More famous as ‘laughing gas’ than for its influence on global warming, Nitrous oxide (N2O) is one of the worst greenhouse gases in terms of its heat-absorption, although its concentration in the atmosphere is relatively low. It’s the third most abundant man-made greenhouse gas after carbon dioxide (CO2) and methane (CH4).
In 2018, its atmospheric concentration was 331.1 ppb (parts per billion), roughly 23 percent higher than its pre-industrial levels. 2 Global annual man-made emissions of nitrous oxide exceed 3 billion tonnes of CO2 equivalent 3 4 , with China accounting for almost one fifth of the world’s total.
At room temperature, nitrous oxide gas is colourless and non-flammable, with a slight metallic smell. As well as being a damaging greenhouse gas, nitrous oxide has important medical uses, especially in dentistry and surgery, where it is valued for its anaesthetic and pain reducing effects. Its nickname “laughing gas”, is due to the euphoric effect it can have on the person inhaling it. Nitrous oxide is the only greenhouse gas that is also on the World Health Organization’s List of Essential Medicines. 9
Does Nitrous Oxide Damage The Ozone Layer?
Yes. It appears so. If it reaches the stratosphere, nitrous oxide reacts with sunlight and oxygen to form nitrogen oxide (NOx), a chemical known to damage the ozone layer which protects us from the harmful ultraviolet rays of the sun. Ozone in the stratosphere is depleted by several different chemicals; most famously by chlorofluorocarbons (CFCs), the chemicals responsible for creating the ozone hole over Antarctica. But now some scientists believe that nitrous oxide may be the biggest threat. What’s more, unlike CFCs and their successors, nitrous oxide is not regulated by the Montreal Protocol. In any event, it seems that reducing nitrous oxide emissions is doubly beneficial – it mitigates climate change and it helps to repair the ozone hole. 10
What Are The Main Sources Of Nitrous Oxide Emissions?
Roughly 62 percent of all nitrous oxide emissions are caused by natural processes in the Earth’s soil, oceans and atmosphere. Historically, these natural emissions were offset by natural sinks, so before the Industrial Revolution levels of nitrous oxide were perfectly stable due to this natural balance.
Uncultivated Soils Under Vegetation
The soil produces 60 percent of all N2O emissions. The most productive sources are uncultivated soils under natural vegetation, which form the bulk of Earth’s land surface. Nitrous oxide emanates from the soil via two biological pathways: nitrification and denitrification. Nitrification in the soil is performed by ammonia oxidizing bacteria who produce nitrate from ammonium, but who also produce nitrous oxide as a by-product. Because this is an aerobic process it works best in well drained and aerated ground.
In lower-oxygen environments such as waterlogged or more compact soils, anaerobic bacteria predominate. These organisms carry out the process of denitrification. This involves the breakdown of nitrate in the soil to gaseous nitrogen (N2). Once again, nitrous oxide is produced on the side – generally a larger amount during denitrification rather than nitrification. How much nitrous oxide a particular soil produces is dependent on several variables, such as temperature and water content, as well as the ammonium and nitrate content of the soil.
Riverside and tropical rainforest soils are prolific contributors to the natural nitrous oxide budget because they contain more nutrients and moisture.
The ocean produces 35 percent of all nitrous oxide emissions. Microbial organisms living in the ocean create these emissions as they break down fecal pellets and other sinking particles of organic matter. Because this decomposition process takes place in an anaerobic environment, it involves denitrification. Aside from its contribution to greenhouse gas emissions, the decomposing actions of these microbes play a huge role in the oceanic nitrogen and energy cycles.
Atmospheric Chemical Reactions
These reactions account for 5 percent of all N2O emissions. Nitrous oxide is produced through the oxidation of ammonia which is a natural occurring gas in the atmosphere. Ammonia is the most abundant alkaline gas in the atmosphere, existing at levels between 1 and 5 ppb. Important sources of atmospheric ammonia include manure from wild animals as well as rotting vegetation.
Nitrous Oxide From Permafrost?
That’s right. In case you thought that Arctic permafrost was only a source of carbon dioxide and methane emissions, rest assured it also emits nitrous oxide. That’s according to a recent study which surveyed 120 square miles of the permafrost surface, during August, using the airborne eddy-covariance method. It found nitrous oxide was being emitted, and at roughly 12 times the rate previously assumed. What’s significant, says study co-author Ron Dobosy, of the NOAA, is that until our discovery, the Arctic was considered to be nitrogen poor. Now it’s clear that nitrous oxide emissions are present, and need to be studied further.” said Dobosy. 12
Human sources account for about 38 percent of N2O emissions. (But see: Greenhouse Gas Statistics Lack Consistency.) These may be smaller than natural emissions, but they are the proverbial straw that breaks the camel’s back, by upsetting the nitrogen balance that existed up until the Industrial Revolution. 13
Prior to 1750, atmospheric levels of nitrous oxide remained stable because of natural sinks. But since then human activity has upset the balance and placed a huge strain on the planet’s ecosphere. Nitrous oxide levels are now higher than they have been for the last 800,000 years. 14 The main causes for this are agricultural practices – chiefly the over-use of nitrogen fertilizers – and fossil fuel combustion, which together account for over three quarters of all emissions. 11
Agriculture accounts for 67 percent of man-made nitrous oxide gases. The use of nitrogen fertilizer to improve crop yields is a major source of nitrous oxide emissions. Fertilizers help feed plants through the addition of nitrogen to the soil. But bacteria living in the soil use this extra nitrogen to produce the energy they need, in order to drive the processes of nitrification and denitrification, which – as we have seen – produces nitrous oxide as a by-product.
