A colorless, sweet-smelling gas with a long history as a medical and dental anesthetic is the next big threat to Earth's protective ozone layer, according to new research.
The culprit: nitrous oxide.
Its role in destroying ozone has long been recognized, as well as its role as a heat-trapping greenhouse gas. But the new study puts nitrous oxide's ability to deplete ozone into numbers comparable to those used for other ozone-depleting gases covered by the 1987 Montreal Protocol.
Nitrous oxide's ozone-depleting clout per pound of gas is comparable to that of a group of chlorine-based gases, HCFCs, that currently are scheduled to be phased out by 2013 under the protocol. And each molecule of nitrous oxide remains in the atmosphere for about 100 years, giving it a lifetime comparable to the compounds covered by the Montreal Protocol.
The new study "puts nitrous oxide onto the same playing field" as other ozone-depleting gases, says John Daniel, one of the trio of scientists at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory that conducted the study. The lab is in Boulder, Colo. The team is reporting its results in Friday's issue of the journal Science.
Near Earth's surface, ozone is considered a pollutant. But in the stratosphere, it shields the surface from much of the sun's ultraviolet light. Scientists have linked excessive amounts of UV radiation to a range of environmental and health problems.
Nitrous oxide has a range of natural and human-made sources. The largest man-made source is agriculture, where the gas gets emitted after bacteria in soil break down the nitrogen in chemical fertilizers as well as in manure-based fertilizers. Nitrous oxide also comes from burning fossil fuels and from burning biomass.
The team notes that while nitrous oxide from all sources is growing by 0.25 percent a year, emissions from human sources are growing at about 1 percent per year.
The Montreal Protocol and its amendments ban the production and use of chlorine- and bromine-based gases once widely used in refrigerators, air conditioners, and as fire suppressants. These gases, especially a group known as chlorofluorocarbons (CFCs), were seen as the main threats to the ozone layer.
The problem was most pronounced over Antarctica, where a special mix of regional weather patterns, 24-hour sunlight, and other factors triggered a dramatic, annual ozone hole during the continent's summer.
Globally, however, Earth's ozone layer has thinned by 5 to 6 percent since 1980, before CFCs and their ilk came into wide use, according to Akkihebbal Ravishankara, who led the study. He and his colleagues note that 6 percent may appear to be a small number, but it still can lead to significant effects on organisms at Earth's surface.
But as negotiators tried to figure out which gases to include in the protocol, they focused on the most destructive gases they knew of at the time. So nitrous oxide, or N2O, wasn't included among the gases. Indeed, N2O initially was seen as beneficial; when it reacts with the chlorine compounds, the reaction in effect neutralizes either other's ozone-depleting traits.
N2O's emergence as the next major threat represents a kind of left-hand complement to the Montreal Protocol's effectiveness in taking the worst chemical villains out of the picture.
"I haven't seen such a clear presentation that N20 would take over" as the biggest threat to Earth's ozone layer, says Drew Shindell, who studies stratospheric chemistry at NASA's Goddard Institute for Space Studies in New York.
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