Where were you on November 11, 2015?

It was only a couple weeks ago, but already you may not remember. It was a humming but unexceptional news day. In the United States, protestors at the University of Missouri had just successfully ousted their president. Marco Rubio had done fine at the fourth Republican presidential debate, held the night before. And much of Europe and the English-speaking world were honoring Armistice Day.  

In two or 10 years’ time, we might recognize that Wednesday as world-historic. On November 11, the Mauna Loa Observatory in Hawaii—which produces some of the most accurate and longest-running measurements of atmospheric carbon in the world—recorded that, of every million molecules in the atmosphere, 399.68 were carbon dioxide. On the following day, November 12, it measured 401.64 parts per million of carbon dioxide. Since then, its measurement of the amount of carbon in the atmosphere has waxed and waned, but always stayed above 400 ppm. (On Monday, they read 400.15 ppm.)

Carbon dioxide is the great engine of climate change: The product of fossil-fuel emissions, it traps heat in the atmosphere, warming the globe and wrecking the weather; and it dissolves in the seas, acidifying the ocean and strangling marine ecosystems.

Two Wednesdays ago, we sailed past a carbon milestone of perhaps geological scale. With CO2 levels above 400 ppm, atmospheric carbon in our era is now higher than it’s been in at least one million years—and perhaps in 25 million years. One of the world’s most respected climatologists now says that November 11 could be the last day of our lifetimes in which atmospheric carbon stays below 400 parts per million.

That climatologist is Ralph Keeling. Keeling is a professor at the Scripps Institution of Oceanography at the University of California San Diego, and he directs the program which oversees the Mauna Loa readings.  

Keeling’s name was famous in climate circles before his career even began. In 1958, his father, Charles David Keeling, began overseeing daily measurements of atmospheric CO2 at both Mauna Loa and at an American science station in Antarctica. Keeling’s precise, continuous measurements were the first to certify that atmospheric CO2 was increasing over time. We now call the chart that shows this rise in CO2 the Keeling Curve.

Why are we hitting the permanent 400 ppm mark now? As the younger Keeling puts it, the Keeling Curve has a “saw-tooth pattern.” From September to May, Mauna Loa’s value increases, as carbon previously trapped in fossil fuel enters the atmosphere. But from May to September, its value briefly falls by about 2 ppm, as plants in the Northern Hemisphere grow with the summer and absorb carbon out of the air.

With the arrival of northern autumn, most of the world’s plants lose their leaves, and much of that carbon again rejoins the atmosphere. The value again increases. It’s not like humanity has stopped burning fossil fuels during that time, either, so every year, the September measurements get a little bigger.

Mauna Loa’s measurement first exceeded the 400-ppm mark in May 2013, but it only remained there for a few days. In 2014, it stayed above 400 ppm for a few months. This year, it only dropped below 400 ppm at the end of the summer. It’s now climbed back above that benchmark.

In October, Keeling predicted that 2015 would be the last year when daily measurements—and especially monthly mean measurements—fell below 400 ppm. ​Barring anything unusual, we would therefore expect next year’s September value to be around 399.3 ppm, just barely below 400 ppm, and we’d expect the lowest daily minima to be around 398 ppm or so,” he said at the time.

But this year there is also a strong El Niño, the most intense since 1997. Among its many effects, El Niño shifts rain away from tropical landmasses and toward the oceans, which in turn sends equatorial forests into drought. Indonesia’s forests suffered fires so bad this year that, every day they raged, they made that country the second-largest producer of atmospheric CO2 in the world. (The last year fires were so bad there? 1997.)

Taking these factors into account, a reasonable forecast for next year’s September minimum is around 402 ppm, with the lowest daily minima also over 400 ppm,” he said.

Last week, Keeling reaffirmed his forecast to Bob Henson, a meteorologist at the Weather Underground. “It’s too early to be 100 percent 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,” he told Henson. (Henson also has an excellent, graph-heavy post on some of the data behind the forecast.)  So even if the daily measurement falls below the milestone in the next week or so, it’s highly likely—and getting likelier, every day—that November’s monthly mean will be at or above 400 ppm. October 2015 could be the last month in our lives that atmospheric carbon was measured in the 300s.

As Henson notes, this is another major climate milestone arriving right before the UN climate talks in Paris. It follows the news that 2015 will be the hottest year ever recorded, with temperatures more than one degree Celsius above pre-industrial levels—fully halfway to the two-degree benchmark that global governments have promised not to exceed.

In the coming two to three years, atmospheric carbon may fall slightly as El Niño ends. “The loss of carbon from tropical forests in El Niño years is temporary as the forests tend to regrow in normal years, building back their biomass and sucking CO2 out of the air in the process,” Keeling said in his October forecast. “But the eventual recovery from this El Niño won’t bring us back below 400 ppm, because its impact will be dwarfed by the global consumption of fossil fuels, pushing CO2 levels ever higher.” In other words, El Niño may end—but the underlying trends won’t.