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A new study has found that emissions of nitrous oxide -- also known as laughing gas -- in the US have been underestimated. It's significant because the gas is a major contributor to global warming and the destruction of the ozone layer. Emissions of nitrous oxide from agriculture in some locations are underestimated by 40%, according to the new research -- suggesting the gas is a bigger problem than previously thought. Our environment correspondent, Matt McGrath explains. (Photo: Cornfields in Indiana, USA. Credit: Saul Loeb/AFP/GettyImages