Ozone Depletion (Page 109)

A gassy threat from above

Sept. 16, 2009

In the chair at the dentist’s office, nitrous oxide is better known as “laughing gas”— it’s used to knock out patients during uncomfortable procedures. That’s not the only place where laughing gas shows up, however.

The nitrogen-based fertilizers spread on farm fields, especially corn fields, add to the nitrous oxide that ends up in Earth's atmosphere. gordo25/iStockphoto

Nitrous oxide from Earth also ends up in the stratosphere, that portion of our atmosphere about 5 to 30 miles overhead. Up there, it’s no laughing matter. Nitrous oxide in the stratosphere is already dangerous for life on our planet, and according to a new study, it may become even more dangerous in the near future. To understand why nitrous oxide is dangerous, it’s important to understand what else is in the stratosphere. The stratosphere is home to about 90 percent of Earth’s ozone — a molecule made up of three atoms of oxygen. This naturally occurring ozone layer acts like a filter that blocks out much of the ultraviolet radiation that comes from the sun. Ultraviolet radiation, which is invisible, can lead to skin cancer and eye damage in people and can damage crops. So the ozone layer is an important protective shield over Earth’s surface.

For decades, scientists have known that chemicals manufactured on Earth, if they end up in the stratosphere, may lessen the amount of that protective ozone. Less ozone up there means that more ultraviolet radiation makes it to Earth. The chemicals most damaging to the ozone layer are called chlorofluorocarbons, or CFCs.

In the new study, three scientists from the National Oceanic and Atmospheric Administration, or NOAA, in Boulder, Colo., show that each molecule of laughing gas depletes about one-fiftieth as much ozone as does a CFC molecule. That may seem like a small number, but consider this: Nitrous oxide stays in the atmosphere for 100 years or so, and enormous amounts of it are released into the air every year. Thanks to an international environmental agreement called the Montreal Protocol, CFC production is going down. In fact, the Montreal Protocol is responsible for eliminating nearly 100 ozone-damaging chemicals. Without the agreement, atmospheric levels of ozone- depleting substances could have increased tenfold by 2050 which in turn could have led to up to 20 million more cases of skin cancer, according to the United Nations. But nitrous oxide is not on the list of pollutants that are reduced by the Montreal Protocol, and levels of the gas are still going up. Nitrous oxide is released into the atmosphere from natural sources, like animal waste and the decomposition of plants. Human activities, like using fertilizer on crops and burning fossil fuels, contribute even more nitrous oxide to the stratosphere. Strangely enough, if there are fewer CFCs in the atmosphere, then nitrous oxide becomes even more damaging to ozone. Even though CFCs and nitrous oxide both destroy ozone, they actually work against each other. So less CFCs means that each remaining molecule of nitrous oxide will destroy more ozone.

In fact, “we found that if you look ahead, nitrous oxide will remain the largest ozone- depleting emissions for the rest of the century,” says A.R. Ravishankara, a scientist at NOAA who led the study. The scientists say that nitrous oxide, not CFCs, are now the biggest threat to the ozone layer. If humans can limit the production of nitrous oxide, more ozone may be spared. Plus, since nitrous oxide is also a greenhouse gas, cutting down on nitrous oxide production could help slow down global warming. There’s another twist to the story. Nitrous oxide in the sky can be destructive, but on Earth the substance can be used as a medicine. Ozone, on the other hand, is helpful when it’s very high up in the sky. But closer to Earth, we know these oxygen molecules as the prime ingredient in another form harmful pollution: smog.

1. How much ozone does a molecule of nitrous oxide affect, compared to CFC molecules?

2. Give two reasons why nitrous oxide (N2O) is a problem, even though reacts with less ozone than CFCs?

3. What is the name of the international agreement created to protect the ozone layer?

4. What human activities release N2O?

5. Why is nitrous oxide now a greater concern for the future of the ozone layer?