"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.
The best available evidence suggests the amount of the gas in the air has not been this high for at least three million years, before humans evolved, and scientists believe the rise portends large changes in the climate and the level of the sea.
The new measurement came from analyzers high atop Mauna Loa, the volcano on the big island of Hawaii that has long been ground zero for monitoring the worldwide carbon dioxide trend.
Devices there sample clean, crisp air that has blown thousands of miles across the Pacific Ocean, producing a record of rising carbon dioxide levels that has been closely tracked for half a century.
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 at Mauna Loa. But the average reading for an entire day surpassed that level at Mauna Loa for the first time in the 24 hours that ended at 8 p.m. Eastern Daylight Time on 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 leaf growth in the Northern Hemisphere 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.
For the entire period of human civilization, roughly 8,000 years, the carbon dioxide level was relatively stable near that upper bound. But the burning of fossil fuels has caused a 41 percent increase in the heat-trapping gas since the Industrial Revolution, a mere geological instant, and scientists say the climate is beginning to react, though they expect far larger changes in the future.
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.
Countries have adopted an official target to limit the damage from global warming, which by most estimates requires that emissions stop by the time the level reaches about 450. “Unless things slow down, we’ll probably get there in well under 25 years,” Ralph Keeling said.
Yet many countries, including China and the United States, have refused to adopt binding national targets. Scientists say that unless far greater efforts are made soon, the goal of limiting the warming will become impossible without severe economic disruption.
“If you start turning the Titanic long before you hit the iceberg, you can go clear without even spilling a drink of a passenger on deck,” said Richard B. Alley, a climate scientist at the Pennsylvania State University. “If you wait until you’re really close, spilling a lot of drinks is the best you can hope for.”
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 .04 percent. “The CO2 levels in the atmosphere are rather undramatic,” a Republican congressman from California, Dana Rohrabacher, said in a Congressional hearing several years ago.
But climate scientists reject that argument, saying it is like claiming that a tiny bit of arsenic or cobra venom cannot have much effect. Research shows that even at such low levels, carbon dioxide is potent at trapping heat near the surface of the earth.
“If you’re looking to stave off climate perturbations that I don’t believe our culture is ready to adapt to, then significant reductions in CO2 emissions have to occur right away,” said Mark Pagani, a Yale geochemist who studies climates of the past. “I feel like the time to do something was yesterday.”