Nitrous oxide has an atmospheric lifetime of approximately 109 years — a fact with profound implications for climate policy. Unlike CO₂, which reaches equilibrium on millennial timescales through ocean and geological processes, N₂O’s fate is primarily determined by stratospheric photodestruction.
Stratospheric Destruction
N₂O molecules are chemically stable in the troposphere, where OH radical concentrations are too low to initiate significant reactions. Transport to the stratosphere via the Brewer-Dobson circulation takes months to years. Once in the stratosphere, UV radiation at wavelengths below 240 nm photolyzes N₂O:
N₂O + hν → N₂ + O(¹D) [primary pathway, ~90%]
N₂O + O(¹D) → 2 NO [produces ozone-depleting NOₓ]
N₂O + O(¹D) → N₂ + O₂ [minor pathway]
Impulse Response
The long lifetime means that N₂O emissions today will still be contributing to warming in the 22nd century. Climate scientists model this using impulse response functions: a pulse emission of N₂O has a long tail, with about 50% still present after 100 years. This long commitment contrasts sharply with methane, which has a 12-year lifetime and offers faster climate response to emission reductions.