For the first time in human history, the average daily concentration of carbon dioxide in the atmosphere has reached 400 parts per million, as recorded Thursday by sensors at the National Oceanic and Atmospheric Administration's Mauna Loa Observatory in Mauna Loa, Hawaii. Scientists estimate that the earth's CO2 concentration hasn't been this high since the Pliocene epoch, between 3.2 and 5 million years ago.
Mounting levels of carbon dioxide, emitted largely from burning fossil fuels, are closely linked to rising temperatures and believed to be the central driver behind climate change. Several years ago, a widely-cited paper by NASA and Yale climate scientists identified [PDF] 350 ppm as the target average annual CO2 concentration for a healthy planet. Remaining over this threshold for a prolonged period of time, the paper's original text states, would risk "seeding irreversible catastrophic effects," like the continued melting of arctic ice and disruption of global environmental systems.
While a daily reading isn't significant in and of itself — CO2 levels fluctuate throughout the year, peaking annually in May — the milestone portends a world where 400 ppm is the baseline. Since the Scripps Institution of Oceanography, another monitoring program in Mauna Loa, began collecting carbon dioxide samples in 1958, atmospheric CO2 levels have risen at a rate never before seen in history.
Scripps geochemist Ralph Keeling said in a release, "At this pace we'll hit 450 ppm within a few decades" — a level last seen an estimated 35 million years ago, when sea levels were over 200 feet higher and the Earth was virtually bereft of ice.
CORRECTION: An earlier version of this story misstated the date of the 400 ppm recording. It was recorded Thursday, May 9. It also misstated the agency that recorded the reading; it was the NOAA, and later confirmed by Scripps.