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, Reuters quoted scientists as saying. 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 fertilisers, fossil fuels, livestock manure and industry. "Laughing gas" is perhaps best known as an anaesthetic. 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.
