No laughing matter: Nitrous oxide rose at end of last ice age

December 10, 2014, Oregon State University
Researchers measured increases in atmospheric nitrous oxide concentrations about 16,000 to 10,000 years ago using ice from Taylor Glacier in Antarctica. Credit: Adrian Schilt

Nitrous oxide (N2O) is an important greenhouse gas that doesn't receive as much notoriety as carbon dioxide or methane, but a new study confirms that atmospheric levels of N2O rose significantly as the Earth came out of the last ice age and addresses the cause.

An international team of scientists analyzed air extracted from bubbles enclosed in ancient polar ice from Taylor Glacier in Antarctica, allowing for the reconstruction of the past atmospheric composition. The analysis documented a 30 percent increase in atmospheric concentrations from 16,000 years ago to 10,000 years ago. This rise in N2O was caused by changes in environmental conditions in the ocean and on land, scientists say, and contributed to the warming at the end of the ice age and the melting of large ice sheets that then existed.

The findings add an important new element to studies of how Earth may respond to a warming climate in the future. Results of the study, which was funded by the U.S. National Science Foundation and the Swiss National Science Foundation, are being published this week in the journal Nature.

"We found that marine and terrestrial sources contributed about equally to the overall increase of nitrous oxide concentrations and generally evolved in parallel at the end of the last ice age," said lead author Adrian Schilt, who did much of the work as a post-doctoral researcher at Oregon State University. Schilt then continued to work on the study at the Oeschger Centre for Climate Change Research at the University of Bern in Switzerland.

"The end of the last ice age represents a partial analog to modern warming and allows us to study the response of natural to changing environmental conditions," Schilt added. "This will allow us to better understand what might happen in the future."

Nitrous oxide is perhaps best known as laughing gas, but it is also produced by microbes on land and in the ocean in processes that occur naturally, but can be enhanced by human activity. Marine nitrous oxide production is linked closely to low oxygen conditions in the upper ocean and global warming is predicted to intensify the low-oxygen zones in many of the world's ocean basins. N2O also destroys ozone in the stratosphere.

"Warming makes terrestrial microbes produce more nitrous oxide," noted co-author Edward Brook, an Oregon State paleoclimatologist whose research team included Schilt. "Greenhouse gases go up and down over time, and we'd like to know more about why that happens and how it affects climate."

Nitrous oxide is among the most difficult to study in attempting to reconstruct the Earth's climate history through ice core analysis. The specific technique that the Oregon State research team used requires large samples of pristine ice that date back to the desired time of study - in this case, between about 16,000 and 10,000 years ago.

The unusual way in which Taylor Glacier is configured allowed the scientists to extract ice samples from the surface of the glacier instead of drilling deep in the polar ice cap because older ice is transported upward near the glacier margins, said Brook, a professor in Oregon State's College of Earth, Ocean, and Atmospheric Sciences.

The scientists were able to discern the contributions of marine and terrestrial nitrous oxide through analysis of isotopic ratios, which fingerprint the different sources of N2O in the atmosphere.

"The scientific community knew roughly what the N2O concentration trends were prior to this study," Brook said, "but these findings confirm that and provide more exact details about changes in sources. As nitrous oxide in the atmosphere continues to increase - along with carbon dioxide and methane - we now will be able to more accurately assess where those contributions are coming from and the rate of the increase."

Atmospheric N2O was roughly 200 parts per billion at the peak of the ice age about 20,000 years ago then rose to 260 ppb by 10,000 years ago. As of 2014, atmospheric N2Owas measured at about 327 ppb, an increase attributed primarily to agricultural influences.

Although the N2O increase at the end of the last was almost equally attributable to marine and terrestrial sources, the scientists say, there were some differences.

"Our data showed that terrestrial emissions changed faster than marine emissions, which was highlighted by a fast increase of emissions on land that preceded the increase in marine emissions," Schilt pointed out. "It appears to be a direct response to a rapid temperature change between 15,000 and 14,000 years ago."

That finding underscores the complexity of analyzing how Earth responds to changing conditions that have to account for marine and terrestrial influences; natural variability; the influence of different greenhouse gases; and a host of other factors, Brook said.

"Natural sources of N2O are predicted to increase in the future and this study will help up test predictions on how the Earth will respond," Brook said.

Explore further: Greenhouse gases: A new group of soil micro-organisms can contribute to their elimination

More information: Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation , DOI: 10.1038/nature13971

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not rated yet Dec 11, 2014
Do you think that man's entrance into the North American continent and subsequent migration to the South American continent (16,000-10,000 years ago) had anything to do with it (anthropogenic sources e.g. wildfires/controlled burns)? Dr.-Guy Robinson et. al. "Landscape Paleoecology and Megafauna Extinction in Southeast New York State", clearly demonstrated the profound influences that early man had on the landscape of this continent BEFORE NOT AFTER the rapid recession and retreat of the large ice masses.  There is no doubt in my mind that the copious amounts of soot spewed into the air from such widespread and vast fires help contribute to the end of the ice age by landing on the ice itself, accelerating the ice melt. This would have been an important secondary contributing factor, the sun being the primary. The increased levels of nitrous oxide (anthropogenic sources), that is being seen in this study only corroborates the soil findings.
1 / 5 (1) Dec 11, 2014
bradfordcutler - I hope I don't attract any vicious ad hominems, but there could very likely be another reasonable explanation for all the light elements and simple molecules such as CO2, N2O, H2, He, CH4, H2S and H2O which come from within the Earth. They vary with the activity of the GeoReactor.

I realize the work of Dr. J. Marvin Herndon has been entirely dismissed since he first published in 1992 (competing scientists refuse to peer review him). Fission and fusion are the complementary forces of the Universe. The naturally occurring fission reactors at the core of every planet are due to accretion of the heaviest elements (the fissionable Actinides) going to the core of all gas giants ( starting point of all planets and all stars) and going critical. Fusion in all stars is due to the thermonuclear fission trigger at the core of the star. Over time the GeoReactor becomes a self-limiting breeder reactor, varying as non-fissionables accumulate (cool), then diffuse out (warm).
not rated yet Dec 12, 2014
If you read the work done at Wisconsin and the initial work done by Dr. Robinson, as well and the work done in the Far East (Indonesia/Malaysia), you will see that man had a profound influence on the landscape through the use of fire. They still do.  Look at the NASA photos of Central America in the spring or any day in China.  We suffer greatly here in Texas due to the soot coming from Central America every spring, call any meteorologist here in the region and they will tell you this. In addition, coal soot is routinely collected and SEEN in Big Bend National Park from OHIO coal fired plants (every year), OK!  You can call the park service in Texas for that one as well. How is it that the glaciers in the Northwestern US are suddenly in an accelerated rate of recession when in fact there hasn't been any concomitant accelerated increase in aerial of ground temperatures that could possibly explain that phenomenon? I think I know why.
not rated yet Dec 12, 2014
I would also like to refer your readers to the Texas Green Report.

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