The level of the most important heat-trapping gas in the atmosphere, carbon dioxide, has passed a long-feared milestone, scientists reported on Friday, reaching a concentration not seen on the earth for millions of years.

Scientific monitors reported that the gas had reached an average daily level that surpassed 400 parts per million — just an odometer moment in one sense, but also a sobering reminder that decades of efforts to bring human-produced emissions under control are faltering.

Governments have been trying since 1992 to rein in emissions, but far from slowing, emissions are rising at an accelerating pace, thanks partly to rapid economic growth in developing countries. Scientists fear the level of the gas could triple or even quadruple before being brought under control.

Indirect measurements suggest that the last time the carbon dioxide level was this high was at least three million years ago, during an epoch called the Pliocene. Geological research shows that the climate then was far warmer than today, the world’s ice caps were smaller, and the sea level might have been as much as 60 or 80 feet higher.