Alister Doyle, Reuters 27 Aug 09;
OSLO (Reuters) - Nitrous oxide or "laughing gas" has become the main man-made substance damaging the planet's protective ozone layer and is likely to remain so throughout the century, scientists said.
The study, by the U.S. National Oceanic and Atmospheric Administration, said tighter limits on emissions of nitrous oxide, which is also a powerful greenhouse gas, would be a "win-win for both ozone and the climate."
"Nitrous oxide emission currently is the single most important ozone-depleting substance emission and is expected to remain the largest throughout the 21st century," the scientists wrote in Friday's edition of the journal Science.
Nitrous oxide has overtaken chlorofluorocarbons (CFCs), formerly used in making refrigerants, which are being phased out under the U.N.'s 1987 Montreal Protocol after they were found to thin the earth's protective ozone layer high in the atmosphere.
About 10 million tonnes of nitrous oxide a year -- a third of world emissions -- come from human activities including fertilizers, fossil fuels, livestock manure and industry. "Laughing gas" is perhaps best known as an anesthetic.
SKIN CANCER
Two-thirds of nitrous oxide comes from nature, when soil bacteria release the gas. It thins the ozone layer, which shields the planet from ultraviolet rays that can cause skin cancers and damage crop growth.
"The main reason for the large role of nitrous oxide is the success of the Montreal Protocol in that it has reduced the emissions of CFCs and other ozone-depleting chemicals," lead author A.R. Ravishankara told a telephone news briefing.
"Limiting future nitrous oxide emissions would enhance the recovery of the ozone layer from its depleted state," the scientists wrote.
The U.N. Environment Programme has said the ozone layer is on the path to recovery in coming decades thanks to the Montreal Protocol, which regulates gases found in everything from hairsprays to air conditioners.
Nitrous oxide is not regulated by the Montreal Protocol but is among greenhouse gases covered by the U.N.'s Kyoto Protocol, which obliges developed nations to cut emissions by 5 percent below 1990 levels by 2008-12.
(Editing by Andrew Roche)
Laughing gas is biggest threat to ozone layer
Lisa Grossman, New Scientist 27 Aug 09;
Nitrous oxide, commonly known as laughing gas, is now the dominant ozone-depleting substance emitted by humans – and is likely to remain so throughout the century, a new study suggests.
Researchers suggest use of the compound – which is produced by the breakdown of nitrogen in fertilisers and sewage treatment plants – should be reduced to avoid thinning the protective ozone layer that blankets the Earth.
The ozone layer shields Earth from the sun's ultraviolet rays, which increase the risk of cancer and threaten crops and aquatic life.
Human-produced chemicals called chlorofluorocarbons (CFCs) made headlines in the 1980s when it became clear they were eating a hole in the ozone layer above Earth's polar regions. An international treaty called the Montreal Protocol regulated production of CFCs and certain other ozone-depleting gases in 1987, and they were phased out completely by 1996.
Since then, Earth's ozone – both the polar hole and the atmospheric layer around the whole planet – has been on the mend. But the emission of nitrous oxide, which is not regulated by the Montreal Protocol, could reverse those gains – and could even make the situation worse.
"Right now, nitrous oxide is the most important ozone-depleting gas that is emitted," says A. R. Ravishankara of the US National Oceanic and Atmospheric Administration, lead author of the new research. "It will continue to be so unless something is done."
Greenhouse gas
Nitrous oxide is also a heat-trapping greenhouse gas in the league of methane or carbon dioxide, so regulating it would also be good for the climate, he says.
Nitrous oxide (N2O) is produced naturally when nitrogen in soil or water is eaten by bacteria. It rises into the stratosphere, where most of it is broken down into harmless molecules of nitrogen and oxygen by the sun's rays.
But some of it remains, and can survive for hundreds of years. The compound reacts with high-energy oxygen atoms to produce a deadlier compound, nitric oxide (NO). This then goes on to destroy ozone, a molecule made up of three oxygen atoms.
Nitrous oxide has no effect on the hole in the ozone layer, Ravishankara points out, but it makes the global layer thinner.
Abundant gas
This chemical process has been known since the 1970s, when scientists were worried about the environmental effects of flying supersonic planes, which emit N2O. Ravishankara and his colleagues are the first to put hard numbers on the role of nitrous oxide in ozone depletion.
To do so, they modelled the atmosphere and the chemical reactions that take place inside it. They found that nitrous oxide's potential to deplete ozone is comparable to other ozone-depleting substances, called hydroCFCs, that replaced CFCs but are also in the process of being phased out.
