A gassy threat from above

In the chair at the dentist’s office, nitrous oxide is better known as “laughing gas.” Dentists and doctors use it to knock out patients during uncomfortable procedures. But that’s not the only place where laughing gas shows up. Nitrous oxide from Earth also ends up in the stratosphere, that portion of our atmosphere about 5 to 30 miles overhead. And 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, it’s important to understand what else is in the stratosphere. That upper layer of the atmosphere is home to about 90 percent of Earth’s ozone. That molecule is made up of three atoms of oxygen. This naturally occurring ozone layer acts like a filter. It blocks out much of the ultraviolet radiation that comes from the sun. Ultraviolet radiation, which is invisible to the human eye, can lead to skin cancer and eye damage in people. It also can damage crops. So the ozone layer is an important protective shield over Earth’s surface.

For decades, scientists had known that chemicals manufactured on Earth could, if they end up in the stratosphere, harm that protective ozone. Less ozone in the stratosphere means more ultraviolet radiation can reach Earth’s surface. 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 about 100 years. And enormous amounts enter the air every year. Thanks to an international treaty 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. And that, in turn, could have led to up to 20 million more cases of skin cancer,  the United Nations says.

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.

POWER WORDS

ozone layer A region of the upper atmosphere, between about 15 and 30 kilometers (10 and 20 miles) in altitude, containing a relatively high concentration of ozone that absorbs solar ultraviolet radiation in a wavelength range not screened by other atmospheric components. Also called ozonosphere.

molecule A group of like or different atoms held together by chemical forces

atom A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons.

stratosphere The region of the atmosphere above the troposphere and below the mesosphere

nitrous oxide A colorless, sweet-tasting gas, made of two nitrogen atoms and one oxygen atom, used as a mild anesthetic in dentistry and surgery.