Nitrous Oxide—No laughing matter

After carbon dioxide and methane, the third most important “greenhouse” gas in Earth’s atmosphere derived from human activities is nitrous oxide (N2O). It is odorless and colorless and most familiar to us as the laughing gas used as an anesthetic in the dentist’s office. Chemically it contains two nitrogen atoms and one oxygen atom in a linear molecule.   It is derived naturally from the action of microbes in soils and seawater, and the rising concentration of N2O in Earth’s atmosphere largely stems from the increasing use of nitrogen fertilizers, which are transformed by soil microbes producing a small amount of N2O as a byproduct. .

Nitrous oxide is found only at low levels—about 320 parts per billion in the atmosphere. But it has increased to that level from concentrations of about 265 parts per billion 100 years ago. The increase has special significance because each molecule of N2O in the atmosphere contributes about 300X the global warming potential as each molecule of CO2. If we could do even something small about the rise of N2O in Earth’s atmosphere, it could have a big contribution to solving the problem of global climate change.

This is easier said than done.   A small amount of nitrous oxide—about 5% of human-derived emissions—is consumed in soils that are not fertilized. The largest fraction of nitrous oxide has a long residence time in Earth’s atmosphere, so it mixes into the stratosphere where it is destroyed in a complex set of reactions that also destroy ozone.   In fact, as the production of chlorofluorocarbons has been reduced, the predominant remaining source of ozone destruction in the stratosphere stems from N2O.   If we were to cut down on nitrous oxide emissions, the recovery of the ozone hole would be more rapid.

All this points to the need to use nitrogen fertilizer judiciously and more efficiently, and to preserve wetlands that naturally convert nitrogen in fertilizer runoff to N2 gas, without a large production of N2O as a byproduct. In practice, it will probably be easier to reduce and capture CO2 emissions, which have known “point-sources” to the atmosphere than to reduce N2O emissions which are produced by microbes in a wide variety of soils. Nevertheless, attention must be paid to nitrous oxide for the dual benefit of reducing its impacts on climate change and on thinning ozone in the stratosphere.



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