Atmospheric levels of carbon dioxide (CO2), one of the primary gases involved in global warming, reached a new, cautionary milestone yesterday. The oldest continuous CO2 measuring station in the world, an observatory on the Mauna Loa volcano in Hawaii, reported that levels of the greenhouse gas have reached 400 parts per million (ppm) — an amount not seen in at least three to five million years, by the observatory's estimate. Levels of CO2 traditionally fluctuated between 200 and 300 ppm between warm periods and ice ages on the earth, but since the industrial revolution, concentrations of the gas have been slowly — and now, rapidly — rising.

When measurements began in Mauna Loa in 1958, CO2 levels were as low as 317 ppm. Though the number 400 is in itself an arbitrary data point, it's certainly a stark reminder of the still-growing problem of greenhouse gas emissions. The earth is now warmer than it has been in at least 11,300 years, and it's leading to more extreme weather patterns that are only predicted to get worse. Reaching 400 ppm is "now a done deal," said researcher Ralph Keeling, whose father began taking the CO2 measurements in 1958. But he told the National Oceanic and Atmospheric Administration that "what happens from here on still matters to climate, and it’s still under our control."