# Projecting nitrous oxide over the 21st century, uncertainty related to stratospheric loss

## TL;DR

This paper examines how changes in the stratospheric sink of nitrous oxide (N2O) affect future projections of its atmospheric abundance and climate impact.

## Contribution

The study highlights the importance of stratospheric sink changes in N2O projections, introducing significant uncertainty comparable to socioeconomic scenarios.

## Key findings

- N2O lifetime decreased by –1.4 ± 0.9% per decade from 2004 to 2024.
- Changes in stratospheric circulation and temperature explain the lifetime decrease.
- The effect on N2O's global warming potential is a –11% shift by 2100.

## Abstract

Projecting atmospheric nitrous oxide (N2 O) abundance is critical for climate and ozone assessments. Research has focused on projecting the changing emissions of N2 O from direct anthropogenic sources, the dominant cause of the recent growth. Earth system models are now projecting natural sources perturbed by climate change. There has been little effort to understand how climate and compositional changes may change the stratospheric sink of N2 O, which balances all these sources and also controls the atmospheric abundance. Here, we review recent observational and modeling evidence for an increase in the sink caused by decreasing N2 O lifetime and show that it introduces uncertainties comparable to shifts across the different shared socioeconomic pathway scenarios used in current assessments.

Extending the N2O lifetime derived from Microwave Limb Sounder satellite observations, we find a mean value of 117 y and a likely decrease of –1.4 ± 0.9% per decade over the period 2004 to 2024. This trend is consistent with the previously published 2004 to 2021 value of –2.1 ± 1.2% per decade. A more careful analysis of uncertainty now provides a more robust likely (one-sigma) range. From analyses of a range of factors controlling the N2O lifetime, we find that the decrease in lifetime can be explained by recent changes in stratospheric circulation and temperature. Projection of the lifetime change to 2100 shows that this effect is comparable to differences across the shared socioeconomic pathways used for climate projections and cannot be ignored. An updated evaluation of the N2O chemical feedbacks shows that this effect produces a relatively small shift in atmospheric abundance over the 21st century, but still an important shift, –11%, in the global warming potential of N2O.