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.)