New York Times' environmental reporter Justin Gillis earned an unusual two-column lead story part in Saturday's paper, part of his long-running scarefest series, "Temperatures Rising." The latest entry: "Heat-Trapping Gas Passes Milestone, Raising Fears ." (Though that headline turns out to be upon further review a bit premature.) Carbon dioxide is closely linked by many environmental scientists to rising temperatures, though there are other factors involved.
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.
But the Los Angeles Times threw cold water on the warming-planet milestone this morning  with an update that renders the New York Times' scary front-page headline officially false: "Carbon dioxide measurements in the Earth's atmosphere did not break the symbolic milestone of 400 parts per million at a Hawaiian observatory last week, according to a revised reading from the nation's climate observers."
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.
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.”
Gillis committed his standard smear  of global-warming skeptics.
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.