STEVE INSKEEP, host:
Scientists are surprised to discover that a gas, produced mainly in agriculture, is actually doing more to damage the Earth's ozone layer than synthetic chemicals. The culprit is a gas called nitrous oxide, known in your dentist's office as laughing gas. NPR's Richard Harris has the story.
RICHARD HARRIS: The Earth is protected from harsh ultraviolet sunlight by a layer of ozone up in the stratosphere. That layer was being depleted by synthetic chemicals used in aerosol spray cans, refrigerators and air conditioners. We averted global disaster by phasing out those chemicals with a treaty called the Montreal Protocol. But the Montreal Protocol is silent about another chemical that can destroy ozone. That's nitrous oxide.
It's always been a normal part of our atmosphere…
Mr. A.R. RAVISHANKARA (National Oceanic and Atmospheric Administration): But since industrialization, its concentration has been going up.
HARRIS: A.R. Ravishankara is a scientist at the National Oceanic and Atmospheric Administration in Boulder, Colorado. He wondered now that the nasty synthetic chemicals are waning in the atmosphere, how big a deal is nitrous oxide, which comes mainly from agriculture. As he reports in Science magazine, it turns out that it's a big deal.
Mr. RAVISHANKARA: In fact, there's so much being emitted, that right now, nitrous oxide emissions would be the largest ozone depleting gas emissions today, and it will continue to be in the future.
HARRIS: Nitrous oxide doesn't threaten to devastate the Earth's ozone layer the way the synthetic chemicals did. But it's still eroding a bit of our planetary sun shield, so it's increasing the risk of skin cancer, among other concerns.
Now, you may recall that the biggest ozone problem is over Antarctica, an annual thinning event called the ozone hole. That hole is slowly on the mend, and Ravishankara says he expects that healing to continue over the coming decades.
Mr. RAVISHANKARA: It turns out that nitrous oxide does not have a deleterious effect on the ozone hole. Its effect is on the global ozone layer.
HARRIS: That's because the ozone hole is influenced by supercold clouds found only over the poles. Those clouds release damaging chlorine, but they actually neutralize nitrous oxide. So that's the good news.
The bad news is that it isn't easy to reduce human production of nitrous oxide. Cindy Nevison at the University of Colorado says controlling chlorofluorocarbons and other synthetic chemicals that destroy ozone was relatively easy, since just a few factories produced them.
Dr. CINDY NEVISON (University of Colorado): Whereas nitrous oxide is produced by microbes in the soil, and the humans have greatly increased the rate of nitrogen available to these microbes.
HARRIS: When we spread nitrogen fertilizer on the soil, we also feed those bacteria. And they produce more nitrous oxide. Bacteria in seawater also produce nitrous oxide when the fertilizer runs down the rivers and out to sea. Nevison says factories and automobile tailpipes produce some nitrous oxide, but not all that much.
Dr. NEVISON: I think that limiting nitrous oxide is going to be more difficult than, for example, limiting carbon dioxide emissions. And we know how difficult that is.
HARRIS: That's because we need nitrogen — it's an essential part of protein. Carbon dioxide comes mostly from smokestacks and tailpipes.
Dr. NEVISON: You can get your energy from other sources than carbon, but you really can't get your food from sources other than nitrogen.
HARRIS: We can't phase out nitrogen fertilizers, she says. And studies show we could make only a modest difference if we used them more carefully.
And it's not just the ozone layer that's at issue here. Nitrous oxide also contributes to global warming — so there are more reasons to pay attention to this often neglected gas.
Richard Harris, NPR News.
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