STEVE INSKEEP, HOST:

If you're interested in what's happening to the planet you live on, you might want to keep an eye on this measurement, which is linked to efforts to track climate change. It is the number of parts per million of carbon dioxide in the Earth's atmosphere. The daily average for that number has just passed 400, which is the highest it's been in the modern age.

And to get an accurate measurement, scientists had to put their CO2 analyzer far from cities or farms that could distort the readings. So they chose a spot in the middle of the Pacific Ocean. Last week, NPR science correspondent Joe Palca visited Mauna Loa Atmospheric Observatory and has this report.

JOE PALCA, BYLINE: The Mauna Loa volcano is on the Big Island in Hawaii. Once you turn off the highway, it's a 16-mile drive on a road that climbs through an undulating field of nothing but rocks and boulders made from lava. When you reach 11,000 feet, you come upon a cluster of small buildings belonging to the National Oceanic and Atmospheric Administration. One of them has a plaque next to the door.

I'm about to walk into the Keeling Building named in honor of Professor Charles David Keeling, Scripps Institution of Oceanography, who initiated continuous CO2 measurements at this site in 1958.

(SOUNDBITE OF DOOR OPENING)

PALCA: Inside, there are bunch of computers and other electronic instruments humming away. In a corner of the room, NOAA atmospheric scientist Aidan Colton is closing off an air line he's just used to fill a glass flask.

AIDAN COLTON: We sample 24 hours a day on most of our instrumentation. But what we're most interested in is actually the extremely clean air coming down from the troposphere that's been well mixed traveling over 2,000 miles in every direction to get here.

PALCA: The troposphere is where the bulk of Earth's atmosphere resides. It's the build-up of carbon dioxide in the troposphere that has climate scientists concerned. Colton takes me outside and points to what looks like a tall antenna tower with several cables attached to it.

COLTON: It's basically strapped down on all sides so that it doesn't blow away.

PALCA: Get windy up here?

COLTON: Yeah, it gets very windy at times. You know, we can get winds in excess of 50 miles per hour.

PALCA: The air Colton analyzes comes from intake ports at the top of the 120-foot tower. Across a small parking lot from the Keeling building is the main observatory building. Inside there are offices, a pantry, and several laboratory rooms. So this is the CO2 room?

COLTON: Yes. We call this the carbon cycle room. So we have a C02 analyzer, a methane analyzer, and a carbon monoxide analyzer.

PALCA: Colton sits down at a laptop next to the CO2 analyzer.

COLTON: On our screen right now we're seeing hourly values of CO2. And the last hourly value was 399.33. And, you know, earlier in the day we had a 400.46.

PALCA: Just to be clear here, the hourly number isn't the headline; it's the daily average that's crossed 400 that's making news. Colton says daily averages are interesting, but they don't tell the whole story.

COLTON: We're a long-term baseline monitoring station. So we're not interested in what happens hour to hour, day to day, even week to week.

PALCA: Far more important is the long-term trend.

COLTON: We don't expect CO2 as a monthly average to reach 400 until probably May of 2014.

PALCA: But no matter what day or week or month you choose to say CO2 exceeded 400 parts per million in the atmosphere, the long-term trends show that the amount of CO2 in the atmosphere is continuing to climb, something scientists say remains a major concern. Joe Palca, NPR News.

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