What’s more, when nitrogen fertilizer is added to the soil to boost plant growth, only about half is used by the plant, according to Neville Millar, a senior researcher at Michigan State University. The rest is washed away in groundwater, or off-gassed as nitrous oxide or other gases.
Rainfall or irrigation water causes part of the reactive nitrogen in fertilizers to seep into groundwater or else be washed away via drainage ditches, into streams and rivers, eventually reaching the ocean. But a significant amount of this fertilizer is left behind along the way, to be broken down by bacteria leading to more nitrous oxide emissions.
NOTE: For more information about greenhouse gas emissions from agriculture & livestock, along with more details about the greenhouse effect, see: 50 Frequently Asked Questions on Climate Change.
The storage and handling of animal waste is another direct source of agricultural greenhouse gas emissions. The same bacteriological processes responsible for nitrification and denitrification in soils, are responsible for the nitrous oxide discharged when the nitrogen in manure decomposes. So, all animal farms produce significant quantities of nitrous oxide emissions.
Intensive livestock farming systems have low nitrogen use efficiency, creating large amounts of losses. Only a small fraction of the ingested nitrogen in animal feed (about 5-30%) is retained in milk, meat, and eggs. The bulk part, 70-95%, is lost via animal waste. This extra nitrogen gets added to the environment and is causing increases in nitrous oxide emissions. Livestock is also a major emitter of methane (CH4). See also: Why are Methane Levels Rising?
Fossil Fuel Combustion
Fossil fuels, along with certain industrial processes, are another important anthropogenic source of nitrous oxide, accounting for 10 percent of emissions.
Nitrous oxide is a by-product of nearly all types of fossil fuel combustion. It happens when part of the nitrogen content of the fuel and surrounding air gets oxidized, thus creating nitrous oxide. Most N2O emissions from stationary sources come from coal, burned inside coal-fired power plants. Most mobile emissions come from petroleum, burned in cars, buses and trucks.
The two main industrial processes that cause nitrous oxide emissions are the production of nitric and adipic acid. Nitric acid is an important ingredient in the manufacture synthetic fertilizers, while adipic acid is used to make nylon and other synthetic fibers. In both cases, oxidization of nitrogen compounds during the manufacturing process creates nitrous oxide emissions.
This category is also responsible for about 10 percent of N2O emissions. Biomass burning involves the burning of wood, vegetation, crop residues or any organic matter. During these fires, some of the nitrogen in the burnt material and surrounding air is oxidized creating nitrous oxide emissions. Biomass fires are mainly started by humans to eliminate crop waste and clear land for cultivation, grazing or other commercial uses. Natural wildfires also fall into this category but, with occasional exceptions such as the 2019 fires in the Arctic, the vast majority of biomass burning is caused by human beings. See: What is the Effect of Wood Burning on Climate Change?
Atmospheric deposition accounts for 9 percent of nitrous oxide emissions. Anthropogenic activities – nitrogen vapors and gases from fossil fuel smoke or burning biomass – release nitrogen compounds into the atmosphere which eventually sinks back down to the Earth’s surface. This supplies terrestrial ecosystems with extra nitrogen which duly stimulates nitrifying and denitrifying microbes to produce more N2O.
Human waste, like animal waste, is a significant source of nitrous oxide emissions. The processes of decay and decomposition that take place in septic tanks and sewage treatment plants create conditions that are highly favorable for nitrous oxide producing bacteria. This is because bacteria use nitrification and denitrification processes to break down the nitrogen-based organic materials found in human waste. Human sewage accounts for 3 percent of N2O emissions.
Why Are Nitrous Oxide Emissions Increasing?
Like methane, nitrous oxide emissions are likely to be strongly affected by global warming. Rising temperatures and higher rates of precipitation lead to more fertile growing conditions, faster decomposition and more active bacteria to drive the twin processes of nitrification and denitrification. Emissions from the pedosphere and from agriculture are almost certain to increase in line with temperature.