But although the depletion potential is roughly equivalent, nitrous oxide could have a more damaging effect because it is much more abundant. Global human emissions of N2O are roughly 10 million tonnes per year, compared to slightly more than 1 million tonnes from all CFCs at the peak of their emissions.
On the rise
Scientists say humans' role in producing the harmful gas has largely been overlooked. Thanks to fossil fuel combustion, which produces the gas, as well as nitrogen-based fertilisers, sewage treatment plants and other industrial processes that involve nitrogen, about one-third of the nitrous oxide emitted per year is anthropogenic.
Although supersonic transport never got off the ground, current emissions are equivalent to flying 500 such planes a day. Emission levels have increased by 0.25 per cent a year since pre-industrial times.
"Nitrous oxide is kind of the forgotten gas," says Don Wuebbles of the University of Illinois at Urbana-Champaign, who invented the method of quantifying a chemical's ozone-depletion potential but was not involved in this work. "It was always thought of as a natural thing. People have forgotten that it's been increasing."
And as CFC levels abate, nitrous oxide could become even more powerful. Nitrogen and chlorine compounds counteract each others' effects on ozone – the more chlorine there is, the less effective nitrogen becomes at destroying ozone, and vice versa. As CFCs are purged from the atmosphere, nitrous oxide will become 50 per cent more potent than it was before, Ravishankara says.
"People were expecting that ozone was just going to recover from the results of human activities that resulted in CFCs," Wuebbles says. "Nitrous oxide could prevent that from happening."
Journal reference: Science (DOI:10.1126/science.1176985)
Laughing Gas Biggest Threat to Ozone Layer, Study Says
Mason Inman, National Geographic News 27 Aug 09;
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.
"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.
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."
New Culprit Seen in Ozone Depletion
Cornelia Dean, The New York Times 27 Aug 09;
Government scientists who study the depletion of Earth’s protective ozone layer are pointing to a previously unheralded culprit: nitrous oxide.
Most of the nitrous oxide in the atmosphere emerges naturally, through the action of bacteria in the soil, the researchers say. But the gas is also produced by human activity, through the use of nitrogen-based fertilizers, the application of livestock manure to fields, the burning of biofuels and in other ways.
Though nitrous oxide is not regulated under the Montreal Protocol, the 1987 agreement to limit emissions of ozone-depleting chemicals, the researchers say it is emerging as the leading artificial cause of ozone loss.
The researchers, from the National Oceanic and Atmospheric Administration, report their findings in Friday’s issue of the journal Science.
They note that the health of the ozone layer has been improving since the adoption of the protocol and that nitrous oxide looms large today as an artificial destroyer of the ozone layer, in part because the emissions of other harmful chemicals have been so sharply reduced. But major chemical targets of the Montreal agreement, chlorofluorocarbons, inhibit the ozone-destroying actions of nitrous oxide, the researchers said. So as their levels fall, the harmful influence of nitrous oxide increases.
The Environmental Protection Agency is already contemplating action on nitrous oxide because it is a heat-trapping gas linked to global warming. In April, the agency declared it and five other gases, including carbon dioxide, to be pollutants that endanger public health, making them subject to regulation under the Clean Air Act.
In a statement, the agency said Thursday that work on a reporting system for emissions of nitrous oxide and the five gases was under way. John S. Daniel, one of the authors of the new report, said scientists had for some time known of the ozone-depleting potential of nitrous oxide. But, Mr. Daniel said in a telephone news conference, “there is a sort of gap between the scientific understanding and the policy.”
The researchers did not make any policy recommendations in light of their finding.
“It is not for us to gauge how much risk there is,” said A. R. Ravishankara, who led the work. In any event, he said, at the moment researchers could not say with confidence “how much nitrous oxide comes from where.”
“The uncertainties are significant,” Dr. Ravishankara said.
Dr. Ravishankara estimated that worldwide the ozone layer had been reduced by about 6 percent from what it was before industrialization.
At ground level, ozone is a pollutant, but in the upper atmosphere it blocks ultraviolet radiation that would harm plants and animals on Earth’s surface. When researchers discovered that chemicals like chlorofluorocarbons were depleting this high-level ozone layer, and especially after the discovery of a highly depleted ozone hole over Antarctica, international negotiators produced the Montreal agreement.
Because of the unusual atmospheric chemistry above Antarctica, nitrous oxide does not affect the ozone hole there, Dr. Ravishankara said.
