Conceptual View of the Nitrogen Cycle on Canadian Farms. Graphic: Agriculture and Agri-Food Canada

By Sam Lemonick
17 October 2017

(Forbes) – A years-long study of Minnesota farm fields has found that emissions of nitrous oxide—a more dangerous greenhouse gas than carbon dioxide—will likely increase as our planet gets warmer. And our climate models aren't ready.

Like carbon dioxide, nitrous oxide (yes, the same stuff as laughing gas) traps heat in Earth’s atmosphere. Although it’s thought to account for only about 6% of the greenhouse effect today, it is about 300 times worse for the climate than CO2.

One major source is runoff from farms and fields. Bacteria breaking down nitrogen compounds from manure and synthetic fertilizers generate nitrous oxide as a waste product.

Climate scientists and policy makers have tried to include nitrous oxide in their models of climate change. But until now we haven’t had a good idea about whether nitrous oxide emissions will get worse or better as global temperatures increase.

“There’s a lot of uncertainty about how nitrous oxide is going to respond to changing temperature and moisture levels,” says David Kanter, a professor of environmental studies at New York University who has studied nitrogen pollution. He was not involved in the new research.

The answer appears to be worse. Tim Griffis, a biometeorologist at the University of Minnesota, led a team that recorded nitrous oxide levels every hour for six years from a tower 200 feet above a local farm field. They found that nitrous oxide emissions were highest during the warming years—a worrying sign of things to come. Their research appears in the Proceedings of the National Academy of Sciences.

The research team monitored the air at the site from 2010 to 2016, tracking air parcels as they rose from the fields and drainage ditches. In 2012, the warmest year of their study (about 2.5 ℃ above normal), nitrous oxide emissions were 50% higher than expected. [more]

Global Warming Could Make This Lurking Climate Threat Even Worse


ABSTRACT: Nitrous oxide (N2O) 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 N2O 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 N2O 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 N2O emissions (316 Gg N2O-N⋅y−1 to 585 Gg N2O-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 N2O emissions that will exceed 600 Gg N2O-N⋅y−1, on average, by 2050. This increasing emission trend in the US Corn Belt may represent a harbinger of intensifying 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.

SIGNIFICANCE: N2O has 300 times the global warming potential of CO2 on a 100-y timescale, and is of major importance for stratospheric ozone depletion. The climate sensitivity of N2O emissions is poorly known, which makes it difficult to project how changing fertilizer use and climate will impact radiative forcing and the ozone layer. Here, atmospheric inverse analyses reveal that direct and indirect N2O emissions from the US Corn Belt are highly sensitive to perturbations in temperature and precipitation. We combine top-down constraints on these emissions with a land surface model to evaluate the climate feedback on N2O emissions. Our results show that, as the world becomes warmer and wetter, such feedbacks will pose a major challenge to N2O mitigation efforts.

Nitrous oxide emissions are enhanced in a warmer and wetter world

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