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Humankind doesn't have a great track record when it comes to cleaning up environmental messes, but there was one time we really outdid ourselves. That was back in 1989, when over 190 nations signed the Montreal Protocol, phasing out the use of chlorofluorocarbons (CFCs). The decade before, scientists had discovered that CFCs were blowing a hole in the ozone layer over Antarctica, exposing us to dangerous ultraviolet radiation and boosting the risk of skin cancer. Today, CFCs are no longer in widespread use, and the ozone layer appears to be on the mend.

But even with that battle all but won, scientists are finding a new man-made threat to the ozone layer: nitrous oxide (N2O), otherwise known as laughing gas. A study published in the Aug. 28 Science found that N2O — a by-product of agricultural fertilizer and a number of other industrial processes — is now the biggest ozone-depleting gas in the air, and could present a real threat to the ozone layer in coming decades. And worse, unlike CFCs, N2O — which also adds to global warming — is not regulated by the Montreal Protocol, meaning there is no global effort to try to reduce emissions.

"Pretty soon human-caused N2O emissions will be greater than all other ozone-depleting substances combined," says John Daniel, an atmospheric scientist with the National Oceanic and Atmospheric Administration (NOAA) and a co-author of the Science study. "It will be the dominant gas in the future."

The idea that N2O poses a threat to the ozone layer is not new, but the Science study is the first comprehensive look at the exact concentrations and consequences of the gas. The investigators found that although N2O is only one-sixtieth as dangerous to the ozone layer as CFCs on a gram-by-gram basis, the sheer amount of N2O — each year nearly 1 billion metric tons of CO2 equivalent are released globally — means that it now poses a more significant threat to the atmosphere. (N2O emissions are calculated in terms of their impact on global warming, and CO2 is used as a kind of base level.)

The news isn't all bad: the fact that scientists can now turn so much of their attention to the dangers of N2O is in part because CFC levels have dropped so low, thanks to the Montreal Protocol. But N2O is likely to prove much more difficult than CFCs to phase out. While CFCs had a relatively narrow range of uses — and chemical companies like DuPont were able to come up with replacements quickly — N2O is all around us, tied intimately to our industrial way of life. The millions of tons of soil fertilizer used in U.S. agriculture alone add N2O into the atmosphere, as do livestock manure, sewage treatment and automobiles. And it's not just our doing: two-thirds of global N2O emissions come from the planet itself, as bacteria in soil and the oceans break down nitrogen. Though N2O is regulated by the Kyoto Protocol of 1997 as a greenhouse gas — and one that is nearly 300 times more potent for global warming than CO2 — that treaty doesn't cover all nations, and will expire in 2012. "The question is how are we going to reduce these gases," says Daniel. "We need to bridge that gap between science and policy."

Such a multifaceted problem will require a multifront solution, and some good ideas might come up at the U.N. Climate Change Summit in Copenhagen in December. Reducing the quantity of fertilizer used in farming, switching to a less meat-heavy diet and lowering the number of cars on the road while boosting fuel economy will all help. The planet itself will continue churning out its own N2O, of course, but the planet did that for eons. It was our N2O production that pushed the gas past the tipping point — requiring that we now push it back. "It can be a win-win phasing out these gases, both for climate and the ozone," says Robert Portman, an atmospheric scientist at NOAA and a co-author of the study. If we fail, we won't be laughing about nitrous oxide.

• Bryan Walsh
• Scientists are finding a new man-made threat to the ozone layer: nitrous oxide (N2O), otherwise known as laughing gas