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From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

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And I'm Robert Siegel.

The Earth's atmosphere is entering a new era. A research station in Hawaii has been tracking average daily carbon dioxide levels for more than half a century. And now, that level has reached a milestone: 400 parts per million.

That number is one of the clearest measures of how human beings are changing the planet. It shows how much carbon we put into the air from burning fossil fuels, carbon dioxide that drives global warming. NPR's Richard Harris has our story.

RICHARD HARRIS, BYLINE: Back in 1958, Dave Keeling started measuring carbon dioxide levels from Mauna Loa, Hawaii, a remote volcano where the air is free of local influences. His first measurements showed carbon dioxide levels at 313 parts per million. And over the lifetime of his son, Ralph Keeling, the number has now reached a big round number: 400 parts per million.

RALPH KEELING: Of course, there's no real planetary threshold at 400. It's the psychological threshold.

HARRIS: Keeling took over the monitoring project at the Scripps Institution of Oceanography after his pioneering father died in 2005.

KEELING: It does feel like we're moving from the past into the future at this juncture. It's one of these milestones where - like the change of a millennium. You know you're going to get there, pretty soon you're going to get there, but you always think of it as being off in the future. And now, here we are moving into what feels like another era.

HARRIS: Keeling says a daily reading of 400 parts per million really marks the beginning of a transition that will play out over several years.

KEELING: The atmosphere doesn't have completely uniform carbon dioxide concentrations. It oscillates up and down the season, and on average it's a little higher in the Northern Hemisphere. And what we're seeing now is the beginnings of flickers up towards 400 parts per million at the iconic Mauna Loa record.

HARRIS: And what does this mean? The physical meaning of the number is quite straightforward.

KEELING: Out of a million air molecules, 400 are carbon dioxide.

HARRIS: That's .04 percent. That small number packs a big punch. That amount of carbon dioxide can trap a great deal of heat in our atmosphere. In fact, the planet would be frozen solid if we didn't have any carbon dioxide in the air.

KEELING: Lots of things that are present in small in amounts can have big impacts. I mean, if you think about blood cholesterol or anything else.

HARRIS: And more to the point, these levels have increased rapidly.

KEELING: These are not small changes in percent terms. Two, three million years ago was the last time we ever had concentrations in this range. So we're moving into territory that's almost outside the scope of human existence on the planet at this point.

HARRIS: And this enormous change has actually occurred during the careers of today's senior Earth scientists. Take, for example, Richard Houghton at the Woods Hole Research Center.

RICHARD HOUGHTON: I graduated from high school in 1961 near the beginning of this record. And concentration then was just a little over 315 parts per million. I came to Woods Hole in 1975, and it was then somewhere between 325 and 330. By year 2000, concentration was over 360.

HARRIS: And the pace of change is actually accelerating.

HOUGHTON: Under business as usual, it's heading to over 600 parts per million. It could go up to 800 parts per million.

HARRIS: That's by the end of this century. And, of course, there is nothing magical about the year 2100 that would stop the trend at that point. Houghton has spent a chunk of his career trying to understand how the planet has dealt with all that carbon dioxide. It turns out that only half of the carbon dioxide that we've put into the air by burning fossil fuels has actually stayed in the air, though it will remain there for centuries. The other half has been soaked up by the oceans, trees, soils, vegetation and so on. But Houghton says we shouldn't assume that nature will be able to keep this up forever. For one thing, as the oceans warm, their ability to soak up carbon dioxide will slow.

HOUGHTON: And my guess is that at some point, the take-up of carbon by oceans and land will at first level off and then actually decline. And it's possible we could have a reverse so that nature, instead of absorbing carbon dioxide, is releasing it. And then we're in big trouble in terms of managing the carbon cycle.

HARRIS: And ask Ralph Keeling at what level we start to get into trouble from climate impacts?

KEELING: It's a little hard to say. But certainly, I'm among those who think that if we really wanted to be prudent about this, we shouldn't have let it go over 350, so we're already into dangerous territory.

HARRIS: And now, reaching 400 parts per million, we're heading deeper into the unknown. Richard Harris, NPR News.

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