For nearly 60 years, a government-run outpost in the remote heights of Mauna Loa has provided among the best data on the world’s rising greenhouse gas levels. On Thursday, data from that station showed the main gas known to cause global warming reaching a worrisome new milestone.

For the first time, air samples taken at the National Oceanic and Atmospheric Administration’s Mauna Loa Observatory measured carbon dioxide at an average daily level of 400 parts per million, a concentration never before seen by humans.

The number is basically a marker, but it shows how efforts to tackle global warming are doing little to stop the trend, officials say.

"It symbolizes that we have failed miserably in tackling this problem," said Pie­ter P. Tans, a Boulder, Colo.-based NOAA employee who runs the agency’s monitoring program.

Carbon dioxide levels recorded at Mauna Loa have creeped up at about 2 parts per million on average each year, said John Barnes, director of the NOAA’s observatory there. When measurements of this chief greenhouse gas were first taken in 1958, carbon dioxide at Mauna Loa was measured at 315 parts per million.

In 2013 the levels are anticipated to fluctuate between 396 and 400 parts per million, depending on the season, Barnes said.

NOAA further anticipates carbon dioxide levels will reach a monthly average of 400 parts per million in May 2014 and then a yearly average at that level in 2015, Barnes added.

The findings this week at Mauna Loa come 55 years after American scientist Charles David Keeling’s analysis of the air samples taken there revealed that carbon dioxide levels were growing in Earth’s atmosphere, Barnes said. Keeling’s measurements from atop the volcano helped alert the world to the greenhouse gas effect. Keeling, who died in 2005, worked with the Scripps Institution of Oceanography at the University of California, San Diego, which also monitors carbon dioxide at the NOAA site on Mauna Loa. A note on the Scripps website Friday confirmed it also recorded a daily average level of 400 parts per million.

Mauna Loa is an ideal place to record samples that reflect what’s happening on average around the world, Barnes said, because of its proximity to the equator and its remote Pacific location removed from major land masses. And, at 11,140 feet — not quite the volcano’s summit of 13,679 feet — the observatory is well above what’s called the "boundary layer," Barnes said, or the first three or so kilometers near the ground where most local pollutants, particles and humidity collect.

It’s obvious when the active volcano emits carbon dioxide because the levels spike, so that effect doesn’t disrupt the scientists’ measurements, Barnes added.

"It’s the original and oldest monitoring station like this," Barnes said Friday. "It’s such a well-known site. The measurements we make are very precise." A seven-member staff of scientists, technicians and engineers mans the observatory, which is about a 75-minute drive from Hilo, Barnes said.

A 40-meter-tall tower above the observatory contains a dozen tubes, each about a quarter-inch in diameter. Those tubes collect the air and direct it to instruments that measure carbon dioxide levels. The samples provide by-the-minute results, which are typically grouped into daily, monthly and yearly averages, Barnes said. The data are then sent to mainland facilities, including a NOAA facility in Boulder, where scientists study the data’s implications and how they fit into the global picture, he added.

Carbon dioxide rises and falls on a seasonal cycle and will likely dip below 400 this summer as leaf growth in the Northern Hemisphere pulls about 10 billion tons of carbon out of the air.

"That was the other key discovery that Charles David Keeling made, that there was this annual cycle" in carbon dioxide levels, Barnes said. But even with the seasonal rise and fall, carbon dioxide levels are rising overall, and experts say 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, carbon dioxide levels were relatively stable near 280 parts per million. But the burning of fossil fuels has caused a 41 percent rise in the heat-trapping gas since the Industrial Revolution, a mere geological instant, and scientists say the climate is starting to react.

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.

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The New York Times and The Associated Press contributed to this report.