Sometime soon, the concentration of carbon dioxide in the atmosphere is expected to hit a scary new milestone: 400 parts per million. That would be higher than at any time in human history — and it’s bad news for anyone who cares about a livable climate.

The latest daily average level recorded by Scripps Institution of Oceanography sensors at Mauna Loa, Hawaii, was 399.5 ppm, on Monday. The CO2 level fluctuates throughout the day, and hourly levels in excess of 400 ppm have already been recorded. The level also fluctuates throughout the year, with May being the month when CO2 reaches its highest concentrations.