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Title: Nitrous oxide emissions are enhanced in a warmer and wetter world

item GRIFFIS, TIMOTHY - University Of Minnesota
item CHEN, Z - University Of Minnesota
item Baker, John
item WOOD, JEFFREY - University Of Missouri
item MILLET, DYLAN - University Of Minnesota
item LEE, XUHUI - Yale University
item Venterea, Rodney - Rod
item TURNER, PETER - Yale University

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2017
Publication Date: 11/7/2017
Citation: Griffis, T.J., Chen, Z., Baker, J.M., Wood, J.D., Millet, D.B., Lee, X., Venterea, R.T., Turner, P.A. 2017. Nitrous oxide emissions are enhanced in a warmer and wetter world. Proceedings of the National Academy of Sciences. 114(45):12081-12085.

Interpretive Summary: Nitrogen fertilizer production and use has been increasing for many years and is projected to continue on that trajectory as global population increases. Approximately 4% of the nitrogen (N) applied as fertilizer returns to the atmosphere as nitrous oxide (N2O), a greenhouse gas that is 300 times as potent per molecule as carbon dioxide. It is also the primary ozone-depleting gas. A key question is how N2O emissions will respond to expected changes in global climate. We used a unique 6-year data set of N2O concentrations measured at half-hourly intervals from a very tall radio tower in conjunction with an inverse model to estimate regional emissions of N2O within the agriculturally intensive US Corn Belt. We found that annual emission was highly sensitive to climatic variations; in the warmest spring, 2012, more than 7 % of the nitrogen applied was emitted as N2O, nearly double the rate assumed by the Intergovernmental Panel on Climate Change (IPCC). Factoring in expected trends in climate and N fertilizer use, we estimate that regional N2O emission will exceed 600 Gg of N per year by 2050. This represents a major challenge as nations attempt to meet GHG reductions that are the focus of the Paris Climate Accord.

Technical Abstract: Production of synthetic nitrogen (N) fertilizer is projected to exceed 200 Tg (1 Tg = 1012 g) N in 2018 – a 25% increase since 2008. The current trajectory of N fertilizer demand is exceeding some of the most aggressive forecasts. Globally, about 4% of the anthropogenic N in agricultural systems is returned to the atmosphere as nitrous oxide (N2O) - a potent greenhouse gas and the predominant ozone-depleting emission. Growing population and demand for food, fiber, and biofuel are accelerating the use of synthetic N, yet it remains uncertain how N2O emissions will respond to changes in climate. Using six years of hourly N2O concentration measurements from a very tall tower, combined with inverse modeling, we show large inter-annual variability in N2O emissions (315 to 555 Gg N2O-N y-1) within the US Corn Belt – one of the most intensive agricultural regions of the world. The large inter-annual variability 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 7.1%, nearly double previous reports. Given current trends in climate and synthetic N fertilizer use, our data and model analyses indicate a strong positive feedback to climate and unabated growth of regional N2O emissions that will exceed 600 Gg N2O-N y-1 by 2050. The increasing emission trend in the US Corn Belt may represent a harbinger of the intensification of N2O emissions from other agricultural regions. Such feedbacks will pose a major challenge to the Paris Agreement, which requires large N2O emission mitigation efforts to achieve its goals.