Laughing Gas Is No Joke: The Forgotten Greenhouse Gas | Article

Nitrous oxide is known as laughing gas for the feelings of euphoria it can induce, but its effect on the climate is no joke. Nitrous oxide has nearly 300 times the warming power of carbon dioxide. Moreover, nitrous oxide stays in the atmosphere for an average of 114 years, where it can be converted into nitrogen oxides that deplete the stratospheric ozone layer and expose the Earth to more solar radiation, thereby damaging crops and human health. While about 60 percent of global nitrous oxide emissions occur naturally, the remaining 40 percent are attributable to human activities. Small yet mighty, nitrous oxide accounted for seven percent of all U.S. anthropogenic greenhouse gas emissions in 2020. And these emissions are accelerating. The global rise in nitrous oxide emissions from 2020 to 2021 was higher than their average annual growth rate over the past 10 years.

Nitrous oxide (N₂O) is not to be confused with the nitrogen oxides: nitrogen dioxide (NO₂) and nitrogen monoxide (NO). NO₂ and NO, both referred to as NOₓ (pronounced like the nox in noxious), are pollutants that create smog. Nitrous oxide, meanwhile, is a colorless gas that is often used in medical or laboratory settings, such as an anesthetic for dental procedures. The gas is often emitted through agricultural processes and wastewater applications, and measures can be taken to reduce such emissions.

Agriculture

Agriculture is one key industry where nitrous oxide emissions can be reduced. Agricultural soil management practices produce 74 percent of U.S. nitrous oxide emissions, while manure management produces an additional five percent. Soil management techniques like nitrogen-based fertilizer application, cropping practices, liquid waste management, and agricultural residue burning can emit nitrous oxide.

Nitrogen is very valuable for boosting plant health, and is a crucial component of many commercial fertilizers. However, in many cases only half of the nitrogen from a fertilizer makes its way into the crop, with the rest seeping into groundwater or rising into the atmosphere as a gas. This nitrogen runoff can pollute waterways or be consumed by bacteria that create nitrous oxide as a byproduct. Similarly, manure fertilizer overapplied to crops can be consumed by microbes that produce nitrous oxide.

Nitrous oxide emissions can become part of a “climate feedback” process. As the Earth gets warmer, nitrous oxide emissions increase, since warmer and wetter conditions stimulate denitrification, the conversion of solid nitrogen into its gaseous forms by microbes. These additional nitrous oxide emissions further contribute to global warming, creating a vicious circle. Granted, the extra carbon dioxide in the atmosphere that causes climate change also causes plant growth, which leads them to take up more nitrogen and actually slightly reduces nitrous oxide emissions. But overall, climate change is leading to more nitrous oxide emissions.

There are many soil management techniques that can help decrease nitrous oxide emissions. For example, remote sensing technology can help implement precision agriculture to sense exactly when nitrogen should be applied to fields. Nitrification inhibitors can be applied to inhibit microbes from creating nitrous oxide. Finally, irrigation methods like drip and subsurface drip irrigation have been shown to decrease nitrous oxide emissions when compared to surface gravity irrigation. Tweaking fertilizer formulation, such as using more urea (a chemical composed of ammonia and carbon dioxide), may also help decrease agricultural nitrous oxide emissions.

Several regenerative agriculture practices can efficiently reduce nitrous oxide emissions. Cutting back pesticide use encourages a diverse microbial community, which in turn leads to natural nitrogen fixation. This converts atmospheric nitrogen into a form plants can use, limiting the need for artificial nitrogen fertilizers. No-till farming aerates the soil, creating oxygenated environments that decrease the production of microbial nitrous oxide. In addition, nitrogen-fixing cover crops, like clover, can help reduce the amount of synthetic nitrogen fertilizer used—although other cover crops like legumes can actually increase nitrous oxide emissions.

Wastewater

Wastewater treatment also creates nitrous oxide as a byproduct of activated sludge processes, which are used to speed up waste decomposition. Wastewater treatment is responsible for six percent of anthropogenic nitrous oxide emissions in the United States.

Nitrous oxide emissions vary significantly between wastewater treatment plants. Low concentrations of ammonia and nitrite in effluent are factors that can decrease nitrous oxide emissions. In order to treat wastewater, treatment plants work to remove nitrogen from the water, which is present in wastewater as ammonium. Through the nitrification and denitrification processes, wastewater treatment plants remove biological nutrients while releasing nitrous oxide. Incomplete nitrification processes can increase nitrite concentrations, causing nitrous oxide emissions to spike. Full-scale nitrous oxide emission mitigation strategies at wastewater treatment plants are still rare, in part due to uncertainty about their effectiveness. However, a 2021 study concluded that maintaining appropriate concentrations of dissolved oxygen and balancing bacterial wastewater feeding practices can mitigate nitrous oxide emissions.

Other Sources

Most nitrous oxide emissions come from agricultural soil management and wastewater treatment. However, transportation, stationary combustion, and industrial production are also contributors. Most nitrous oxide emissions from global transportation result from the exhaust control systems of light-duty vehicles. While catalytic converters help reduce pollutants like carbon monoxide and nitrogen oxides from vehicles, they can also end up generating nitrous oxide.

Nitrous oxide is also emitted from stationary combustion sources, mainly coal plants. Therefore, developing cleaner energy sources for electricity will be beneficial in decreasing nitrous oxide emissions.

Finally, reductions to industrial nitrous oxide emissions are considered “the lowest-hanging fruit in our arsenal of N₂O mitigation” according to Stanford University researcher Rob Jackson. Nylon and polyurethane, materials widely used in a variety of industries, require adipic acid—the chemical production of which leads to dangerous levels of nitrous oxide emissions. These emissions, however, can be captured at the source and purified for reuse, as some companies are already doing.

Moving Forward

Historically neglected, nitrous oxide emissions are finally receiving some much-needed attention because of the harm they cause to the climate. In fact, reducing non-CO2 emissions like those of nitrous oxide is listed as one of the pathways for the United States to reach net-zero greenhouse gas emissions by 2050. The recently-enacted Inflation Reduction Act (P.L. 117-169) invests over $16 billion into practices that sequester, capture, and avoid greenhouse gas production associated with agriculture—including nitrous oxide emissions. An additional $300 million has been allocated to quantify nitrous oxide emissions from agriculture using field-based data and assess the impact of conservation activities. Investing in the reduction of nitrous oxide emissions is becoming a crucial tool in our arsenal for the fight against greenhouse gas emissions.

Authors: Nathan Lee and Molly Brind’Amour

Read the other articles in our Non-CO2 Greenhouse Gases series on Methane and Fluorinated Gases.