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 …