’m not confident that it will be enough to prevent 2°C of warming.


Figure 3. One of the major protest marches held during the Copenhagen climate summit in December 2009. Image credit: Bob Henson.

In wake of attacks, a global climate march gets new attention
Several experts involved with the upcoming UN meeting have speculated that global leaders now have even more incentive to make the talks a success in the wake of the deadly November 13 attacks in Paris. At the same time, the French government announced on Wednesday that massive climate marches planned for Paris on November 29 and December 12 would not be allowed, due to the heightened state of alert following the deadly attacks of November 13. Organizers led by Avaaz.org and 350.org have responded by intensifying their efforts toward a Global Climate March, scheduled for the weekend of November 28-29. More than 2000 events are on tap in towns and cities around the world. You can see what’s happening near you by using the interactive tool at this Guardian article.