Nitrous oxide poses a growing atmospheric problem.
Nitrous oxide (N2O) has become the greatest threat to the ozone layer, a new analysis suggests. The ozone-destroying abilities of the gas have been largely ignored by policy-makers and atmospheric scientists alike, who have focused on the more potent chlorofluorocarbons (CFCs) — historically the dominant ozone-depleting substances in the atmosphere.
But CFC levels have been falling since the 1989 adoption of the Montreal Protocol on Substances that Deplete the Ozone Layer, an international agreement that mandated the phasing out of CFCs, and more recently hydrochlorofluorocarbons (HCFCs). Meanwhile, nitrous oxide levels have been climbing as a result of increased emissions from agricultural fertilizers, biomass burning and animal waste.
Atmospheric chemist A. R. Ravishankara and his colleagues at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, have now used a chemical model of stratospheric ozone to calculate the ozone-depleting potential (ODP) of nitrous oxide. That provides a measure of how much ozone is depleted by a particular gas, relative to that destroyed by the same amount of trichlorofluoromethane (CCl3F, also known as CFC-11), one of the most significant ozone-depleting substances.
"We wanted to see how nitrous oxide stacked up as an ozone-depleting gas," says Ravishankara. "People haven't looked at it this way before."
They computed the ODP of nitrous oxide at 0.017, or about one-sixtieth of that of CFC-11. This seems like a pretty feeble punch, but when the authors took into account the large scale of human-related emissions of nitrous oxide — as estimated in the latest report from the Intergovernmental Panel on Climate Change — they found that nitrous oxide has the greatest impact of the ozone-depleting substances emitted by human-related activities today.
Nitrous oxide is also a major greenhouse gas which is controlled under the Kyoto Protocol on climate change, although emissions are not currently expected to fall significantly in the coming century. The authors project that if nitrous oxide emissions are not reduced, they could be 30% more destructive to ozone in 2050 than the combined CFC emissions from 1987, when these were at their peak (see 'The threat of nitrous oxide'). The team's results are published online by the journal Science1.
"This is the first time someone has dealt with nitrous oxide in isolation like this," says atmospheric chemist Susan Solomon of the NOAA, who was not involved in the study. "It's one of those things that has simply been overlooked."
Atmospheric scientists have known since the 1970s that nitrous oxide depletes the ozone layer, but did not group it with other ozone-depleting substances because it seemed to be impotent relative to CFCs.
Atmospheric scientist Don Wuebbles at the University of Illinois in Urbana-Champaign agrees that nitrous oxide deserves more attention. "In a sense, nitrous oxide is almost a forgotten gas. When we talk about ozone, we talk about halocarbons. When we talk about climate, we talk about carbon dioxide and methane. We forget that nitrous oxide is the third largest-growing gas in the atmosphere."
The findings won't come as a surprise to most atmospheric scientists, says Ravishankara. "Everyone's going to say they knew it. But that's not the same as showing it."
That distinction has important implications for policy-makers, who use the ODP to make quantitative comparisons between ozone-depleting substances. "Without this information, decision-makers do not have the tools to evaluate the role of nitrous oxide in ozone-layer depletion. In that sense, we have bridged the gap between policy relevance and atmospheric science," says Ravishankara.
But not everyone is concerned about nitrous oxide's impact on the ozone layer. "Nitrous oxide sort of died out as a problem [for the ozone layer] in the 1970s, because we knew it was increasing at such a slow rate," says atmospheric chemist Richard Stolarski at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "In our chemical climate models, where nitrous oxide increases by 15 or 20 per cent by 2100, we still end up with more ozone than we had in 1960 [before mass production of CFCs]."
Ravishankara notes that ozone-depleting gases should still be a cause for concern. "Now it's up to the decision-makers on how they're going to deal with this," he says. "This is just one piece of information to feed into the discussion."
Ravishankara, A. R., Daniel, J. S. & Portmann, R. W. Science advance online publication doi:10.1126/science.1176985 (2009).