Chances are that in the next few days, the concentration of carbon dioxide in Earth’s atmosphere will exceed 400 parts per million (ppm) for the first time in about 4 million years.

Recently, Mauna Loa Observatory on the big island of Hawaii has been regularly recording daily CO2 levels above 399 ppm, with several hours already exceeding 400.

Atmospheric CO2 as recorded last week at Mauna Loa Observatory. Source: Scripts Institution of Oceanography.

Considering that carbon levels tend to peak in mid-May, one or more daily averages above 400 in the next few weeks is a near certainty. Yesterday’s reading, May 5, was 399.54 ppm.

While crossing the 400 ppm threshold is largely symbolic, the rate at which atmospheric carbon is increasing is anything but. When Mauna Loa began measuring CO2 in 1958, CO2 was running @ 317 ppm. Unless we begin to seriously slow the rate of greenhouse gas emissions now, we’re on track to surpass 450 ppm within 30 years.

The Keeling Curve shows concentration of atmospheric CO2 at Mauna Loa since measurements began in 1958. The sawtooth nature of the curve reflects the annual cycle of seasonal differences. Source: Scripts Institution of Oceanography.

Mauna Loa Observatory, operated by the US National Oceanic and Atmospheric Administration, not only has the longest continuous history of monitoring CO2 concentrations, but thanks in part to its location, its measurements are regarded as the baseline standard for atmospheric carbon.

At 11, 335 feet (3,397 m) above sea level, Mauna Loa’s sits above low-level, local pollution and temperature inversion layers. Its location in the mid-Pacific also isolates it from major sources of pollution.