Nitrogen emissions have a net cooling effect. But researchers warn against a climate solution.

Published today in Nature, the paper found that reactive nitrogen released in the environment through human activities cools the climate by minus 0.34 watts per square metre. While global warming would have advanced further without the input of human-generated nitrogen, the amount would not offset the level of greenhouse gasses heating the atmosphere. 

The paper was led by the Max Planck Institute in Germany and included authors from the University of Sydney. It comes one day after new data from the European Union’s Copernicus Climate Change Service indicated that Sunday, 21 July was the hottest day recorded in recent history. 

The net cooling effect occurs in four ways: 

• Short-lived nitrogen oxides produced by the combustion of fossil fuels pollute the atmosphere by forming fine suspended particles which shield sunlight, in turn cooling the climate; 

• ammonia (a nitrogen and hydrogen-based compound) released into the atmosphere from the application of manure and artificial fertilisers has a similar effect; 

• nitrogen applied to crops allows plants to grow more abundantly, absorbing more CO2 from the atmosphere, enabling a cooling effect; 

• nitrogen oxides also play a role in the breakdown of atmospheric methane, a potent greenhouse gas.  

The researchers warned that increasing atmospheric nitrogen was not a solution for combatting climate change.  

“Nitrogen fertilisers pollute water and nitrogen oxides from fossil fuels pollute the air. Therefore, increasing rates of nitrogen in the atmosphere to combat climate change is not an acceptable compromise, nor is it a solution,” said Professor Federico Maggi from the University of Sydney’s School of Civil Engineering.  

Sönke Zaehle from the Max Planck Institute said: “This may sound like good news, but you have to bear in mind that nitrogen emissions have many harmful effects, for example on health, biodiversity and the ozone layer. The current findings, therefore, are no reason to gloss over the harmful effects, let alone see additional nitrogen input as a means of combatting global warming."

Elemental nitrogen, which makes up around 78 percent of the air, is climate-neutral, but other reactive nitrogen compounds can have direct or indirect effects on the global climate – sometimes warming and at other times cooling. Nitrous oxide (N2O) is an almost 300 times more potent greenhouse gas than CO2. Other forms of nitrogen stimulate the formation of ozone in the troposphere, which is a potent greenhouse gas and enhances global warming.  

Professor Maggi said the research was important as it helped the team gain an understanding of the net-effect of the distribution of nitrogen emissions from agriculture.  

“This work is an extraordinary example of how complex interactions at planetary scales cannot be captured with simplistic assessment tools. It shows the importance of developing mathematical models that can show the emergence of nonlinear – or unproportional -- effects across soil, land, and atmosphere,” he said.  

“Even if it appears counter-intuitive, reactive nitrogen introduced in the environment, mostly as agricultural fertilisers, can reduce total warming. However, this is minor compared with the reduction in greenhouse gas emissions required to keep the planet within safe and just operational boundaries. 

“New generation computational tools are helping drive new learnings in climate change science, but understanding is not enough – we must act with great urgency to reduce greenhouse gas emissions.” 

Gaining a holistic understanding of the impacts of nitrogen

The scientists determined the overall impact of nitrogen from human sources by first analysing the quantities of the various nitrogen compounds that end up in soil, water and air.  

They then fed this data into models that depict the global nitrogen cycle and the effects on the carbon cycle, for example the stimulation of plant growth and ultimately the CO2 and methane content of the atmosphere. From the results of these simulations, they used another atmospheric chemistry model to calculate the effect of man-made nitrogen emissions on radiative forcing, that is the radiant energy that hits one square metre of the Earth's surface per unit of time. 

DECLARATION

The authors declare no competing interests. The research was supported by the European Commission, US National Science Foundation, USDA CBG, US National Science Foundation, US Department of Treasury, JSPS KAKENHI, Office of Biological and Environmental Research of the US Department of Energy Office of Science.