Abstract
Nitrous oxide (N(2)O) has a global warming potential that is 300 times that of carbon dioxide on a 100-y timescale, and is of major importance for stratospheric ozone depletion. The climate sensitivity of N(2)O emissions is poorly known, which makes it difficult to project how changing fertilizer use and climate will impact radiative forcing and the ozone layer. Analysis of 6 y of hourly N(2)O mixing ratios from a very tall tower within the US Corn Belt-one of the most intensive agricultural regions of the world-combined with inverse modeling, shows large interannual variability in N(2)O emissions (316 Gg N(2)O-N⋅y(-1) to 585 Gg N(2)O-N⋅y(-1)). This implies that the regional emission factor is highly sensitive to climate. In the warmest year and spring (2012) of the observational period, the emission factor was 7.5%, nearly double that of previous reports. Indirect emissions associated with runoff and leaching dominated the interannual variability of total emissions. Under current trends in climate and anthropogenic N use, we project a strong positive feedback to warmer and wetter conditions and unabated growth of regional N(2)O emissions that will exceed 600 Gg N(2)O-N⋅y(-1), on average, by 2050. This increasing emission trend in the US Corn Belt may represent a harbinger of intensifying N(2)O emissions from other agricultural regions. Such feedbacks will pose a major challenge to the Paris Agreement, which requires large N(2)O emission mitigation efforts to achieve its goals.