A recent study of global N2O emissions, using atmospheric data and nitrous oxide concentrations from over 50 stations around the world, found that global N2O emissions had increased substantially since 2009 and at about twice the increment reported to the UN Framework Convention on Climate Change (UNFCCC). The study authors argue that this discrepancy is due to the fact that emissions do not increase in line with the amount of fertilizer used, exactly as the IPCC rulebook states. They believe that the IPCC method, which assumes a constant rate of emission, may understate the situation and that N2O emissions are more damaging to our climate system than we thought. East Asia and South America were responsible for the largest contributions to the increase of N2O, almost certainly because of the booster effect of warming on sub-tropical growth. 8
Nitrous Oxide Emissions Are A Food Issue!
For years, climatologists have sounded warnings about the risks from nitrous oxide and its contribution to the greenhouse effect, and yet there’s been little remedial action taken by climate organizations or governments.
The reason: “It is intimately connected to food,” said Ravi Ravishankara, an atmospheric chemist at Colorado State University who co-chaired a U.N. panel on stratospheric ozone from 2007 to 2015.
The largest source of nitrous oxide emissions is agriculture, chiefly fertilized soil. Trouble is, it’s soil that is being used to grow the food needed for the 4 billion new human beings due to be born before 2100. “One could imagine limiting carbon dioxide or methane. But nitrous oxide is so much a food production issue,” Ravishankara said.
Not everyone is convinced that more fertilizer is the always the right answer. A growing number of farmers are discovering that fertilizer use is not always compatible with good soil health.
Among the leading proponents of the soil health movement in America, is U.S. Department of Agriculture (USDA) researcher Rick Haney. Haney travels around the country teaching farmers how to create healthy soil. His message is simple: years of agricultural abuse have depleted the soil of essential nutrients as well as valuable bacteria and fungi that create vital organic material needed for plant health. “Our mindset nowadays is that if you don’t put down fertilizer, nothing grows,” says Haney. “But that’s just not true, and it never has been.” 15 For more, see: Why is Soil So Important to the Planet?)
The last word belongs to the U.S. Environmental Protection Agency, who states: “The application of nitrogen fertilizers accounts for the majority of N2O emissions in the United States. Emissions can be reduced by reducing nitrogen-based fertilizer applications and applying these fertilizers more efficiently.” 16
- IPCC. Fifth Assessment Report (2013), cited in “What is a Global Warming Potential and which one do I use?” Michael Gillenwater. GHG Management Institute. April 1st, 2015.
- “Greenhouse gas concentrations in atmosphere reach yet another high.” World Meteorological Office (WMO) November 25, 2019.
- “Nitrous oxide emissions.” World Data Atlas. Knoema (2012)
- Global Carbon Project (2018)
- On a CO2eq basis, assuming a 100-year time span. Intergovernmental Panel on Climate Change (IPCC) Working Group III report on the Mitigation of Climate Change. Fifth Assessment Report. 2013.
- “Acceleration of global N2O emissions seen from two decades of atmospheric inversion.” R. L. Thompson, et al. Nature Climate Change, 2019.
- “Overview of Greenhouse Gases: Nitrous Oxide Emissions”. United States Environmental Protection Agency. 6 October 2016.
- “Acceleration of global N2O emissions seen from two decades of atmospheric inversion.” R. L. Thompson, L. Lassaletta, P. K. Patra, C. Wilson, K. C. Wells, A. Gressent, E. N. Koffi, M. P. Chipperfield, W. Winiwarter, E. A. Davidson, H. Tian and J. G. Canadell. Nature Climate Change. Vol 9, pp 993-998. December 2019.
- WHO 21st Essential Medicines List (2019)
- “Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century”. Ravishankara, A. R.; Daniel, J. S.; Portmann, R. W. (2009). Science. 326 (5949): 123–5.
- IPCC – Fourth Assessment Report (2007)
- “Permafrost nitrous oxide emissions observed on a landscape scale using the airborne eddy-covariance method.” Jordan Wilkerson, Ronald Dobosy, David S. Sayres, Claire Healy, Edward Dumas, Bruce Baker and James G. Anderson. Atmospheric Chemistry and Physics, 19, 4257–4268, 2019.
- Denman, K.L. et al. “Couplings Between Changes in the Climate System and Biogeochemistry.” In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press.
- “Glacial–interglacial and millennial-scale variations in the atmospheric nitrous oxide concentration during the last 800,000 years.” Schilt, Adrian, Matthias Baumgartner, Thomas Blunier, Jakob Schwander, Renato Spahni, Hubertus Fischer, and Thomas F. Stocker. Quaternary Science Reviews 29, no. 1-2 (2010): 182-192.
- “Why It’s Time to Stop Punishing Our Soils with Fertilizers.” Richard Schiffman. May 3, 2017. Yale Environment 360. Yale School of Forestry and Environmental Studies.
- “Overview of Greenhouse Gases.” EPA.