An instrument near the summit of Mauna Loa in Hawaii recorded a long-awaited climate milestone today: the amount of carbon dioxide in the atmosphere there has exceeded 400 parts per million (ppm) for the first time in 55 years of measurement—and probably more than 3 million years of Earth history.

The last time the concentration of Earth’s main greenhouse gas reached this mark, horses and camels lived in the high Arctic. Seas were at least 30 feet higher—at a level that today would inundate major cities around the world.

The planet was about 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit) warmer. But the Earth then was in the final stage of a prolonged greenhouse epoch, and CO2 concentrations were on their way down. This time, 400 ppm is a milepost on a far more rapid uphill climb toward an uncertain climate future.

Two independent teams of scientists measure CO2 on Mauna Loa: one from the U.S. National Oceanic and Atmospheric Administration (NOAA), the other from the Scripps Institution of Oceanography. The NOAA team posted word on its web site before dawn Hawaii time: The daily average for May 9 was 400.03 ppm. The Scripps team’s data for the day was too variable to report an average.

The Scripps team is led by Ralph Keeling, son of the late Charles David Keeling, who started the Mauna Loa measurements in 1958. Since then the “Keeling curve,” showing the steady climb in CO2 levels caused primarily by burning fossil fuels, has become an icon of climate change.

When the elder Keeling started at Mauna Loa, the CO2 level was at 315 ppm. When he died in June 2005, it was at 382. Why did he keep at it for 47 years, fighting off periodic efforts to cut his funding? His father, he once wrote, had passed onto him a “faith that the world could be made better by devotion to just causes.” Now his son and the NOAA team have taken over a measurement that captures, more than any other single number, the extent to which we are changing the world—for better or worse.

Setting the Record Straight

Since late April that number had been hovering above 399 ppm. The Scripps lab opened the vigil to the public by sending out daily tweets (under the handle @Keeling_curve) almost as soon as the data could be downloaded from Mauna Loa, at 5 a.m. Hawaii time. NOAA took to updating its website daily. The two labs’ measurements typically agree within .2 ppm. Both measure the amount of CO2 in an air sample by measuring how much infrared radiation it absorbs—the same process by which CO2 in the atmosphere traps heat and warms the whole planet.

Click here for the full story.