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Laughing Gas Biggest Threat to Ozone Layer, Study Says

Mason Inman
for National Geographic News
August 27, 2009
 
Laughing gas—known to scientists as nitrous oxide—is now the biggest threat to Earth's ozone layer, according to a new study.

The ozone layer, part of Earth's upper atmosphere, protects plants and animals from harmful ultraviolet radiation from the sun.

In 1987 countries around the world united to ban chlorofluorocarbons, or CFCs—gases that were commonly used in refrigerators and air conditioners. These gases made their way into the atmosphere and thinned the ozone layer by about 5 percent worldwide.

CFC emissions drastically dropped following the ban, and the ozone layer has been on track to largely recover by mid-century, according to the World Meteorological Organization.

But nitrous oxide emissions, which are being released at a rate of about ten million tons a year, may thwart that progress.

An expansion in farming and soaring numbers of livestock may increase emissions of the gas, which comes mostly from fertilizer and animal waste.

"The ozone layer would be prevented from recovering by the time we thought it would," said study leader A. R. Ravishankara of the National Oceanic and Atmospheric Administration in Boulder, Colorado.

Win-Win Situation

Using a computer simulation of the atmosphere, Ravishankara and colleagues calculated how big of an effect nitrous oxide will have on the ozone layer.

The team found that nitrous oxide's effect is as potent as it is for many of the banned CFCs.

Nitrous oxide emitted today will have a lasting effect: "The overall lifetime of nitrous oxide is about a hundred years, comparable to many CFCs," Ravishankara said.

Chlorofluorocarbons, which are still found in the atmosphere, continue to damage the ozone layer.

For instance, millions more cases of skin cancer caused by UV exposure are expected to occur over the 21st century due to the CFC-depleted ozone layer, according to the World Health Organization.

What's more, nitrous oxide is also a greenhouse gas, which means it traps heat and fuels global warming. (See global warming fast facts.)

"That's why cutting nitrous oxide emissions is a win-win situation in terms of greenhouse gases and ozone depletion," said John Daniels, a co-author of the new study in tomorrow's Science.

Farmland Squeeze

Modern farming practices are responsible for most of the rise in human-generated nitrous oxide. (Take a pollution quiz.)

Nitrous oxide is also released to a lesser degree by sewage and transport, including vehicle exhaust, said Detlef van Vuuren of the Netherlands Environmental Assessment Agency in Bilthoven, who was not involved in the new study.

But as populations climb and incomes rise, so does the use of chemical fertilizers and meat eating—contributing to the release of more nitrous oxide.

The growing population is also putting a squeeze on the world's existing croplands, so changing what we eat would make a big difference, van Vuuren said.

"Eating less meat would reduce not only the number of [farm] animals," he said, "it also requires less fertilizer for feed production."

Researchers are also testing different ways of growing food that might release less nitrous oxide, such as farming without tilling fields to prevent nitrogen in the soil from escaping.

Another approach is to add a form of charcoal called biochar to soils, enriching croplands and reducing the need for fertilizers.

(Read about other sustainable agriculture projects.)

Taking such measures may "reduce [overall] emissions by 30, maybe 40 percent," van Vuuren said.

But "I don't see ways to easily reduce them to zero."
 

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