Nitrous oxide (also called dinitrogen oxide or dinitrogen monoxide, or sometimes laughing gas) is a chemical compound with the chemical formula N₂O. At room temperature, it is a colorless non-flammable gas, with a pleasant, slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic effects. It is commonly known as "laughing gas" due to the euphoric effects of inhaling it, a property that has led to its non-medical use as an inhalant drug. It is also used in motor racing as an oxidizer to increase the power output of engines.

Unlike other nitrogen oxides, nitrous oxide is a major greenhouse gas. While its radiative warming effect is substantially less than CO₂, nitrous oxide's persistence in the atmosphere, when considered over a 100 year period, per unit of weight, has 296 times more impact on global warming than that per mass unit of carbon dioxide (CO₂).

Control of nitrous oxide is part of efforts to curb greenhouse gas emissions, such as the Kyoto Protocol. Despite its relatively small concentration in the atmosphere, nitrous oxide is the third largest greenhouse gas contributor to overall global warming, behind carbon dioxide and methane. (The other nitrogen oxides contribute to global warming indirectly, by contributing to tropospheric ozone production during smog _formation).

Nitrous oxide is emitted by bacteria in soils and oceans, and thus has been a part of Earth's atmosphere for eons. Agriculture is the main source of human-produced nitrous oxide: cultivating soil, the use of nitrogen fertilizers, and animal waste handling can all stimulate naturally occurring bacteria to produce more nitrous oxide. The livestock sector (primarily cows, chickens, and pigs) produces 65% of human-related nitrous oxide. ^[2] Industrial sources make up only about 20% of all anthropogenic sources, and include the production of nylon and nitric acid, and the burning of fossil fuel in internal combustion engines.

Human activity is thought to account for somewhat less than 2 teragrams (this is multiplied by about 300 when calculated as an equivalent amount of carbon dioxide) of nitrogen oxides per year, nature for over 15 teragrams. The global anthropogenic nitrous oxide flux is about 1 petagram of carbon dioxide carbon-equivalents per year; this compares to 2 petagrams of methane carbon dioxide carbon-equivalents per year, and to an atmospheric loading rate of about 3.3 petagrams of carbon dioxide carbon-equivalents per year.

Nitrous oxide also attacks ozone in the stratosphere, aggravating the excess amount of UV light striking the earth's surface in recent decades, in a manner similar to various freons and related halogenated organics. Nitrous oxide is the main naturally occurring regulator of stratospheric ozone.

Recent research by Nobel Laureate Paul Crutzen suggests that emissions of nitrous oxide in the production of biofuels are more than enough to offset the advantages that biodiesel was hoped to have in terms of carbon dioxide emissions. More generally this concerns the use of all nitrogen fertilizer