The amount of heat-trapping carbon dioxide in the atmosphere reached a milestone on Thursday - the daily average topped 400 parts per million, a level not seen since the middle Pliocene era, some 3.6 million years ago, when the Arctic was wetter, warmer and ice-free.
Scientists have been measuring CO2 data for more than half a century, gathered from analyzers mounted high atop the Hawaiian volcano Mauna Loa. When the project began in 1958, the CO2 level was 315 parts per million. Since late April that number had been hovering above 399. Many scientists say that to avoid the worst impacts of climate change, the CO2 concentration must be stabilized at 450 parts per million, while others argue it should be no higher than 350.
The atmospheric level of CO2 fluctuates with the seasons, dropping during the Northern Hemisphere's spring, as plants absorb CO2 to fuel their new growth, and rising in the winter. But analysis of the Mauna Loa measurements show a long-term increase superimposed on the seasonal cycle.
Carbon dioxide is released by the burning of fossil fuels, and policymakers worldwide have been unable to reach a global agreement on steps to reduce those emissions. Ralph Keeling of the Scripps Institution, whose father began the monitoring program, said a continuing rise could be catastrophic. "It means we are quickly losing the possibility of keeping the climate below what people thought were possibly tolerable thresholds (of CO2)."