Bad news for dentists and their patients? Scientists at the National Oceanic and Atmospheric Administration today released results of a study (published in the journal Science) showing that nitrous oxide is now the top human-caused source of ozone-layer depletion, knocking chlorofluorocarbons (CFCs) out ot the top spot.

Although nitrous oxide emissions are slightly on the rise, the change is mostly due to the successful regulation and reduction of CFCs (remember how we used to have spray-on deodorant?) since the Montreal Protocol of 1987. But this still could be a major policy flashpoint, because unlike CFCs, there has to date been no regulation of nitrous oxide as an ozone threat. With the release of this report we should see increased discussion of the role of agriculture, and specifically animal agriculture, in terms of global environmental threats.

Why agriculture? What about all that laughing gas, which goes in, does its crazy work on the brain, and then is exhaled into the air? Well, in relative terms, the overall use of nitrous oxide as a pain suppressant is a drop in the bucket compared to agricultural sources of nitrous oxide emissions. According to the EPA's "U.S. Emissions Inventory 2009," the biggest human-caused emissions source for nitrous oxide, far outpacing all the others combined, is "Agricultural Soil Management." Direct use as an inhalant (whether legit or otherwise), represents less than two percent of the total.

When you look further (PDF) into what the EPA defines as "Agricultural Soil Management," this is what you find:

management practices that add, or lead to greater release of, mineral nitrogen to the soil, including fertilization; application of managed livestock manure and other organic materials such as sewage sludge; deposition of manure on soils by domesticated animals in pastures, rangelands, and paddocks (i.e., by grazing animals and other animals whose manure is not managed); production of nitrogen-fixing crops and forages; retention of crop residues; and drainage and cultivation of organic cropland soils.

Animal manure from livestock plays a pretty large part in this category of emission, in addition to having its own category, "Manure Management," which ranks fourth out of the 15 categories of nitrous oxide emissions sources. (Many people believe farm-animal manure is a necessary component of effective fertilization, but it isn't.) Add to this the fact that a huge portion of plant agriculture is feed crops for food animals, and the latter issue is certainly one that will need more investigation by scientists as well as regulators to exactly quanitfy manure's effect in this area.

Study author A.R. Ravishankara said as much in a conference call yesterday about the findings: "It's important to know how just much of this comes from that source [animal agriculture]," he said, adding that the latter will undoubtedly be a major area of research in coming years among environmental scientists. Author J.S. Daniel chimed in that since it's also a prominent greenhouse gas (one 300 times stronger than carbon dioxide, by the way), "it's a win-win to phase out nitrous oxide as both" a GHG and ozone-depleting substance. This is, of course, all on top of the well-established prominent role of animal agriculture in causing global warming.

If we're serious about stopping ozone-layer depletion and fighting climate change, governments will enact policies to address this major contributor to both phenomena. Sure, that could eventually mean it's harder to get laughing gas at the dentist, but right now, what makes the most sense in terms of the raw numbers is to greatly reduce the amount of manure being generated and used by the agricultural sector - in other words, reducing the number of animals raised for food.