Nitrous oxide (N2O) is commonly known as laughing gas and is widely used as an anaesthetic in the field of medicine. It has a slightly sweet odour and was first described by Joseph Priestley in 1772. It is a relatively inert gas and, like the other greenhouse gases (GHGs) - carbon dioxide (CO2) and methane (CH4) - its mixing ratio in the atmosphere has risen markedly in the last 200 years, increasing from ~0.270 ppm in the pre-industrial era to 0.315 ppm currently, with the rate of increase estimated to be ~0.25% per year. Although its concentration is very small relative to that of CO2, N2O has two attributes that make it a potent GHG: (i) it is chemically rather inert, with a lifetime of ~120 years in the atmosphere; and (ii) each molecule of N2O has a much greater radiative forcing potential than a molecule of CO2.

Together, these attributes mean that N2O has a global warming potential (GWP) of 296 on a 100-year time horizon (see Chapter 5), relative to CO2 on a mass basis (i.e. 1 kg of N2O is equivalent to 296 kg of CO2). Thus it has contributed a significant amount to the enhanced radiative forcing from GHGs since pre-industrial times -~6% of the total. A doubling in its concentration would lead to a global temperature rise of ~0.7°C.

Before considering the major sinks for N2O let us first examine where the N2O that enters our atmosphere comes from. Global emissions total ~18 Tg N2O-N/year. Of this, ~10 Tg arises from 'natural sources'.

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