The concentration of carbon dioxide in our atmosphere reached a record high Thursday, attaining a level the planet hasn't experienced for the last three million years, according to scientists, and worsening the risk of climate change from greenhouse gas emissions.
Researchers at the Mauna Loa Observatory in Hawai'i reported that they recorded average CO2 concentrations above 400 parts per million (ppm) over 24 consecutive hours on Thursday, May 9. That's the first time the observatory has recorded that high and persistent a CO2 concentration since the observatory started measuring in the 1950s.
Local CO2 concentrations above 400 ppm have been measured in a number of places, but the spikes were likely the result of local fires or decomposing organic matter, which emits CO2 Mauna Loa Observatory's measurements are the best proxy we have for a global average CO2 concentration. At a spot on the Island of Hawai'i more than 11,000 feet above sea level, the observatory is high enough in elevation to avoid local concentrations of the gas, and samples air blown across thousands of miles of the Pacific Ocean.
Before we started adding CO2 to the atmosphere by burning fossil fuels, the concentration stood at about 280 ppm. Boosting CO2 concentrations from there to 400 ppm is an increase of 43 percent in atmospheric carbon. That increase has already caused the earth's climate to warm, and further increases may well push warming to catastrophic levels.
The last time there was this much CO2 in the earth's atmosphere was about 3 million years ago, scientists hypothesize, during the Pliocene epoch. Global sea level was about 80 feet higher during much of the Pliocene, and sea ice melted nearly completely during summers. (Here's what would happen to California if sea level rose not quite that high.)
The 400 ppm benchmark is mainly significant as a warning. Many analysts have suggested 350 ppm as a relatively safe level at which to stabilize our atmosphere's CO2, but the two nations that emit the most CO2 -- the U.S. and China -- have refused to agree to emissions caps. We're likely to be looking at 400 ppm in fond retrospect before too long.