Fertilizers used to nourish biofuel crops, such as the grasses pictured here, release ozone-destroying nitrous oxide into the atmosphere (Credit: alh1, Flickr user)

Already in doubt as an alternative to fossil fuels, biofuels might also contribute to destruction of the protective layer of ozone in the upper atmosphere, a new study says. Some fertilizers used in biofuel production can release ozone-destroying chemicals. And if the rate of carbon dioxide emissions were to decline as biofuels replaced fossil fuels, according to the new research, those chemicals could be even more damaging and stall any potential ozone recovery achieved through cutting back on other ozone-depleting gases.

Previous studies have indicated that biofuel production contributes to climate change. They found that greenhouse-gas emissions from converting land to biofuel crops such as corn and soybeans, and from vehicles and other emissions associated with cultivating those crops, exceeds reductions from burning the biofuels in lieu of fossil fuels.

Farmers typically fertilize biofuel crops with nitrogen-based fertilizers that release nitrous oxide into the atmosphere when they decompose. This new research examines possible negative effects from a combination of increased nitrous oxide emissions from biofuel fertilizers and somewhat restrained growth of atmospheric carbon dioxide concentrations because of replacement of considerable fossil fuels by biofuels, said Laura Revell, a PhD candidate at the National Institute of Water and Atmospheric Research in Christchurch, New Zealand. Revell is the lead author of the study published May 18 in Geophysical Research Letters, a journal of the American Geophysical Union.

For the purposes of their analysis, the researchers treated biofuels as if they don’t add net carbon emissions to the air from land-use conversion and crop production.

Nitrous oxide is already well known for harmful effects in the atmosphere: It  destroys stratospheric ozone, can linger for up to 120 years, and ranks above both methane and carbon dioxide in the potency with which it contributes to global warming. A previous study projected it to be the dominant ozone-depleting gas emitted this century.

Fossil fuel burning releases carbon dioxide that warms the lower atmosphere, trapping in the heat. When that happens, the heat doesn’t leak farther up, into the stratosphere, as it otherwise would. With the stratosphere losing heat to the vacuum of space, but not gaining it from the lower atmosphere, it cools down.

This cooling slows down the frequency with which nitrous oxide reacts with ozone. If, however, there was less carbon dioxide trapping heat in the lower atmosphere, then the stratosphere would be warmer. Then, nitrous oxide would react with and destroy ozone molecules more frequently.

Revell and her co-authors ran simulations to explore the effects on the atmosphere over the next century of three greenhouse-gas emissions scenarios. They adapted two of the scenarios – one a relatively high carbon-dioxide emission scenario, the other with a more modest rate of carbon dioxide output – from the United Nations Intergovernmental Panel on Climate Change (IPCC). To create their third scenario, the scientists adjusted the more modest of their IPCC scenarios to include a lower carbon dioxide emission rate but also a higher nitrous oxide emissions rate – both from biofuel use.

In the high-emissions scenario, atmospheric carbon dioxide increases by 77 percent (up to 700 parts per million (ppm)), and nitrous oxide increases by 14 percent (up to 370 ppm) by the end of the century. While the greater amount of carbon dioxide would increase the greenhouse effect, the scenario is actually a plus for ozone, the simulation indicates: Ozone’s concentration would grow by 3 percent over its current level due to a cooling upper atmosphere.

In their biofuel-scenario simulation, the research team looked at what would happen in a future in which biofuels production and consumption became widespread. While carbon dioxide increased by a smaller amount (by 35 percent, up to 525 ppm)  by 2100, an accompanying 36 percent rise (up to 440 ppm) in the concentration of nitrous oxide would take a unwelcome toll on stratospheric ozone: an almost 1 percent drop in its average global concentration.

There has been a steady increase in ozone levels since 1998, and scientists expect ozone will recover and return to normal (pre-1980) levels by 2075 because of the Montreal Protocol, a United Nations treaty created in 1987 that stipulates the phasing-out of ozone-destroying gases. But under the biofuels scenario, this ozone recovery is stymied to some extent, due to increased nitrous oxide in a warmer stratosphere, Revell said.

Revell, L., Bodeker, G., Huck, P., & Williamson, B. (2012). Impacts of the production and consumption of biofuels on stratospheric ozone Geophysical Research Letters, 39 (10) DOI: 10.1029/2012GL051546

-Eric Villard, AGU science writing intern