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 instruments showed that the gas had reached an average daily level above 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.
“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.
Virtually every automobile ride, every plane trip and, in most places, every flip of a light switch adds carbon dioxide to the air, and relatively little money is being spent to find and deploy alternative technologies.
China is now the largest emitter, but Americans have been consuming fossil fuels extensively for far longer, and experts say the United States is more responsible than any other nation for the high level.
The new measurement came from analyzers atop Mauna Loa, the volcano on the big island of Hawaii that has long been ground zero for monitoring the worldwide trend on carbon dioxide, or CO2. 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. The two monitoring programs use slightly different protocols; NOAA reported an average for the period of 400.03 parts per million, while Scripps reported 400.08.
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.
“It feels like the inevitable march toward disaster,” said Maureen E. Raymo, a scientist at the Lamont-Doherty Earth Observatory, a unit of Columbia University.
From studying air bubbles trapped in Antarctic ice, scientists know that going back 800,000 years, the carbon dioxide level oscillated in a tight band, from about 180 parts per million in the depths of ice ages to about 280 during the warm periods between. The evidence shows that global temperatures and CO2 levels are tightly linked.
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.
Indirect measurements suggest that the last time the carbon dioxide level was this high was at least three million years ago, during an epoch called the Pliocene. Geological research shows that the climate then was far warmer than today, the world’s ice caps were smaller, and the sea level might have been as much as 60 or 80 feet higher.
Experts fear that humanity may be precipitating a return to such conditions — except this time, billions of people are in harm’s way.
“It takes a long time to melt ice, but we’re doing it,” Dr. Keeling said. “It’s scary.”
Dr. Keeling’s father, Charles David Keeling, began carbon dioxide measurements on Mauna Loa and at other locations in the late 1950s. The elder Dr. Keeling found a level in the air then of about 315 parts per million — meaning that if a person had filled a million quart jars with air, about 315 quart jars of carbon dioxide would have been mixed in.
His analysis revealed a relentless, long-term increase superimposed on the seasonal cycle, a trend that was dubbed the Keeling Curve.
Countries have adopted an official target to limit the damage from global warming, with 450 parts per million seen as the maximum level compatible with that goal. “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 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 0.04 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.”
This article has been revised to reflect the following correction:
Correction: May 10, 2013
An earlier version of this article misstated the amount of carbon dioxide in the air as of Thursday’s reading from monitors. It is .04 percent, not .0004 percent.