Nitrous oxide (N2O) is commonly known as "laughing gas". Its building blocks are the two dominant gases in the atmosphere: nitrogen and oxygen. Both play important roles in the chemistry of living things (nitrogen goes into amino acids, the "building blocks" of protein and key to metabolism, growth, and repair of tissue).

Although nitrogen and oxygen make up some 99 per cent of the atmosphere, nitrous oxide is scarce - with a mean concentration of 310 ppbv (parts per billion by volume) in 1980, according to the Intergovernmental Panel on Climate Change (i.e. less than one-thousandth as abundant as carbon dioxide).

The scarcity of nitrous oxide in the atmosphere is countered by its effectiveness as a greenhouse gas: a molecule of nitrous oxide is 200 to 300 times more effective than a molecule of carbon dioxide in the greenhouse warming it produces. Furthermore, it is longer-lived in the atmosphere than carbon dioxide and some other radiatively active gases. Nitrous oxide is eventually broken down by light in the stratosphere into nitrogen and oxygen, but its atmospheric lifetime is about 150 years. As a result, the cumulative effect of any human-induced or anthropogenic emissions of nitrous oxide will be greater than those of carbon dioxide.

Nitrogen is constantly being taken out of the atmosphere and put into the soil by micro-organisms (either alone, or in symbiosis with plants). Nitrogen in the atmosphere (N2) is fairly inert and largely unusable by living things. The process by which micro-organisms convert atmospheric nitrogen into forms such as ammonia (NH3) which can be used by plants is called nitrogen fixation. After nitrogen is fixed, it may be cycled from the soil into plants and animals many times.

All the while, other micro-organisms, in a process called denitrification, are constantly taking nitrogen out of its fixed form in the soil and putting it back into the atmosphere. Besides yielding molecular nitrogen (N2), denitrification produces nitrous oxide.Scientists estimate that soil denitrification is the dominant source of nitrous oxide going into the atmosphere. Another important contribution comes from natural ocean processes, which are not so well understood.

At the beginning of the industrial age, atmospheric concentrations of nitrous oxide were steady at about 285 ppbv, so the current 310 ppbv concentration represents an 8.8 per cent increase. This increase is believed to result from human activities.

Combustion is one source of nitrous oxide, whether burning fossil fuels, wood, or other biomass. But scientists today think combustion may be less important a source of anthropogenic nitrous oxide than the use of fertiliser.

Anyone who has ever struggled with a front lawn knows that nitrogen is the key to making leafy green things grow abundantly. Farmers know this, too. Worldwide, fertiliser production puts some 55 Teragrams (Tg, or 1012 grams) of nitrogen into the soil each year. Nitrogen fertiliser is made either by mining nitrates or by "fixing" atmospheric nitrogen (into the usable form of nitrate or ammonium) by industrial processes. When this artificially enriched soil is denitrified, or when fertilisers leach into groundwater, nitrous oxide goes into the atmosphere.

Estimates of the amount of nitrous oxide put into the atmosphere from fertiliser use vary wildly - from as low as .01 TgN/year to as high as 2.2 TgN/year. This kind of uncertainty makes it hard to assess the role of nitrous oxide in any human-induced greenhouse warming. The estimated range of N2O going into the atmosphere annually from all sources is 4.4 to 10.5 TgN/year. That total includes N2O from oceans, tropical and temperate forest soils, fossil fuel combustion, biomass burning, and fertiliser.

Nonetheless, both combustion and intensive agriculture are part of human culture. Whatever the details of possible human-induced increases in atmospheric concentrations of the greenhouse gas nitrous oxide, they still offer a reminder that human population cannot grow endlessly and still sustain living conditions on Earth.

Related Articles:

- Gases Introduction
- Carbon Dioxide
- Halocarbons
- Methane
- Nitrous Oxide
- Ozone
- Water Vapour