The level of the most important heat-trapping gas in the atmosphere, carbon dioxide, has passed a long-feared milestone, scientists reported 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 -- a sobering reminder that decades of efforts to bring human-produced emissions under control are faltering.
The best available evidence suggests that the amount of the gas in the air has not been this high for at least 3 million years, before humans evolved, and scientists believe that the rise portends large changes in the climate and sea level. "It symbolizes that, so far, we have failed miserably in tackling this problem," said Pieter P. Tans, who runs the monitoring program at the National Oceanic and Atmospheric Administration that reported the new reading.
Ralph Keeling, who runs another monitoring program at the Scripps Institution of Oceanography in San Diego, 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," he said.
Carbon dioxide above 400 parts per million was first seen in the Arctic last year and had also spiked above that level in hourly readings. But the average reading for an entire day surpassed that level for the first time in the 24 hours that ended at 8 p.m. Eastern time Thursday, according to data from both NOAA and Scripps.
Carbon dioxide rises and falls on a seasonal cycle, and the level will dip below 400 this summer, as Northern Hemisphere leaf growth pulls about 10 billion tons of carbon out of the air. But experts say that will be a brief reprieve; the moment is approaching when no measurement of the ambient air anywhere on Earth, in any season, will produce a reading below 400.
Climate-change contrarians, who have little scientific credibility but are politically influential in Washington, point out that carbon dioxide represents only a tiny fraction of the air -- as of Thursday's reading, exactly 0.0004 percent. "The CO2 levels in the atmosphere are rather undramatic," Rep. Dana Rohrabacher, R-Calif., said in a congressional hearing several years ago.
Climate scientists reject that argument, saying it is like claiming that a tiny bit of arsenic or cobra venom cannot have much effect.